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Marine Biological Laboratory Library

Woods Hole, Mass.

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Presented by

IN MiiFiOiY OF

DR. HiilRBERT W. RAND
1872 - i960

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THE SENSES

OF

ANIMALS

SCOrXISH WILD CAT

THE SENSES

OF

ANIMALS

By

L. HARRISON MATTHEWS, Sc.d., f.r.s.

Scientific Director, The Zoological Society of London

and

MAXWELL KNIGHT, o.b.e., f.l.s.

PHILOSOPHICAL LIBRARY
NEW YORK

Published ig6j, by Philosophical Library, Inc.,
/J East 40th Street, New York 16, N. Y.

All rights reserved

(§) L. Harrison Matthews and Maxwell Knight, 1963

PRINTED IN GREAT BRITAIN

BY EBENEZER BAYLIS AND SON, LTD,

THE TRINITY PRESS, WORCESTER, AND LONDON

PREFACE

The senses of animals — and of man — are what keep them,
and us, in touch with the environment; they enable the
naturalist to observe the animals he is studying and the
animals to observe him and, very often, to avoid being
observed by him. We have long been interested in this
fascinating aspect of natural history and we hope that our
book will be acceptable to all who are interested in animals,
and in particular that it may be useful to the younger people
who are entering upon this delightful subject in their school
biology classes. Perhaps, too, those with more experience will
find some matters set forth that will suggest new lines of
profitable study.

In writing the book it seemed to us best to divide it into
two parts : the first explaining what the naturalist can observe
in the field ; and the second giving the more technical back-
ground of the way in which the senses of animals function.
In both parts we have tried to avoid the heavy going of too
many technical terms, and wherever it has been necessary
to use the language of science we have, we hope, sufficiently
explained it.

All branches of natural history are absorbing, but we think
that attention directed to the link between ourselves and the
subjects of our study will be rewarding to all naturalists
whether they be ornithologists, entomologists, mammalogists,
or specialists in any of the other numerous subdivisions of
knowledge about the living world. We trust they will find it
as full of interest and problems as we have.

L. Harrison Matthews
Maxwell Knight

ACKNOWLEDGEMENTS

The Authors and Publishers wish to acknowledge with
thanks permission to reproduce photographs from the follow-
ing: frontispiece and Nos. 4, 5 and 7, Geoffrey Kinns;
No. I, S. Beaufoy; No. 2, Philip Street; No. 3, Jane Burton;
Nos. 6 and 8, L. Hugh Newman's Natural History Photo-
graphic Agency, from H. Bastin and W. J. C. Murray re-
spectively; Nos. 9 and 12, Sdeuard C. Bisserot; No. 10,
Eric Hosking; No. 11, Douglas P. Wilson; No. 13, J. H. D.
Hooper; No. 14, Marine Studios, Marineland, Florida;
No. 15, Marineland of the Pacific, California; No. 16,
H. R. Hewer; Nos. 17 and 18, Sarawak Museum; No. 19,
Zoological Society of London; No. 20, R. M. Lockley.

CONTENTS

CHAP. PAGE

Preface 5

Acknowledgements 6

PART I

ANIMALS IN THE FIELD

[Maxwell Knight)

I The Dominant Senses 13

II Sight — General 21

^ III Sight — Field work and Experiments 30

IV Hearing — General 69

V Hearing — Field work and Experiments 76

VI Smell — General 89

VII Smell — Field work and Experiments 95

VIII Taste 109

IX Touch — General 114

X Touch — Fieldwork and Experiments 117

PART II

HOW DO THE SENSES WORK?

[L. Harrison Matthews)

XI Senses and Nerves 125

XII Sight 137

82988

8 CONTENTS

XIII What do Animals See? 146

XIV Sight in Invertebrates 156
XV Hearing 169

XVI Echo-location 182

XVII Smell and Taste 195

XVIII Touch 208

XIX Monitors 218

Index 23 1

ILLUSTRATIONS

FIG. PAGE

1. Oscillograph tracing of "action potential"

running along a nerve fibre 132

2. The jelly-fish ^wr^/m 133

3. A human eye cut in half to show the internal

structure 138

4. The eye of a bird cut in half to show the internal

structure 148

5. The cephalothorax in three different sorts of

spiders to show the ocelli 161

6. Front view of head of Privet Hawk Moth

caterpillar, showing ocelli 163

7. Front view of a Honey Bee's head, showing

ocelli and compound eyes 163

8. A section of the compound eye of a Honey Bee 164

9. A single ommatidium from the eye of a Honey

Bee 165

10. A diagram of the human ear 170

11. A cross-section of one turn of the spiral cochlea 1 72

12. A Lung-fish, showing the lateral line 181

13. The head of a Fulmar Petrel, showing the

tubular nostrils 199

14. Section of part of the tongue, showing taste

bulbs 203

15. A single taste bulb, showing ends of the receptor

cells 203

16. A Catfish, showing the barbels 210

17. The head end of a marine worm, showing the

four "feelers" beside the four eyes 212

18. A diagram of the three semi-circular canals,

showing their connection with the cochlea 219

19. (a) Freshwater crayfish, showing the second

pair of feelers 222

lO ILLUSTRATIONS

19. (b). Enlarged view of a "feeler", showing the

entrance to the statocyst 222

(c). The arrangement of the sensory cells and

sand granules inside the statocyst 222

20. A mosquito at rest with the hind feet raised

from the perch 223

Half-tone Illustrations

{Between pages 120 and 121)

Scottish wild cat Frontispiece

1. Dragonfly, showing the huge compound eyes

2. South American giant toads

3. Head of a Tawny Owl, showing third eyelid

4. Marsh Frog, showing external ear-drum

5. Fox-cubs with ears pricked

6. Emperor Moth, showing large feathery antennae

7. A Grass Snake's tongue — its organ of smell

8. Convolvulus Hawk Moth drawing in sweet nectar

9. Tongue of a frog touching a worm

10. A Ringed Plover beside its nest

1 1 . Quin scallops swimming away from a starfish

12. A Mouse-eared Bat in flight

13. Close view of the face of a Greater Horse-shoe Bat

14. A tame dolphin towing a girl on a surf-board

15. A whale tragedy

16. A mother Grey Seal identifying her pup by snifling

17. Cave Swiftlets on their nests

18. Close-up of Swiftlet nests

19. A Western Diamond-back Rattlesnake, showing the

sensory pit

20. The Manx Shearwater that homed across the Atlantic

PART I

ANIMALS IN THE FIELD

by Maxwell Knight

THE DOMINANT SENSES

Not all field naturalists are biologists and, it must be
admitted, by no means all biologists are field naturalists —
more's the pity ! Every field worker would be better for some
knowledge of biology, including as it does, both morphology
and physiology; while the biologist, whether student or
teacher, would be more fully equipped if he included a
regular amount of field natural history in his working
routine.

High on the list of subjects which those who prefer to
indulge in observations out of doors should embrace, is the
fascinating and essential one of the senses of animals. Not
only is this aspect of the behaviour of living creatures well
worth while in itself, it is necessary because without some
knowledge of the senses which govern, to a greater or lesser
extent, the lives and habits of animals, true field work
becomes impossible.

Unless a student has at least some general idea as to which
senses play the most important part in the activities of the
animals to be studied, it will mean that observation of many
kinds of creatures will be either abortive or incomplete. If,
for instance, one is set upon finding out more about the
behaviour of badgers, there will be little or no success if that
attractive mammal's sensory equipment is not understood.

Now all groups of animals have some sense or senses which
are more highly developed than others, and these are
generally referred to as dominant senses. In the case of certain
groups it may be that one sense is far more highly evolved
than all the others; but there are instances where it is
difficult to be precise about this because some other sense
may also be of a marked degree of acuteness ; and our know-
ledge may not yet be advanced enough for us to be able to
say with certainty that such-and-such an animal depends

14 THE SENSES OF ANIMALS

more on — say — sight than anything else. Their hearing may
be as sensitive as their eyes, but this sense may require much
more investigation both in the laboratory and in the field
before a true assessment can be made.

However, from a practical point of view it is possible and
desirable to have some idea as to which sense is most probably
the dominant one. We humans are so much dependent upon
our eyes that it is often difficult to appreciate that there are
many animals which have little or no seeing abilities; yet
they conduct their lives most efficiently, making up for the
lack of sight or poorly developed sight by an enormously
increased sense of hearing or smell.

It is almost impossible to place animals neatly into some
convenient category as regards their senses ; but one can give
what is certainly more than a rough outline as to which senses
play the most important part in the lives of the various
Classes, Orders and Families.

Before attempting to do this, it is probably wise to remind
students that no animal possesses an effective sense that is of
no practical use to it. Hence one can approach the subject in
two different ways : one can observe an animal's behaviour,
and from some action or actions one can deduce that a
certain sense is clearly present or absent. For example, frogs
croak ; and even without going deeply into the physiological
side of the frog's vocal behaviour, it is obvious that frogs
would not croak unless other frogs could detect the sounds
uttered. This may seem all too obvious ; but it is surprising
how often the question is asked, "Do frogs have ears?" There
is a pitfall here for those who cannot rid themselves of con-
stant comparisons between human beings and other animals.
If hearing is mentioned, we too often think of an outer ear
like our own, and fail to realize that what we call hearing is
a matter of receiving and interpreting vibrations. These
vibrations are conducted to our brains, in the first instance,
by means of our outer ear ; but this does not mean that other
creatures cannot detect sounds and react to them even when
they do not possess an outer ear at all.

Be this as it may, observation in the field can draw our
attention to the fact that frogs utter sounds, and by deduction
we can soon appreciate that these sounds would not be made

THE DOMINANT SENSES I5

unless they could be received by other individuals of the
species ; and therefore it may be said that frogs can "hear"
or, if one puts it more pedantically, frogs have some auditory
apparatus.

The other way of reaching the same conclusion is to study,
with the help of an expert, the actual auditory mechanism
and nervous system of a frog. Once demonstrated this will
show, by quite different means, that there really is some
hearing apparatus by which the frog receives the sounds
made by other frogs.

Now as to the senses themselves: these are sight, smell,
taste, hearing and touch. Nearly all actions performed by
animals are governed by these, sometimes in combination
and sometimes even singly. In implying some reservation by
using the words, "Nearly all actions . . .", we must remember
that there are some kinds of animal behaviour which at
present we cannot explain. The way in which dogs and cats
can find their way home over what is quite unfamiliar ground
is a good example of this. It may well be that in the course
of time these extraordinary journeys will be explained in
terms of the senses we already understand, but at present
students of animal behaviour are baffled by the many well-
proven accounts of these amazing performances. More will
be said about this subject later on.

Let us now consider the matter of the dominance of
certain senses in various groups of animals and see in what
kind of world such animals live.

The lowest forms of animal life, the Protozoa, being uni-
cellular (one-celled) will not be expected to possess anything
but the simplest of senses — if indeed they can be truly said
to have any at all. They respond to certain stinmli : light and
touch (or vibrations in their environment) but as they have
neither brains nor proper nervous systems, they are very
limited in their sensitivity.

The still simple, but somewhat higher kinds of animals —
mostly aquatic — such as Rotifers, Hydra, Planarians (flat-
worms), though not unicellular, are also very limited in their
sensory equipment. They cannot "see" even if they react
positively or negatively to light ; they cannot "hear" but they
will react to vibrations both from the air and the water in

l6 THE SENSES OF ANIMALS

which they live ; they neither smell nor can they taste but
they respond to chemical stimuli in a primitive fashion. It is
not until we reach the spiders, insects, and similar Classes
of creatures that we really come across signs of sensory
perception in the way in which we normally use the term ;
and even among these groups it is by no means easy to indi-
cate which senses predominate; for a certain sense in one
kind of insect, for instance, may be lessened or subordinate
to another sense in a closely related kind.

Spiders can certainly see, otherwise they would not per-
form such complicated courtship dances as some species are
known to carry out. It is obvious that web-building spiders,
with greatly varying types of webs, are able to detect the
slightest touch or vibration that affects their snares. They
can also taste-by-touch as insects do, for they will taste
certain food items some of which will be discarded at once
— but can they smell? At first thought it is sometimes con-
sidered that, as in many higher animals — notably ourselves —
the sense of smell and that of taste are very closely linked,
there must be an ability to smell if taste is present ; but this
is not always so.

It will be seen, therefore, that web-spiders probably rely
mostly on their tactile sense, except during some certain
types of courtship ; but hunting spiders can see their prey
well so long as it is moving. This question of sight and move-
ment will arise again in connection with other kinds of
animals.

In the world of Insects we come across many variations in
regard to dominant senses. Dragonflies, for example, have
wonderful visual acuity as all can see for themselves if they
watch one of the "hawker" species flying up and down its
regular beat and at times suddenly going swiftly off course
only to return to the original beat a moment later. This
deviation from course is for the purpose of catching some
insect prey which is frequently as small and fragile a thing
as a gnat. Such a performance denotes eyesight of a high
degree. On the other hand, there are beetles such as the
burying beetles which have a fantastic ability to detect
odours at long range ; while the scenting powers of some of
the moths seem to be even greater. A male Emperor Moth

THE DOMINANT SENSES I7

can detect and locate a female giving off her scent from over
a mile away. Of course, as will be fully explained in Part II
of this book, the reception of these scent particles is carried
out by the antennae— those marvellous structures which
serve as organs of touch and smell and taste. It is as well to
emphasize here that when referring to touching or smelling
or hearing in relation to many of the animals to be dealt
with, it is not implied that all creatures which respond to, or
have the ability to touch or smell or receive sounds, have
fingers, noses, and ears. One must at times fall back on
words or terms of convenience. (See Plate 6.)

In the Amphibians and Reptiles there are again variations
in respect of the senses on which some groups rely ; and it is
not possible to generalize without taking families into con-
sideration. A few comments will suffice to make this clearer.
The amphibia include frogs, toads, newts and salamanders.
In all these sight plays the most important role in feeding
behaviour so long as the prey is moving. No amphibian will
feed unless it has been attracted to the particular prey by its
movement and in some cases smell, and this suggests that
these animals do not perceive a clear picture of what is in
front of them — it is the movement that stimulates attack.
However, the fact that some of the creatures they snap up
are rejected quickly certainly points to a sense of taste ; the
sense of smell would seem to be present in frogs and toads,
while newts are known to smell well in water and probably
to some extent on land also.

It has already been shown that frogs and toads can utter
and detect sounds, but newts, being voiceless, have no need
to hear.

In the reptiles the eyes are of quite complicated structure
but, as in the amphibians, it would seem that movement is
once again an important factor in connection with the feed-
ing behaviour, although prey which is still will sometimes
be attacked.

This is explained by the complex roll of the tongue plus
Jacobson's organ about which more will be said in due
course. The tongues of snakes and lizards act as organs of
scent and also as most delicate organs for the perception of
vibrations. Vibrations are also picked up through the bones

l8 THE SENSES OF ANIMALS

of the lower jaw, and it can thus be said that even if these
reptiles do not have ears, they can "hear" in a rather
specialized way. At the same time it is very doubtful if
snakes truly hear the sounds made by a snake charmer's
pipe; it is more likely that the rhythmic movements made
by a snake under these conditions are due to the eyes
being able to follow the side to side motions made by the
charmer himself. My own experiments will be referred to
later.

Although snakes use their tongues to pick up particles of
scent, there is no evidence that taste is present. Many snakes
eat toads which are known to have an unpleasant taste. It
is therefore probable that snakes and most likely lizards as
well, depend much upon their sense of smell, though lizards
rely on scent to a lesser degree, their eyesight being very
acute indeed.

The vast world of fishes presents some interesting sense
features. Generally speaking, most of the fishes can see well,
but there are some puzzles in the cases of some deep sea
species which live in almost total darkness yet they have
quite large eyes. There are, too, certain fishes that dwell in
caves and which are known to be blind. The sense of smell
is very well developed in fish as widely different as sharks
and trout ; and it seems more than likely that while sight is
used (and used efficiently) when water is clear; in muddy
water, through which light penetrates indifferently, fish fall
back on their olfactory sense to locate food. The sense of
taste is well developed.

It is a commonplace occurrence to hear anglers say that
voices should be kept low when fishing, in case the fish will
thereby be disturbed ; but throughout a lifetime of anghng
I have never had reason to think that ordinary conversation
has any effect at all on the behaviour of fish. Nevertheless,
fishes are extremely susceptible to vibrations — particularly in
the water, or on the land immediately adjoining the water ;
and they possess highly specialized nervous equipment which
responds to vibrations and thus brings about escape be-
haviour of one kind or another. The mechanism involved in
this kind of sensory perception is known as the "lateral line"
and it will be fully described in Part II.

THE DOMINANT SENSES I9

Marine life, other than fishes, includes an enormous
number of greatly varying creatures ranging from the minute
organisms which make up the animal part oi plankton to the
largest mammals of all, the whales ; and, of course, seals as
well. A complete book could be written about the senses of
all the numerous groups of animals that live in the sea ; but
reference will only be made to the most commonly studied
kinds in dealing with the separate senses in following
chapters.

Birds, like ourselves, for the most part live in a "seeing
world", and the complex way in which the eyes of different
Families work is one of the most interesting aspects of bird
life. Sight plays a major part in feeding, courtship, and
escape from enemies, but in considering this sense it must not
be forgotten that, as is the case in other groups of animals,
the sense of hearing plays a big part too.

It seems quite certain that many kinds of birds have a
well-developed sense of taste, yet it is thought that with the
exception of geese and ducks, they are very deficient in
respect of smell. This is curious considering that the two
senses of smell and taste are so closely linked. To me it seems
so curious that I am tempted to suggest that much further
research is needed before accepting the frequently expressed
opinion that very few birds can smell at all.

The sense of touch is not one that at first would seem to be
important to birds; but their beaks (bills) and even their
bodies are capable of very delicate tactile perception. The
nestlings of many species are, in the first few days of their
lives, very dependent upon vibrations to stimulate a feeding
reaction ; and there are other examples of this kind of thing
which will be given in greater detail later.

When we reach the Mammals we are dealing with animals
most of which live in a world of smells ; but hearing once
more plays a large part in mammalian life and to a greater
extent than is often thought. We may see a mammal in a
zoo, apparently asleep, and under conditions of complete
safety; yet deep sleep as we know it is most likely a rare
phenomenon, for even when no enemy can threaten, the
ears are constantly on the alert. This, of course, shows how
deep-seated is the necessity for self-preservation. It is most

20 THE SENSES OF ANIMALS

unusual for a mammal of even the largest size to shut off its
delicate senses when at rest. The constant twitch of the outer
ear (when present) will show this to be true, and there is
more than a little in the phrase "cat-nap" — watch your own
domestic cat and see for yourselves.

Sight, in the vast majority of mammals, is certainly subor-
dinate to smell and hearing ; taste is well developed in many
greatly differing species, but it is interesting to note that even
the apes, which without doubt have a very fine appreciation
of taste, always smell an unfamiliar piece of food before put-
ting it into their mouths.

All mammals have the sense of touch, though not always
by means of the parts of their anatomy one would expect.
Whiskers, the external nasal processes, and the lips are used
where there are no fingers or prehensile toes.

This brief survey of the most important senses, in some
kind of order of priority, will, it is hoped, prepare the student
for the more detailed account of the way in which the senses
are used by the diverse groups of animals with which we
shall deal throughout this book.

II

SIGHT— GENERAL

Quite apart from the anatomical and physiological aspects
of the senses which will be dealt with fully in Part II, it will
be helpful to the field naturalist to have a broad idea of the
subject under the heading of each sense. It seems right
to commence with sight, but before going into the visual
abilities of the different groups of animals it may be as
well to consider the limits within which we can study this
sense.

When dealing with the higher animals it is comparatively
easy to convey what one means in talking of "sight" ; but it
must be kept in mind that if the word is taken literally it
implies the reception of some more or less definite image.
However, among lower forms of life there are light receptive
organs which produce positive or negative reactions in the
creature concerned, but these cannot by any means be
termed "eyes".

Some brief reference to these organs must be made if the
complicated eyes of animals which are more highly developed
are to be properly appreciated. Fieldwork and the collection
of these lower organisms will be made easier if a student
knows where best to find a given type in relation to its
toleration or otherwise of light.

In the Protozoa, for instance, there are many kinds to be
found in areas of water which receive a considerable amount
of light ; on the other hand, there are some simple aquatic
forms which seek the dark. The various kinds of water-fleas
(which are really crustaceans) can also be caught in well-
lighted areas; but on land earthworms will return to their
burrows if a light is flashed on them.

All these creatures have a means of perceiving light, and
water-fleas have "eyes" which can be clearly seen under a
low-power microscope — yet these "eyes" do not present

22 THE SENSES OF ANIMALS

images to help in getting food or in avoiding obstacles and
enemies.

It is probable that some readers of this book will have less
knowledge than others; and, therefore, it is reasonable to
include some fairly elementary matter which, it is hoped,
will not put off the better-informed student.

Light is one of the stimuli which brings about a reaction —
either positive or negative ; and we speak of animals being
positively or negatively phototactic. Very simple experiments
will demonstrate this kind of behaviour ; and one such experi-
ment can be carried out with water-fleas and earthworms
with no more elaborate apparatus than a jar of water con-
taining some of the "fleas" and a roughly made cylinder of
black paper with a hole the size of a halfpenny cut in it.
In the case of earthworms one requires a box of earth and
some worms. A source of strong light will be necessary for
each experiment.

Having got the jar of water-fleas ready, observe how these
tiny crustaceans swim about in a very haphazard way. Then
place the cylinder over the jar, and after a few minutes direct
a beam of light on to the place where the hole has been cut.
In a very short time it will be seen that the spot where the
light penetrates will be crowded with the water-fleas. They
have been affected positively by the light and have gathered
in numbers where the light is strongest.

Where earthworms are concerned, if some of them are
placed on top of some loose earth in a normally lighted room
there will be immediate reaction by the worms, they will
start to burrow. If the strong light source is switched on so
that it falls directly on the worms their burrowing move-
ments will be speeded up and it will not be long before all
of them have disappeared ; they have been negatively affected
by light and have moved to get away from it as quickly as
possible.

The larvae of some insects will behave in the same manner
as earthworms ; and if some maggots are put in a box, half
of which is in darkness and half well lighted, the maggots will
wriggle their way to the dark portion to avoid the light.
Certain caterpillars of moths and butterflies will show that
some tolerate light while others do not. The larva of the

SIGHT — GENERAL 23

elephant hawk moth feeds almost entirely after dusk, and
during the day can be found only down on the bottom stems
of willow-herb, which is its food plant. Such behaviour is
protective.

In spiders the eyes are of considerable interest. A spider is
not sufficiently well-served merely by a pair of eyes — either
six or eight eyes being present. Four families have six and
the remainder have eight. W. S. Bristow in his World of
Spiders (New Naturalist Series) considers that spiders can
alter the focus of their eyes to suit given situations or require-
ments.

All the pairs of eyes may be used in catching prey, but
which ones are in operation depends upon the situation of
the pairs — these being differently situated in different species
— and also on the position of the prey in relation to the spider
itself Naturally, those spiders which are hunting species
depend more on ocular ability than those with webs of one
kind or another where the delicate sense of touch takes first
place.

Among the insects there are two distinct kinds of eyes.
There are the simple eyes or ocelli, and the compound eyes.
The simple eyes are themselves split into two types, the dorsal
ocelli and the lateral ocelli. The former are found in adult
insects and in the nymphs of some of them, while the lateral
ocelli are, in nearly all cases, confined to the larvae.

These simple eyes are only capable of perceiving objects
which are very near, and are very different from the mar-
vellous compound eyes of adult insects, which are made up
of a great many lenses or facets — in dragonflies, for instance,
the number of lenses may amount to some thousands. There
are two compound eyes, one on each side of the head, and
the type of vision these provide is called "mosaic vision".
This means that the brain of the insect receives "pieces" of
the picture of the object in view, and these "pieces" are
eventually put together to form a single picture. The word
mosaic describes very well the kind of ultimate picture that
reaches the brain. This brief description of the optical
apparatus of typical insects may help the field worker to
understand that part of some insect behaviour in which the
eyes are used, and will enable him to interpret much which

24 THE SENSES OF ANIMALS

is of interest in relation to the sight of insects both aquatic
and terrestrial.

The Amphibia — frogs, toads and newts — are very ade-
quately endowed with vision; but it is necessary that an
object be moving for the eyesight to be fully used. On the
whole the Amphibia are short-sighted; and though not
enough work has been done on this aspect, it is probable that
about three feet is the maximum distance for effective use
of the eyes, which are almost exclusively concerned with the
catching of prey. A frog, toad or newt may be able to detect
a moving object at a slightly greater distance, but the animal
will approach nearer before showing by its alert attitude that
it recognizes the moving object as something to eat. There
is some variation in length of sight as between species, but
my own experience in the field and with captive amphibians
leads me to think that my views are correct. Nearly all
amphibians have good nocturnal vision since so many of
them feed at night ; but tree-frogs also feed well during day-
light. (See Plate 2.)

In the Reptiles the role of the eye is subject to some varia-
tions and modifications as between tortoises and turtles;
crocodiles ; and lizards and snakes.

Tortoises and turtles have good vision; the latter being
able to see well under water, and land tortoises in their
terrestrial environment. But it seems likely that, so far as
acuity of vision is concerned, the aquatic species use their
eyes with greater effect than those species that live entirely
on land. Tortoises, with a few exceptions, are vegetarian and
their food requires locating not capture ; and as these reptiles
have good powers of scent they depend equally upon their
powers of sight and smell. However, as is the case with most
snakes and lizards, turtles, terrapins and pond tortoises are
much stimulated by movement on the part of their prey.

Snakes and their vision offer some nice problems which
ought to provide the naturalist with much scope for work in
the field. Many kinds have the vertically elliptical pupil
usually associated with nocturnal vision, yet others without
this particular feature are also known to hunt their prey at
night. Much further study is required on this subject, and
there is no reason why the field worker should not provide

SIGHT GENERAL 25

the answers ; in fact, a field worker is more likely to provide
the clues in the first instance leaving the question of anatomi-
cal proof to the laboratory worker.

Much more will be said about snakes in the chapters deal-
ing with scent and hearing; but there is no doubt that they
are assisted by their eyesight when feeding, especially those
kinds that have to catch swiftly moving prey.

Lizards are very dependent upon their efficient eyes and
few of them, with the exception of a minority of vegetarian
species, take any notice of prey which is not moving. This
does not mean that they have no powers of scent; but it does
mean that eyesight is the dominant sense as will be demon-
strated later when dealing with field observations. Lizards
also have a good sense of hearing.

Crocodiles, too, have fine vision both on land and water ;
but their keen sense of smell must not be ignored when study-
ing their behaviour.

Although Part II of this book will be devoted to the
anatomy and physiology of the sense organs, it seems wise
to include a brief reference here to the eye-coverings of
reptiles, since these vary. It is a distinguishing feature as
between snakes and lizards that all snakes are devoid of
eyelids, their place being taken by a scaly window covering
the true eye and which is incapable of movement. Most
lizards, on the other hand, have eyelids which are easily seen
in a tame specimen ; there is a third eyelid as well. Turtles,
crocodiles and their close relatives also have a "third" eye-
lid, more properly called the nictitating membrane. This is a
transparent covering which moves across the eye and, in
addition to protecting the eyeball when under water, also
permits full vision under the same circumstances. This nicti-
tating membrane is present in the Amphibia, too, where it
serves the same purpose.

In studying the senses of Fishes, a little field observation
will soon demonstrate the important part played by sight in
the majority of groups. If the student is a fisherman — par-
ticularly a fly-fisherman — he will quickly learn that the eyes
of the fish he hopes to catch are most efficient. Even though
the eyes are flattish and are norm.ally placed at right angles
to an imaginary line drawn from the snout down the back,

26 THE SENSES OF ANIMALS

such kinds as trout, grayling, chub and many others which
can be tempted by the artificial fly have a very wide area of
vision. To approach these fish with any hope of success the
fisherman must keep well out of sight — a position which is
almost directly behind the fish being necessary.

The acuity of a fish's vision can also be observed by
watching a feeding fish that may be poised just above mid-
water (which itself may be quite turbulent) yet the fish will
"rise" and take a minute insect, such as a gnat, that is in-
visible to the angler ; the circle on the surface of the water
made when the fish comes up being the only clue the angler
may have to its presence and position.

In some kinds of sea fish, too, sight plays a great part in
hunting or seeking food, though naturally it is not so easy
to make observations of marine fish as it is in rivers and
lakes.

It will pay the really keen student of the senses of fishes to
keep some kind of aquarium where observation is much easier
than in the natural habitat; and it is worth pointing out
here that in an aquarium it is possible to create natural con-
ditions more easily than is the case with birds or mammals.
But while it is true that the arrangement of conditions most
closely resembling the normal habitat will add value to the
observations made, it is not, in my opinion, correct to say — as
many authorities maintain — that observations on behaviour
are valueless if the animal studied is in captivity.

That Birds can see colours should be well known even to
the less experienced, for if colour vision played no part in
the lives of birds as a whole it is unlikely that so many would
have bright and distinctively coloured plumage. But the fact
that birds' eyes are capable of seeing colours must be quali-
fied to some extent in relation to the type of life various birds
have to live. A nocturnal bird will clearly have less use for
colour vision than a diurnal one ; and the eye will be adapted
in structure to meet the necessary needs. In brief, a bird of
the dusk or dark must be able to make the most of whatever
light there may be (no bird can see in complete darkness),
and will, therefore, have little or no use for an ability to see
colours. A bird that is active in daylight only, will normally
have no need for extra light-receiving mechanisms, but

SIGHT — GENERAL 2']

colours may play a big part in its existence and so the eye
must be capable of detecting them.

It is a most curious thing that, in this country at least, the
study of mammals lags far behind the study of birds, insects,
and even amphibians and reptiles. There may be more than
one reason for this but the most likely is that mammals, being
far more secretive than birds, and being in many cases
nocturnal in their behaviour, are much more difficult to
observe. However, the comparatively recently formed
Mammal Society of the British Isles has already done much
to encourage more widespread and detailed work on mam-
malian life in which there are many aspects not fully under-
stood, while some still await any explanation.

Mammals in the main are far more dependent upon their
marvellous senses of smell and hearing than they are on
sight; yet the seeing abilities of mammals present some
fascinating features.

As a whole mammals are short-sighted and colour-blind.
Their noses and ears fulfil their long-range requirements,
and their visual powers are subordinate to their powers of
scent and hearing. Amphibians and reptiles may seem well
separated from the highest forms of living things, but there
seems to be one factor in respect of sight that links them.
This is that movement plays as an important a part in the
use of eyesight by mammals as by reptiles and amphibians.
The prey of mammals frequently "freeze" (stay motionless)
when the proximity of an enemy is detected, while the pre-
dator, even though it may scent its potential meal, will often
wait for further movement before attacking. This, of course,
refers only to those mammals which are carnivorous in the
widest sense of that word. Herbivores do not wait for the
breeze to move the succulent leaves of bushes or grasses,
because scent will tell where and what the food plants are.
Eyesight, in all probability, plays only a subsidiary part in
their feeding behaviour.

All this does not mean that the eyes of mammals are less
worthy of study than the eyes of birds or fishes ; but it shows
that where some other sense or senses are particularly acute
there is less reason for great sharpness of vision.

The degree of visual acuity may vary with different

28 THE SENSES OF ANIMALS

mammal families, or even within a species. Among dogs,
greyhounds must have better eyesight than spaniels; while
squirrels will take alarm at something they see more quickly
than shrews. Moles and bats have no real need for good
vision, and although both have eyes which are sensitive to
light, it is hardly possible to say that they possess sight in the
ordinary meaning of the word.

The other senses of mammals are so very delicately and
highly developed that it is by no means easy to say of any
one kind what precise use is made of sight. We know so little
about the world of sounds and smells by which mammalian
life is governed, that to dogmatize as to when one sense takes
over from another would be misleading and probably futile.

The position of the eyes and the size of the eyeballs are
indications of habit and behaviour. Hares, with their promi-
nent eyes so set that they can look behind them as efficiently
as they can look in front, are clearly using sight as one of
their means of defence ; but a hedgehog, although it can see
well, has not the same need for all-round vision — it has its
own very specialized way of defending itself! In the wild,
rats, mice and voles have prominent eyes with good "fore
and aft" angles of vision, but it is worth while mentioning
that tame albino or partially albino fancy rats and mice
depend less on their eyes because albinism carries with it
ocular defects, and of course they do not have enemies; but
albino and normally coloured rats alike react to movement.
Albino mice and voles in the wild are rare, and this could
be because they are more vulnerable than their normally
coloured brethren and so are captured more easily.

Nocturnal vision is well developed in mammals and some
clue to the possession of this ability is to be seen in the
elliptical pupil of those mammals that use their eyes at night
to a greater degree than others — the cat family comes to
mind at once. It must not be thought, however, that an
elliptical pupil is confined to mammals ; certain snakes have
such a pupil and our British adder is a good example. To
make possible confusion worse it is by no means certain that
adders make use of nocturnal vision any more than snakes
without it, and this specialized pupil in the adder requires
more investigation.

SIGHT — GENERAL 29

Badgers are very largely nocturnal yet all the evidence
points to the probability that their eyes are not much relied
upon in darkness, though once again it is true that move-
ment is detected quickly enough. It almost seems as if
badgers use their eyes at night as a very general aid and that
sight plays a minor part in their activities.

Cats are so often quoted as examples of mammals that can
"see in pitch darkness" that it seems necessary to say once
more that no animal can see in complete darkness. Cats have
good nocturnal vision and can catch mice under circum-
stances where a dog would be useless; but some degree of
light must be present for even a cat to see enough for its
purposes.

Marine mammals — whales and seals — can see quite well
whether on land (seals) or under water (both) ; but whereas
it is obvious that the killer whales which are carnivorous, or
seals which also take prey beneath the surface, must use their
eyes, vision is not so important to the baleen whales which
feed by swimming through myriads of plankton organisms
with wide open and specially adapted mouths and throats.

Ill

SIGHT— FIELDWORK AND EXPERIMENTS

Much information about the part that vision plays in the
general behaviour of animals may be obtained from observa-
tion in the field. This does not lend itself well to an instruc-
tional style of writing, and as a great deal of what one wishes
to draw attention to depends upon personal experiences it
is, perhaps, better to adopt the narrative form in cases where
this style will make for greater clarity and interest.

I have already made reference to quite simple experiments
with lower organisms — mostly aquatic — and it is only neces-
sary to say that a student can see for himself the way in
which light affects many of these animals, some of which are
microscopic and a few which are just visible to the naked
eye. In either case a microscope is necessary; and though
the instrument need not be an elaborate one, it is most
satisfactory if one uses a microscope that will take an objec-
tive of one-sixth for high-power work, but lower powers
down to one inch or even two inches will be invaluable.

An old-fashioned type of instrument — one with what is

called a Wenham tube — is really excellent for pond life. This

type has the advantage over up-to-date models that it will

rack back sufficiently to allow a low-power objective of two

inches to be used. Many modern instruments will not allow

of this. The Wenham tube type is a binocular instrument

working with a prism which can, at will, be put out of action,

thus allowing monocular use for higher power work. The

advantage of using the binocular mechanism is that a degree

of stereoscopic effect is present when using objectives up to

half an inch. This is a great help when observing pond life,

and gives a truer picture of the organism under view. Simple

accessories such as live-boxes, glass troughs, and slides with

cavities ground in them are necessary; also setting needles,

fine forceps and so on.

so

SIGHT — FIELDWORK AND EXPERIMENTS 3I

These older, but beautifully made examples of the optical
worker's art can still be bought second-hand ; and provided
that the objectives, eye-pieces and condensers have lenses
free from scratches, and the working parts are not too slack,
they will be capable of many more years of use. They may
often be bought at a reasonable price, but if the student is
not fairly well acquainted with microscopes the help of a
knowledgeable friend when buying the instrument will save
pounds in repairs and adjustments.

It is hardly necessary to add that such an aid as a micro-
scope will not be limited to studying the visual organs of
small animals ; it can be equally useful for the examination of
sensory hairs, insects' antennae and limbs, and other objects.

Now, it is often said by some authorities that observations
carried out on captive creatures in aquaria, vivaria, cages
and so on do not give accurate indications of an animal's
true behaviour. After many years of experience, I do not
accept this as wholly true. Most animals will display many
of their basic activities under captive conditions so long as
their requirements as to space, dryness or dampness, ventila-
tion and temperature are provided. This means that the
student can, with much advantage, keep many kinds of
animals under constant observation. However, the captive
specimen will not provide all the information desired, and
nothing quite takes the place of fieldwork. A combination
of field observations and captive creatures will give the best
possible results.

I should like to recall a number of personal experiences
in the field, and with animals which I have kept, because
I think these accounts may be of value. They may also serve
to stimulate those who are interested in the senses of animals
to see for themselves and to learn that observations in the
laboratory, the animal house or room, and in the wild, will
provide the wide range of study necessary if a sense of pro-
portion and a complete picture is to be acquired. It will be
obvious that only a few examples in each group of animals
will be possible in a single volume ; but the accounts I shall
give have been carefully chosen so as to present some prac-
tical instances of the operation and use of the sensory abilities
of the creatures dealt with.

32 the senses of animals

Worms

Among the lower animals, the earthworm is worthy of some
field study, because it is possible with a minimum amount of
time, trouble and apparatus to observe some of its behaviour
in its natural surroundings. Earthworms of all kinds are
burrowers by nature, but the Lob-worm {Lumbricus terrestris)
is the easiest one on which to base one's experiments.

Every coarse-fisherman has at times to collect numbers of
these worms, and as I am myself a keen angler, I have on
countless occasions obtained dozens of these on warm and
preferably dewy nights. This is less hard work than digging
for them since it is always possible to find out whether they
are present in numbers under the surface of your lawn by
observing the characteristic worm "casts" which show them-
selves on any well-laid lawn so long as it has not been treated
with any "worm-killer" — a most reprehensible practice from
the naturalist's point of view !

I have already pointed out that earthworms are negatively
affected by light, and in spite of statements to the contrary,
I have found that moonless nights are better for capturing
earthworms. All one wants is a good electric torch, a deep
tin for the worms, and considerable agility and swiftness of
movement.

On such nights as I have described the worms will come
out of their burrows — at times leaving the burrows com-
pletely — but usually lying out on the grass with the muscular
flat tip of their tails still in the mouths of the holes leading
to their burrows.

Long ago I learned that to be successful in collecting
worms under these conditions I had to proceed with caution ;
otherwise my footfalls would set up vibrations in the ground
which would be instantly detected by the worms, which
would then vanish before I had time even to start to capture
them. I would walk stealthily round the margins of the lawn
to begin with, leaving the centre area until I had explored
the edges. I would switch on my torch and play it slowly
over the grass in front of me. Should a worm be seen lying
out away from its burrow a quick grab would usually secure
it; but it is very necessary to do this without much fuss or

SIGHT — FIELDWORK AND EXPERIMENTS 33

Stamping, otherwise the combination of the light and earth
vibrations would lose me the worm.

More likely to be spotted are those worms with their tails
still in their holes, and catching them is no easy task. It is
no use flashing your light to guide you and trying to pick up
the worm by its body at the same time. I have always found
that the worm could withdraw into its burrow before I could
get it. Put your torch gently on the ground and put your
thumb firmly on the place where the worm's tail enters the
hole. Then if you are lucky you can grasp the worm with
your other hand before it can get too strong a hold with its
tail. The light, without which you can't do much, will
stimulate the worm to make a retreat from it; so quick
movement and quiet approach is the only way of defeating
the earthworm's fantastically swift reaction to strong light.
This may seem to be an involved and lengthy description
of how to catch worms, but nothing that I know of will
demonstrate so well how sensitive worms are to light, even
though they do not possess "sight" in its true sense.

Insects

There are so many species of insects that it is not easy to
select one or two which will in the field demonstrate their
ocular efficiency and, at the same time, be possible to observe.
Binoculars are so associated with bird watching that it may
seem almost absurd to think of using them to watch the
behaviour of insects ; but as I consider that dragonflies, for
example, bring home to one their amazing powers of sight
better than any other insect, I can strongly recommend the
use of field-glasses as an aid to observation.

The larger dragonflies are, of course, the best kinds to
watch, not only because their size makes following their
flight easier but because they will take larger prey than their
smaller relatives. Of these large species, the ones known as
the long-bodied Hawker Dragonflies, are the most satis-
factory. (See Plate i.)

They measure about three and a half inches across the
wings and two and three-quarter inches in body length.
They are to be found flying near and over ponds, canals,

34 THE SENSES OF ANIMALS

and also at times some distance from water ; they will often
come into gardens to hunt their insect prey. Some of these
long-bodied Hawkers will be found on the wing from mid-
June to September, or even in October in one or two species.

They often have "beats" up and down which they will
fly regularly — particularly in woodland rides and in gardens.
This habit makes them easier to watch. Their prey will
vary from occasional butterflies and true flies to mos-
quitos and gnats.

I have spent many hours watching them when feeding,
and I have found that it is best to begin by relying on the
naked eye so that a general idea of their beat and the time
they take to cover it may be obtained. They fly fast, and
every now and then they will deviate suddenly and at in-
creased speed from their normal course. This is when they
have seen a possible victim to catch in their first pair of legs.
Should the insect that has attracted their attention be a
butterfly, there will be no difficulty in seeing the dragonfly
seize its prey and there will be no need to use the binoculars ;
but as, more often than not, it is a tiny flimsy insect that is
scarcely visible to our human eyes, the binoculars will enable
one to see the capture if it is not made at too long a range.

The mere fact that the dragonfly can sight and then seize
insects varying in size from butterflies to gnats, shows the
focal adjustments that can be made instantaneously ; and it
will also demonstrate the keenness of vision that enables the
hunter to see a gnat at all when flying at considerable speed.
Dragonflies are quite long-sighted and can see prey at ten
or fifteen yards' distance.

I must point out that the dragonflies do not always catch
the intended victim : they do at times miss their mark ; but
I have gained much satisfaction from my observations, and
have increased my knowledge of the eyesight of dragonflies
at the same time.

Spiders

Seeking practical evidence in the field of the way that spiders
use their eyes is not at all simple, and I think that field
observations on spiders are more productive of results when

SIGHT — FIELDWORK AND EXPERIMENTS 35

we come to deal with the sense of touch. However, I think
it may be appropriate here to say a Uttle about observations
on captive creatures when field observations are not readily
to be made.

I think that in certain cases it is quite permissible to carry
out observations and experiments with animals in captivity
when it is unlikely that in the field the observations could be
carried out at all. The key to success is to make sure that the
conditions under which one's captive creatures are housed
resemble as closely as possible the correct natural habitat.
A well-balanced aquarium for observations on small fishes
or other aquatic life is a good example of this.

To return to spiders : if the student wishes to see for him-
self how a spider of a suitable kind uses its eyes, there can
be no better way of doing this than by catching some water
spiders from a pond and then keeping them in a small
aquarium well planted with weeds and supplied with water
from the pond of origin. These spiders will adapt themselves
very well indeed to aquarium life and will not only feed but
will breed under these conditions. They would do neither if
the place of captivity were not satisfactory, and so the student
can be confident that any observations he may make will be
worth while and his conclusions will be well based.

Amphibians

I have found that amphibians are very profitable for use in
sense observations, and it is easy to discover the way in
which frogs, toads and newts use their eyes in getting food.
The Common Frog, being so agile, is unsatisfactory to watch
in the field not only because most of its feeding is done at
dusk or at night, but because this frog, being terrestrial once
its breeding season is over, is not always easy to locate. This
being the case it is better to rely on a captive frog or two for
one's investigations. It is frequently said that Common Frogs
are difficult and even impossible to keep in captivity for any
length of time. In the laboratory this may be so because, as
far as my own experiences of seeing frogs in other people's
laboratories are concerned, they are often housed in a tank
with nothing but a few inches of water at one end and some

36 THE SENSES OF ANIMALS

sloping gravel at the other. The laboratory is either too hot
or too cold and there is much too much movement going on
to suit a nervous creature.

It is perfectly possible to keep Common Frogs in captivity
and to watch them feed if a minimum of trouble is taken to
provide the correct surroundings. The most important thing
is to keep no more than two frogs; the tank must measure
not less than twenty-four inches — preferably more — in length
and it should be about fourteen inches high and at least the
same in width. Rough turf and moss from a pond side should
cover two-thirds of the tank area, and as far as the provision
of water is concerned, this is better done by having a flattish
shallow dish which will take up the remaining third of the
tank. This is more satisfactory than just putting some water
in and tilting the tank a little so as to keep it at one end. If
this is done, it so often happens that the water becomes foul
and to clean it out means disturbing the frogs which is the
last thing you want to do. The shallow dish can be removed
daily and the water renewed.

In addition to the turf and moss, it is a good idea to have
a clump of growing tall grasses planted. This will mean that
some of the insects you put in for food will climb up the
stems and so give you an opportunity to see how accurately
a frog will jump and seize an insect which is out of its normal
feeding range. This demonstrates the way in which a frog
must adjust the focus of its eyes very quickly.

Most laboratory workers use worms for feeding the frogs,
but in my experience earthworms do not form any substantial
part of a Common Frog's diet. Blow-flies, immature grass-
hoppers, moths and crane-flies are more natural prey ; while
small snails and the grey and white slugs from cabbages and
other garden produce will be taken — at ground level. A
great variety of insects can be collected with a sweep-net,
the catch being transferred to a jam-jar until a considerable
quantity has been collected. A large jarful of varied insects
will keep two frogs going for two or three days, and blow-
flies can be used as a stand-by.

To observe the manner in which a frog spots and captures
its food you must tip a supply of insects into the tank (which
must have a cover or lid of perforated zinc) after which a

SIGHT — FIELDWORK AND EXPERIMENTS 37

cloth should be draped gently over part of the tank to reduce
light. You must have only enough light for you to be able
to watch the frogs easily. Having done this, take a chair and
sit patiently until the frogs have quietened down.

Before long, you will see an insect that has attracted the
frogs' attention by its own movement. The insect may be
well behind the frog's head, but the frog, if hungry, will
swivel round — thus showing its angle of vision. If the insect
is very near, the frog will then make a grab with open mouth
and, more often than not, will catch its victim. As I have
just said, an insect may climb up a grass stem some distance
from the frog, and if this is seen by the frog a quick jump and
open-mouthed attack will result in the insect disappearing
almost faster than your own eyes can see what has happened.
I once saw a most interesting piece of feeding behaviour in
a reptiliary which showed how careful one should be not to
accept everything stated in many books, e.g. that our
Common Frog cannot climb.

It so happened that in this reptiliary there were some
dwarf trees and shrubs — two to three feet in height. There
were plenty of insects about — house flies, small moths and a
few cabbage white butterflies. I watched the frogs feeding
normally for a while, and then saw that one had been
attracted by two butterflies which had first flown into one
of the little shrubs and then come to rest. To my eyes it
seemed as if these insects were quite still ; but the frog could
obviously detect some motion for it moved slowly, more like
a toad than a frog, and started to climb into the very lowest
branches of the shrub. It did this almost as easily as a tree-
frog could do ; it moved higher until it was about eighteen
inches or so from the ground. It then waited for a moment
of two when it must have perceived some small movement
on the part of the butterfly. The frog tensed and then
launched itself about a foot forwards and upwards and
caught the butterfly with ease. The force of its leap made the
frog lose its balance and down it tumbled to the ground.
It was quite unhurt but looked rather ridiculous with the
tips of the butterfly's wings sticking out on each side of its
mouth ! There was a slight pause, a gulp and the meal was
finished.

38 THE SENSES OF ANIMALS

This incident was not only interesting as an example of the
way in which an amphibian can see and judge distance so
long as its intended prey is moving; it was also a lesson in
not always accepting as a rule statements made regarding the
activities of an animal. It proved that a Common Frog, in
spite of assertions to the contrary, can climb.

Tree-frogs are also good subjects for observations on feed-
ing behaviour ; and any student travelling in countries where
tree-frogs are to be found can see their amazing ability to
focus an object which is itself moving while the frog is also
in motion. Moths which fly at dusk are very commonly eaten
by many species of tree-frogs, and if an observer provides
himself with an electric torch he will, with patience, be able
to watch tree-frogs launching themselves into the air to seize
a flying moth. It is true that they sometimes miss their prey,
but it is a cardinal error to imagine that predators always
make a satisfactory kill whether it be a lion attacking an
antelope or a tree-frog a moth. The fact that animals possess
senses far keener than our own does not rule out accident or
misjudgement.

Should a student not be in a position to travel abroad, it
is still possible for the foregoing kind of observation to be
carried out with captive tree-frogs which not only do quite
well in suitable cages or tanks, but make most attractive
"pets" as well.

Any cage or tank for tree-frogs must be roomy ; one for a
pair of frogs should be at least two feet in height, and not less
than eighteen inches in breadth and twelve inches long.
Some sand in the bottom with growing moss on top of it, a
pie-dish of water and some branches of a broad-leaved shrub
will be adequate — rhododendron or laurel is good. The tank
must have a perforated zinc cover to prevent escape and to
provide ventilation.

I have at present a White's Tree-frog from Australia which
was adult when I first obtained it and which has lived and
thrived with me for over ten years. This large and handsome
bright green frog will give striking evidence of its keen sight
— both in daylight and at night. I have found that observa-
tions at night are the most impressive because, in addition to
demonstrating acuity of vision, they also indicate a very

SIGHT FIELDWORK AND EXPERIMENTS 39

marked degree of nocturnal vision. Two simple experiments
will show what I mean.

The first is to introduce into the cage or tank a decent-
sized moth — in daylight. If the moth comes to rest as soon
as it is put in, the tree-frog takes no notice of it. A slight
shaking of the foliage will usually make the moth take flight,
and one has to be well on the alert to see the actual capture,
so quick is the frog to detect the slightest movement. The
second experiment is more dramatic. The moth should be
put into the cage when the room is in darkness and this must
be done quickly and gently in order to ensure as far as
possible that the moth does not fly at once. A torch must be
at hand, and as soon as the cover is secure you must remain
still and in darkness. Listen carefully, and before long you
will hear a kind of "plop" as the frog moves to seize the
moth which will probably have taken wing. As soon as you
hear this noise switch on your torch, when you will see the
frog making its last gulp — sometimes having quite a job to
get the wings of the moth into its mouth. To do this, the frog
will use its front legs as hands to tuck the protruding wings
into its mouth.

You will note that the capture has been made in the dark,
yet more often than not the tree-frog will secure the moth as
easily as it did in daylight.

While all the instances I have given show the sharp sight
of frogs when their prey is moving, it is diflacult to be precise
as to the length of sight possessed by frogs as a whole, since
the fact that they themselves move from their original posi-
tions, and by leaping enable the insect to be caught, makes
it harder to be precise as to the actual length of sight which
frogs enjoy.

Certain other frogs, those which are aquatic rather than
terrestrial, will give further evidence of their visual powers
under quite different conditions. In Britain we have two
such frogs — both introduced from Europe. These are the
Edible Frog and the Marsh Frog.

The former are not numerous though there are still a few
colonies to be found in Kent, Surrey, Middlesex and Essex.
The Marsh Frog is less erratic and is confined to the Romney
Marsh in Kent where it inhabits the dykes and the canals

40 THE SENSES OF ANIMALS

which run through that area. It is a large frog, twice as big
as our Common Frog, and as it spends some of its time
basking on the banks in spring and summer (as well as being
submerged in the water) its feeding behaviour may be seen
under both conditions. The Marsh Frog is also nocturnal
and observations at night, if you are armed with a strong
electric torch, are well worth while.

This frog does not move away from the vicinity of the
dykes and canals and it can be captured with a deep net
once its habits have been watched and if you are prepared
to be agile and quick yourself. You can easily see these frogs
feeding in the daytime if you can spot one and sit quietly
watching. On land the food of these frogs is varied : all kinds
of insects are taken, and the tremendous leaps that these
frogs sometimes make suggest that they have longer sight
than other less agile species.

It is of course very difficult to make field observations
when the frogs are feeding under water ; but if some frogs
are captured and painlessly killed, examination of their
stomach contents will show that their underwater prey is as
varied — if not more varied — than that on land. I have taken
from the stomachs of Marsh Frogs such diverse organisms as
dragonfly larvae, water beetles, fresh-water shrimps, may-
fly nymphs, water spiders, newts, tadpoles (including those
of its own species) and on one occasion a two-inch young
Marsh Frog! Fish are also taken, and the late Malcolm
Smith records a fish of three inches in length. Now the point
here is that all these creatures move and some move very
quickly, yet the Marsh Frog, with the aid of its third eyelid,
can capture all these victims as easily under water as it can
on land with better light conditions.

Toads are easier than frogs to experiment with; this is
largely due to their longer tongues and also their sluggish
movements which give one a better chance of assessing their
length of sight — their acuity would seem to be just as good
as frogs' and their angle of vision is, if anything, even better.

I have experimented with both our Common and Natter-
jack Toads in the field and in captivity, and with many
foreign species also under captive conditions. I have not
found a great deal of difference in their seeing abilities except

SIGHT — FIELDWORK AND EXPERIMENTS 4I

in SO far as the difference in agility and the size of the toad
reflects some variations. It will be clear to the observer that
a really large toad such as the South American Giant Toad
will have a much longer tongue, and will, therefore, be able
to take in prey which may be two to five inches away from
it; while our own Natterjack Toad, being a running toad
capable of quite a good speed, will be able to compensate
for its much smaller size and shorter tongue by its quickness
of movement. Like frogs, all toads take only moving prey,
and stories that they will take prey which is still are due to
faulty observations. A moth at rest in good light conditions
may appear to be still, but its wings or the antennae, delicate
though they are, will be moving slightly. This is enough for
a toad to be stimulated to flick out its tongue and capture
the insect. A mealworm which is about to pupate is often
apparently motionless, but even if it is lying on its back tiny
movements of its legs will seal its doom. Many large tropical
toads will eat mice ; and if a mouse is put into a tank or cage
having been just previously killed, it will still produce small
reflex movements quite sufficient to attract the notice of the
toad. A live mouse in the wild will "freeze" if it senses danger,
but it will not stop breathing, and the almost imperceptible
breathing movements will again suffice to activate the toad.
At short distances a toad can see movement which would be
invisible to a hunting owl.

One caution is necessary when carrying out experiments
of this nature. Toads being crepuscular and nocturnal
creatures, cannot instantly adjust themselves from natural
environmental Hght conditions to those brought about by
using a torch to aid in observations in the ordinary habitat ;
and still less can they make the necessary optical adjustment
when in captivity and where from being in a nice shady tank
in a dark room they are subjected to the sudden beam from
an electric lamp or torch. This means that before settling
down to make observations of feeding behaviour it is neces-
sary to give the toads at least five minutes to accustom them-
selves, or rather their eyes, to what to them must be a
dazzling light. I have also noticed that toads will often try
to take a moving insect soon after their light conditions have
changed, but they will miss their mark. This does not reflect

42 THE SENSES OF ANIMALS

adversely on their seeing powers, it merely means that they
must always — even in nature — have a chance of "getting
used to more light". Not only toads are affected in this way;
human beings under reversed light conditions have to do the
same : if we go from a well-lighted room into the darkness we
cannot see at all ; but if we wait a few moments, we find that
we can often see dimly the objects around us which we should
have bumped into had we attempted to move about without
this precaution. Another example is provided by a cricketer
who has been awaiting his turn to go in to bat and who has
been sitting in a dark pavilion. A wicket falls, and out into
the sunlight he has to go, perhaps to face a fast bowler.
Many a cheap wicket has been gained because the next man
in has omitted to seat himself in good light while waiting.
He just cannot adjust his eyes to a sudden increase of light.
The great Don Bradman never made such mistakes; this
may have accounted for the rapidity with which he was often
in full cry after runs — even in the first overs he received !

To return to our experiments : the best way of ensuring
plenty of opportunities for observation is to have a friend,
who is a moth collector, as an assistant. Such persons often
use a strong lamp and a white sheet on the ground in order
to bring to them moths which are drawn to the illumination.
Such conditions are ideal for watching the manner, speed,
and accuracy with which a toad will deal with the moths,
beetles, craneflies and so on which flutter about or pitch on
to the sheet. I have many times seen a toad from a garden
come slowly from its hiding-place and hop ponderously to
the sheet. There it will wait for a time while it gets used to
the brilliance, after which it will set about feeding in earnest.
Of course, you will not always be lucky in having a hungry
toad conveniently visiting you; but a "pet" toad — whether
British or foreign — can be introduced for the purpose.
Foreign toads will not lose their body heat so quickly as to
inhibit feeding behaviour so long as the night in question is
not too cold; and as cold nights are not conducive to
"mothing" with a lamp and sheet you need not worry much
about this problem.

There is, however, one risk in conducting this experiment
which, though it has its amusing side, will not endear you to

SIGHT — FIELDWORK AND EXPERIMENTS 43

your entomological assistant. I well remember once carrying
out some observations with a South American Giant Toad
when a friend of mine, a keen moth collector, had set up his
lamp on a very favourable night. He was intrigued when my
toad sat on the sheet picking up with great rapidity common
moths and beetles. He was not so enthusiastic when a par-
ticular moth which he much wanted for his collection
alighted just to the side and rear of the avid toad. My friend
was advancing with a pill-box all ready for capture — but the
toad beat him to it. Turning round with a speed with which
one would not credit a toad as big as a small plate, its tongue
flicked out quite four inches and my friend's prize was
snapped up right from under his outstretched hand ! His
language was livid, and I removed the toad. Fortunately for
our friendship, and future co-operation, he did get another
individual moth of the same species ; but he looked on my
amphibian pets with a rather jaundiced eye for ever after.
However, this incident taught him something about a toad's
eyesight and speed of tongue movement so he did add to his
knowledge if not to his collection.

Newts and salamanders, for all practical purposes, may
be regarded as equally useful for experiments designed to
test their acuity and range of vision. Salamanders spend little
time in water, going there only for the purpose of spawning ;
newts, of course, stay in the water longer — over-wintering
there in some cases — but both are creatures that can feed on
land. Our newts demonstrate their visual ability well if
observed in water during the months of April, May and
June. They can be satisfactorily kept in well-planted aquaria
with perforated zinc covers to prevent escapes, for they are
great climbers. These conditions need in no way be regarded
as abnormal, for newts will feed quite naturally in tanks and
any observations made are quite reliable. To satisfy myself
as to which is the dominant sense in newts I have kept many
of various species; and though their sense of smell is good, it
is their eyes that are mainly depended upon for catching
food. Newts are equally at home in feeding by day and by
night. Not more than one newt should be housed in one tank,
otherwise it will be very hard to keep track of the movements
and behaviour.

44 THE SENSES OF ANIMALS

To watch newts feeding in their normal surroundings is
difficult, because it is only by very good luck that you will
catch a glimpse of one doing so ; and this will be when a
surface insect is taken. As, however, newts must breathe
atmospheric air they have to rise to the surface at frequent
intervals to obtain it. When they do this they cause a ring
to form on the water somewhat similar to that made by a
"rising" fish, and this may be mistaken for the snapping up
of an insect on top of the water. Such faulty observations
will render any records useless. Newts in tanks are much
more reliable as indicators of feeding behaviour.

Small earthworms are good food items to start with be-
cause they can be easily seen. A worm should be placed in
the tank about eight inches from where the newt is lying.
Unless the newt is too well fed, it will at once take notice and
will usually approach slowly, then with a swift dart it will
seize the worm. Repeat this experiment with the worm
farther and farther away from the newt and note the greatest
distance over which the newt will sight the moving worm.

For more exacting tests use "bloodworms", which are the
larvae of certain gnats ; they can be bought at most good
aquarists' stores. They look like tiny, deep scarlet worms,
and though small they are active in their movements. The
best way to test sight with these larvae is to transfer your
newt to a large (24 X 12 X 8 inches) tank with nothing in
it except some clean water. Then place one bloodworm in
the tank as far from the newt as possible and watch. If the
newt is turned away from the place where the bloodworm is
wriggling, gently touch the newt near its head with a pencil.
This often turns it round without disturbing it too much. As
soon as it catches sight of the bloodworm, it will chase it,
and though it may not be able to catch it at once, it is the
distance from which the prey was spotted that is worthy of
note. The quickness with which a newt will respond to the
attraction of such a moving food item will surprise most
students who view it for the first time.

To experiment with newts feeding on land it is, of course,
necessary to find one or two which are already in the terres-
trial state. They may be discovered from October to Feb-
ruary under logs of wood, old tin sheets, sacks and so on

SIGHT — FIELDWORK AND EXPERIMENTS 45

which may be in the vicinity of ponds. They will, in the case
of our largest newt, the Great Warty Newt, look Hke a very
dark brown lizard without scales, but with a dull — not shiny
— body. They will usually be sluggish and easily caught with
the hand. The Common or Smooth Newt, and the rarer
Palmate Newt are smaller in size and their skins have a
velvety look. In colour they may vary from dark brown to
a pale tawny yellow. These newts are equally easy to catch.
Any newts, of whichever species, should be transported in a
linen bag with some slightly damp moss. When you transfer
them to a tank (with cover) the bottom of the tank should
have about two inches of soil, and a few pieces of growing
moss must be placed on it. A shallow dish of water and a
piece of tree bark for shelter will complete the experimental
tank.

I have always found that newts in their terrestrial stage
feed mostly at night or at dusk, and this poses quite a nice
problem. When in water it is well known that newts, in addi-
tion to their keen sight, can detect smells very well and it is
not Hkely that on emergence from the water they will lose
this sense — nor is there any evidence that this happens.
When, therefore, I have left newts overnight in pitch dark-
ness with a dishful of suitable food which cannot crawl out
of the dish, I have been struck by the fact that on inspecting
the tank in the morning the food has vanished. Under the
conditions which I arrange, the degree of nocturnal vision
which newts undoubtedly possess cannot supply the solution
to the disappearance of the food. The newts must detect it
by scent, not vision. At the same time there is no doubt at
all that newts use their eyes for finding food whenever the
light conditions permit. They can be watched feeding in day-
light so long as they are not fed during the previous night.
Their food consists of small worms, any soft-bodied grubs,
slugs and even snails. Their jaws are shown to be powerful
in that they can cope with the shells of young snails. Young
woodlice are also taken, as are tiny flies and beetles which
are often so minute that they are not easily seen with the
naked eye. Newts detect the slightest movement of their prey
and are very quick to grab it so long as it is within reach.
They can move along quicker than one would suppose when

46 THE SENSES OF ANIMALS

they have to shift position in order to get near enough to the
creature they are attracted to.

Salamanders, being less aquatic than newts, are very suit-
able for observation in a vivarium, and will feed readily
under captive conditions. The Spotted Salamander is easy
to obtain and, when fully grown, may be eight inches or so
from snout to tip of tail. They prefer larger prey than newts
do, and worms are a good staple diet; but they will take
smaller prey of the same kinds as those taken by newts. Their
vision is good both by day and night and observations on
their ability to catch worms, insects and so on can easily be
carried out.

Before moving on to the next group of animals it is worth
noting that though it may be thought that the use of sight
in the feeding habits of newts must be well known and under-
stood, surprisingly little work has been done on this. Further
experiments would be worth while in order to determine not
only the distances at which newts can perceive food items,
but also to find out how wide a range of insects and other
invertebrates newts will eat. It would be interesting to have
the results from a number of observers on whether newts are
put off by the scent of certain invertebrate creatures and if
so which ones ; it would also add to our knowledge of the
senses of newts to experiment with different species of worms
to gain some idea of newts' abilities to discriminate between
one species and another by means of smell and taste. These
senses will, of course, be referred to again when I come to
deal with them in future chapters.

Reptiles

The word "reptile", to the layman, often merely means "a
snake", and it is surprising to some that among the reptiles
are included, in addition to snakes and lizards — crocodiles,
alligators, tortoises, terrapins and turtles.

In all these groups the eyes play quite an important part
in their lives, though I would hesitate to say that sight is
always the truly dominant sense. The differences in the
structure of the eyes of different kinds of reptiles belong,
quite properly, to Part II of this book; but to make the

SIGHT FIELDWORK AND EXPERIMENTS 47

explanation of some of the field observations I have carried
out more clear, I feel it is necessary to make brief references
to certain aspects of the optical mechanism of reptiles.

It may be fairly stated that all reptiles are short-sighted
rather than long-sighted, but this does not mean that any-
thing which is some distance away — yards rather than feet
or inches — cannot be detected, though some degree of move-
ment is necessary. Reptiles, however, have apparatus for
accommodating to close vision since all of them must be able
to see their prey clearly when it is near at hand.

It is certain that most reptiles, with the exception of croco-
diles and alligators, can see colours if only to a limited
extent. Any student can prove this for himself, as I have
done, with tortoises and some common species of lizards,
e.g. Viviparous Lizard, Sand Lizard, and Green Lizard.

I have often demonstrated to interested friends that when
dandelions are in flower, if placed with some lettuce near
a tortoise, the tortoise will nearly always make straight for the
dandelion plant and eat the flower before it eats the foliage or
the lettuce. Curiously enough most of my tortoises have pre-
ferred dandelion and clover to lettuce, however succulent the
latter may appear. I have also watched my tortoises, when
placed on a lawn, move off on what might almost be de-
scribed as a tour of inspection. My lawn, I fear, often has
nearly as much clover and dandelion and plantain as it has
grass ; but this suits the tortoises even if it doesn't make for a
good lawn.

However, the point here is that when the tortoises are
moving around they can be seen examining the plants in
front of them and selecting the ones they intend to eat. They
go first for clover, then dandelion and then plantain. They
can clearly recognize one plant from another; and if the
patch of lawn immediately in front of them contains no
clover they will move on until they detect some. Of course,
it must be admitted that the sense of smell is well developed
in tortoises, but from close observation I feel sure that when
covering a small area of ground in front of them it is by sight
that they find the desired food.

No flower-bed containing pansies is safe from a tortoise,
but it will be seen that it is the yellow ones they go for first.

48 THE SENSES OF ANIMALS

Pansies of other colours will be taken in due course, but the
attraction of yellow is most marked and would seem to be
the colour which is seen best.

The turtles, terrapins, and pond tortoises take their food
under water, and from such experiments as I have been able
to make it would appear hkely that colour-sight plays little
part in feeding behaviour, movement of the prey being the
main stimulus.

From many observations on terrapins and pond tortoises
in captivity, I am also of the opinion that vibrations set up
in the water by tadpoles, worms, small fish, and some aquatic
insects (on which most of these reptiles feed) are a means of
attracting attention to the presence of food. I have frequently
placed an earthworm in a tank well behind and out of sight
of a terrapin or pond tortoise. The worm on being submerged
will start to move, often wriggling though remaining in the
same spot. As soon as this takes place the reptile will make
a turn or half-turn, thus bringing the worm within sight. If
hungry enough, the pond tortoise will at once move and
seize the worm. It has been argued that smell may explain
this behaviour, but I am inclined to think that such a quick
reaction to the currents set up by the movements of the worm
is due to a perception of water vibrations. Though I shall
say more about this when dealing with the sense of smell, it
is worth recording that I have placed dead worms into a
tank in order to test my theories, and I have found that the
time taken by a terrapin or pond tortoise to get some scent
from a dead worm which is out of sight is much longer than
that taken to detect a living worm.

In the crocodiles and alligators it is difficult to decide
which sense plays the most important part, since these reptiles
have good powers of scent, sight and hearing. Most of the
food is eaten out of the water though, of course, such food
items as fish must be taken beneath the surface.

The eyes are well equipped for seeing on land and beneath
the water, and this can be easily observed by anyone who
can provide suitable tanks and water temperature for the
keeping of young crocodiles and alligators. A word of warn-
ing here: young alligators up to about eighteen inches to
two feet in length can be tamed to some extent and will even

SIGHT — FIELDWORK AND EXPERIMENTS 49

feed from the hand, but crocodiles — almost without excep-
tion — are more vicious in character and seldom become as
docile as alligators. I do not know why this is and I have
never come across any explanation in the course of my read-
ing. Both crocodiles and alligators can be trained to take
dead fish or meat according to species, but this kind of food
never seems to produce such healthy specimens as those
which can be fed on live fish, frogs, snails and so on. This is
curious when one considers that "high" meat is relished by
some kinds of crocodiles and alligators — perhaps those of us
who have kept such reptiles as pets or as subjects for class
observation are averse to allowing food to become putrid
before offering it !

Both crocodiles and alligators have good nocturnal vision,
though their eyesight is quite acute in anything but really
bright sunlight.

In the lizards we meet the reptiles which are endowed
with the best sight of all. There are so many species of lizards
which can be kept in captivity that it is simple to carry out
experiments to prove the great acuity and accommodation
which their eyes are capable of Our own Common Lizard,
Sand Lizard and Slow Worm will demonstrate this soon after
capture, while among the foreign species the Wall Lizards,
Eyed Lizards, and Green Lizards of Europe are equally satis-
factory and can all be kept in vivaria provided that the
temperature is not less than 60° F.

The Geckos are worth observing because they feed almost
entirely at night or at dusk, and if fed with grasshoppers,
moths, craneflies, etc., their bodily agility and keen sight can
be observed and studied so long as the room in which they
are kept is dimly lighted. It is my experience that Geckos will
feed more readily in subdued Hght, although those who have
lived in hot climates know that the Geckos, which so often
take up residence in houses and bungalows, will display their
powers in well-lit rooms where moths, beetles, and other
flying insects are drawn to the source of light.

Special mention must be made of the chameleons, not just
because they have the same sharp sight as other lizards, but
because of the unique structure of their eyes. The eyeball is
"housed" in a cone-shaped turret-like structure; and in the

50 THE SENSES OF ANIMALS

apex of the cone the comparatively small eye is visible. These
eye housings are capable of independent movement so that
the chameleon is able to direct one eye forwards and the
other eye backwards ; or one eye upwards and the other
downwards, with intermediate positions as well. This curious
adaptation is said by some to enable the chameleon to per-
ceive an insect in front of it with one eye, while keeping a
look out for possible enemies behind it with the other eye.
This has always seemed unlikely to me because chameleons
are so slow-moving by comparison with other lizards that
they would be unable to escape from a marauding snake, for
example, if the backward-looking eye did spot the poten-
tial foe.

It is more reasonable to consider that this independent
movement of the eyes allows the chameleon a very wide
range of vision when seeking food, and this all-round sight,
coupled with the long, sticky and swiftly operated tongue,
compensates for the slowness of locomotion. Chameleons
stalk their prey, they do not catch it by swift darts and
rushes as do so many other lizards.

As some students may well wish to have a chameleon for
observation, it must be pointed out that they are not the
easiest of reptiles to keep in health. The real reason for this
is not thoroughly understood even by zoo authorities; but
one reason for the failure of most novices to keep chameleons
alive is because they are not able to heat the vivarium effi-
ciently. A temperature of 70° F. is desirable so long as plenty
of foliage is provided and kept alive ; some of the heat should
come from a lamp — even a reading-lamp will do — so that
the chameleons may bask at will.

Another reason for failure is connected with the drinking
habits of chameleons. It is no use placing a little dish of
water in the vivarium and hoping for the best. Personally
I have never known a chameleon drink from such. In nature
they drink by lapping with their tongues the dew on leaves,
or raindrops. In captivity the foliage in the vivarium should
be sprayed with a fine spray at least every other day. Ventila-
tion must also be good otherwise an over-damp tank will lead
to rotting vegetation which is not, for some reason, good for
chameleons.

SIGHT — FIELDWORK AND EXPERIMENTS 5I

As to food for captive chameleons : an assortment of insects
secured by means of a sweep net will provide some variations
of diet, while grasshoppers, moths, cabbage white butterflies,
and craneflies will be taken freely. Blow-flies can be bred
for feeding, but these are not so nutritious as wild caught
insects. Some — but not all — chameleons will take meal-
worms, but they will seldom feed from a dish. Each meal-
worm offered must be lightly attached to a wire and pre-
sented so that it will wriggle and attract attention. This is a
slow and often time-wasting performance, and the time spent
could be more usefully employed in catching insects in fields
or hedgerows.

To return to the use of the eyes in other, more usual kinds
of lizards, the British Viviparous or Common Lizard is the
easiest to obtain for it is widely distributed and can be caught
in the hand once a little practice has been gained. The
British Sand Lizard is another species which can be used for
close observations, but as it is extremely local in distribution
over-collecting must be avoided. It is perhaps better to buy
one or two Continental Sand Lizards from a dealer; these
will do just as well as our own species. Other lizards suitable
for observations under captive conditions are Wall Lizards
and Green Lizards. Both these may be bought from dealers
during the spring and summer months.

Whichever kind is chosen it is as well to give varied insect
food obtained by sweeping; mealworms are excellent as a
stand-by but as they have to be confined in a dish from which
they cannot escape they are less useful as tests for feeding
experiments than moths, smooth caterpillars, grasshoppers
and spiders, all of which will move freely around the vivarium
without burying themselves as mealworms do.

All lizards mentioned will be found to have the most
amazingly acute eyesight for moving prey; and compared
with some reptiles they certainly have reasonably long sight.
I have seen a Green Lizard detect very slight movement in a
caterpillar from a distance of four feet. The speed with which
these lizards will cover the distance between them and their
intended victim is surprising to those who have not watched it
take place, and the sureness with which the insect or spider will
be seized shows clearly that change of focus is extremely rapid.

52 THE SENSES OF ANIMALS

Regarding the colour vision of lizards, there are some
interesting observations to be made. Dr. Angus d'A. Bellairs,
in his excellent little book Reptiles (Hutchinson's University
Library), states that experiments with food placed on
coloured discs show that lizards seem to prefer grey and white
to colours ; but that green, which he says is the commonest
colour in their natural environment, was selected more often
when four coloured discs with food on them were offered
simultaneously.

I would not question this except to say that the British
Viviparous Lizard and the Sand and Wall Lizards are, more
often than not, found in places where the yellow of sand, and
the brown hues of soil where heather grows, and the various
shades of colour of walls contain very little green. This does
not disprove that lizards see greens well, but there is a good
deal of evidence that lizards can see yellow, orange and red
— or at least some lizards do. This brings up another interest-
ing point that applies to certain lizards — the Green and Wall.
These species have a weakness for some fruits and will feed
on orange, peeled greengages (yellowish in colour) and on
strawberries. Of course, these fruits can be detected by their
scents ; but if a tank containing such lizards is divided into
two by means of a glass plate, and in the part shut off from
the lizards themselves one or other of the named fruits are
placed, the lizards (if hungry) will at least appear to recognize
them by sight and will run up to the glass partition, often
scratching at it when they discover the invisible barrier.

There is a further interesting point about this fruit eating :
there is no doubt that lizards are normally attracted by the
movement of insects and so on ; but as fruit is static it is
obvious that movement is not essential — at least for some
species — and food is recognized by sight and scent.

Some mention must be made of our Slow Worm which is,
of course, a legless lizard. In spite of its other common name,
Blind Worm, this reptile has excellent sight. Its natural food
consists mainly of small slugs — the grey and white species,
not the black or brown which would appear to be un-
palatable. In addition. Slow Worms will eat grubs and
earthworms, the latter being found under logs and stones
where Slow Worms frequently lie up. The slightest move-

SIGHT — FIELDVVORK AND EXPERIMENTS 53

ment of a slug or worm will attract the Slow Worm which,
having tested the prey with its tongue, will seize it and eat it.

Slow Worms, being easy to find and keep in a vivarium,
are good subjects for study, and though their speed of seizing
prey cannot be compared with other lizards, they can be
shown to have considerable acuity of vision. I have proved
that a Slow Worm can see the wriggling of a worm at a
distance of nearly three feet, while the slightest movement
of even the "horns" of a slug will be detected from twelve
inches away.

Snakes have been subjected to much faulty observation
and are spoken about with fluent ignorance by those who
have learned their snake-lore from country-folk. It is true
that their senses are not easy to assess without close observa-
tion, and it is easy to fall into error if the tales of uninformed
travellers abroad are taken as a basis of information. The age-
old stories about snake-charmers are the root of many quite
inaccurate views on the behaviour of snakes, and it is some-
what amusing to reflect that the music of the "charmer's"
pipe is said to be the reason for his power over snakes when
in fact it is the eyesight of the snake that makes it respond to
the charmer's actions. What happens is that the charmer sits
on the ground with his pipe while a cobra is a yard or two
away from him. He plays his pipe and sways to the oriental
rhythm of his music. Soon the cobra lifts the front part of
its body, extends its hood and appears to make side-to-side
movements as if following the plaintive notes from the pipe.
In reality it is the sight of the moving pipe that stimulates the
snake into action. It will be noted by any acute observer
that the snake is never more than a few feet away from the
charmer, and thus it is enabled to see the man and his pipe
as they sway to and fro. If it were truly the sound of the
music that influenced the cobra, there is no reason why it
could not be a considerable distance from the charmer, but
he is unlikely to risk losing his snake in order to satisfy the
scientific curiosity of one member of his audience.

Snakes on the whole are short-sighted, and they would
have little use for long sight since they are often in under-
growth and normally could hardly be closer to the ground.
Prey is trailed by scent ; and further reference to this will be

54 THE SENSES OF ANIMALS

made later. Once the snake has come up on its prey the eyes
will take over, and provided that the victim is not beyond
the striking distance of the particular snake in question the
aim will usually be sure and capture certain.

It is perhaps worth referring here to the encounters be-
tween snakes and mongooses, since these show how the
snake's sight can in such cases lead to error which would not
occur when the snake is pursuing prey such as rodents,
lizards, frogs and so on.

Being short-sighted, the snake is at a disadvantage when
encountering a mongoose, for the mongoose's defence is its
great agility coupled with the fact that it can, by muscular
action of the skin, cause its fur to stand out all over its body,
thus making it appear bigger than it really is. The snake
keeps striking at the swiftly moving outline of the mongoose's
body but fails to get home, more often than not, being unable
to detect the actual body of its opponent and "striking short"
in consequence. The mongoose keeps darting at the snake
and then, by reason of its quickness, jumping back to avoid
the attack of the snake. This goes on until the snake is
literally tired out, when the mongoose will dash in and bite
the snake — usually just behind the head.

Returning to the question of what part the eyes play in
capturing food, it is clear that many species of snake, once
having located their prey, will quickly detect any kind of
movement, and will then seize it ; the last part of this move-
ment or series of movements is visual in origin. At fairly
close quarters the eyesight is keen and even the pulsating skin
under a frog's lower jaw is quite enough to stimulate a snake
to strike. The actual strike is very quick indeed ; it has to be,
since many of the animals eaten by snakes — voles and mice
and rats for instance — are themselves very quick in their
movements ; and though at the first sight of an attacking
snake a rodent will "freeze" and remain still as if hypnotized,
any delay in attacking may cause the rodent to scuttle away.
The snake, therefore, must strike instantly if it is not going
to miss. This particularly applies to those snakes which, in
feeding, merely swallow their prey whole; if the prey is
missed at the first attempt the snake may not get another
chance. On the other hand, venomous snakes, such as our

SIGHT — FIELDWORK AND EXPERIMENTS 55

British Adder, do not at once eat their prey when it has been
struck by the poison fangs. The object of the strike is to inject
sufficient poison into the victim and paralyse it so that it
cannot escape. I have had the good fortune to witness this
several times when voles and lizards have been the prey. The
adder strikes, and if it gets home it may wait quite two or
three minutes before following up its attack and proceeding
to swallow. Where the victim is a vole or mouse there is an
additional advantage in this waiting period, because if the
snake went in at once to try to engulf its prey it would stand
a chance of being bitten by its intended victim. I once saw
this happen when a venomous snake struck at a rat and
evidently either did not get its fangs into the rat, or perhaps
its poison was temporarily depleted. Whichever was the
reason, the rat bit the snake in the head and it subsequently
died — a case of the biter being bit !

Notwithstanding these accidents, which must often occur
in nature, the close-range vision of snakes is good. Nocturnal
snakes can, of course, see quite well in dim light and their
eyes are well adapted for this form of life.

Birds

Field observations and experiments designed to test the eye-
sight of birds are not so easy to suggest and carry out as one
might think. This kind of limitation must also occur in rela-
tion to certain aspects of other senses than sight, because the
very nature of a sense in a particular animal may make it
less susceptible to experimentation.

The flight of birds, and their general activity when on the
ground, makes them less suitable for tests ; while field obser-
vations — except in respect of birds of prey — are not very
satisfactory. There are, however, some observations which
will at least serve to bring home to the student the extreme
acuity of sight possessed by birds.

First we must remember that all diurnal birds are able to
see colours, and this ability helps them to select food ; while
in courtship colours sight plays its part. Of course the fact
that these birds can see colours does not play the main part
in courtship, as may be gathered from the fact that many

56 THE SENSES OF ANIMALS

birds are dullish in plumage ; but birds would not be brightly
coloured in so many species if these colours had no signi-
ficance.

However, colour-sight alone is not the most outstanding
feature of the seeing abilities of birds. It is the sharpness of
sight and the way in which the eyes can accommodate them-
selves to swift changes of focus that excite one's interest.

When a song-bird perceives the shape of some creature
which is an enemy, it will react at once, and will take certain
self-protecting steps. It will either freeze in order to lessen
the chance of capture, or it will resort to flight ; or, when
defending eggs or young, it may become aggressive and
threaten the enemy. In other instances several birds — often
of different species — will simultaneously mob the predator,
frequently driving it away.

In all these varied actions the keen sight of the bird is all
important; for whatever defensive manoeuvre is put into
operation must be almost instantly carried out. To do this
it is necessary for the bird's eyes to accommodate themselves
quickly to differing circumstances. The threat may come
from the ground if a stoat or rat is hunting ; it may be an
owl in a tree that sets small birds off mobbing ; or a hawk
high up in the sky may be the cause of alarm.

Now colour-sight must play an almost negligible part in
this kind of behaviour ; but the inborn instinct which enables
a bird to recognize shape as a threat is vital. For many years
it was thought that backwardly-curved wings showing up as
silhouettes against the sky were the danger signals that
alerted a bird to the presence of a hawk ; but recent experi-
ments are said to have proved that it is an outline which
shows a short neck which acts as a warning. Doubtless this is
so ; but I, myself, am not convinced that it is the shortened
neck that is the only feature which causes alarm.

I have carried out some experiments on this subject which,
simple though they were, seem to me to have some weight.
I have cut out cardboard shapes which were nothing more
than crescents and have hung these high up over such plants
as peas, dwarf beans and young lettuces. I have then watched
the behaviour of sparrows, tits and similar birds to see what
happened. There has been alarm almost at once, and on one

SIGHT — FIELDWORK AND EXPERIMENTS 57

occasion my model was mobbed. I hope no one will think
that from this the distribution of crescent-shaped pieces of
cardboard will keep birds off vegetable gardens for ever — I
don't have much faith in any of these devices except for a
short while. If no subsequent attack on birds ever takes place,
these threatening outlines lose their powers. The birds soon
get to learn that in spite of their shape they are harmless,
but that is not the point. My rough experiment was for the
purpose of finding out if apparent wing shape, with no head
or neck (either long or short) would produce alarm among
small birds — it did.

I should also like to relate a quite accidental observation
I was able to make a few years ago when a baby cuckoo,
which I had hand-reared, was first set at hberty in my garden.
I must explain that my garden is small but contains some
fruit trees, while at one end there is quite a tall birch. I must
also remind readers that a cuckoo — even a newly fledged one
— has very hawk-like wings and it has not got a particularly
short neck; also the distance across the garden was less than
thirty yards of flight space.

I had the young cuckoo on my hand and I had been feed-
ing it. To be truthful I thought that as it was so tame it might
fly from my hand into a nearby tree and then either come
to hand again or let me catch it up. I was wrong, however,
because the bird took wing very suddenly and flew straight
from my hand, across the garden and landed in the birch
tree. The flight cannot have taken more than a few
seconds.

Notwithstanding the short duration of this flight, the
cuckoo had hardly perched itself before about a dozen small
birds — tits, chaffinches, greenfinches, and sparrows — ap-
peared as if by magic and set about mobbing the cuckoo.
My bird stood this for about a minute, when it flew back
towards me and took refuge in an apple tree by which I was
sitting. It was pursued by the mob, most of which sheered
away when they saw me ; but a pair of greenfinches did not
give up. They took up station in the tree above my head;
the cock at one end of the branch on which the cuckoo had
landed, and the hen at the other. Then they sidled rather
stealthily towards the cuckoo. I watched for a moment or

58 THE SENSES OF ANIMALS

two and then reached up and caught the cuckoo, which I
removed to a safer place.

Whether this most interesting happening has any bearing
on the "short neck" theory, it is difficult to say. All I would
point out is that the cuckoo was in flight for only a few
seconds, but its curved wings were much in evidence, and
the reaction of the small birds to the sight of it was instan-
taneous. In contrast to the indifference to my cardboard
models on the part of the birds around my house, I can state
that a little later, when my cuckoo had complete freedom in
the garden for three weeks, I never knew the vegetables to
be freer from damage. During that three weeks, hardly a
bird was to be seen around.

It is known that certain species of birds react differently
to threats to themselves or their eggs and young. Many
ground-nesting birds — plovers for instance — go in for a kind
of distraction behaviour by appearing to simulate injury,
thus diverting the attention of the enemy away from the
nest ; but whatever kind of reaction to danger takes place, it
is the eyes of the threatened birds that come to their aid.

Avoidance of danger is not the only reason why a bird
relies so much on its eyes. The selection of food, or the mere
perception of it, depends upon sharp sight when those birds
which feed on either seeds or insects are at work. A creature
as small as an aphid can be spotted and eaten in a flash ; an
earthworm some distance away, which is out of its burrow,
will be seen at once, though when the worm is in its burrow
it is the sense of hearing that is all important as will be shown
later.

A piece of simple field observation may be carried out by
spending some time watching a kestrel feed. These lovely
little birds of prey doubtless take some food which may be
near at hand and on the ground — beetles and worms for
instance; but the typical feeding behaviour of kestrels is
carried out by the bird hovering high up in the sky — as far
as two hundred feet being not unusual. From this vantage
position the bird can survey a large area of the ground
beneath it, and a kestrel may be seen quartering a meadow
or a piece of waste-land just like a good gun-dog.

If the kestrel is watched through binoculars, you may have

SIGHT — FIELDWORK AND EXPERIMENTS 59

the luck to see it suddenly drop like a plummet down to
earth. Transfer your gaze to the spot where the bird landed
and, if you can see where it is, you should be able to tell
whether it has made a capture or not. If the kestrel takes
wing again almost at once, it will have missed its prey ; but
if it has seized the vole which it sighted from its position in
the air, it will stay where it is for a while.

What is remarkable in this behaviour is that the keen eye
of the kestrel not only had to be able to detect the vole far
below it, it must have been able to keep its intended prey in
focus during the whole of its long drop. Naturally kestrels
and other birds of prey do not always succeed in their efforts at
capture, but if you carry out enough of these observations, you
will find out that far more victims are caught than missed.

It may seem a big step to go from the small and beautiful
kestrels to the huge and ugly vultures ; but no commentary
on the eyesight of diurnal birds would be complete without
some mention of these great carrion-eating birds.

It is not uncommon to hear the view expressed that
vultures must smell the odour of decaying flesh in order that
they may find the corpse from which the scent reaches them
when they are circling round at very great heights. Now first
of all it must be understood that there is little evidence that
any but a few species of birds have an appreciable sense of
smell ; furthermore, a little thought or some field observations
by those lucky enough to be in countries where vultures are
to be found, would soon convince anyone that sight is the
sense employed by these birds when searching for food.

Vultures often give evidence of anticipating a "kill" as
when a lion kills an antelope. They arrive on the scene
almost as soon as the kill is made, and as they will normally
have been flying above the area, surveying it as they fly
round and round, it is obvious that no scent will have reached
them ; but their eyes will have sufficed to indicate that in due
course a meal will be there. These large gatherings of vultures
in the neighbourhood of the kill have not in most instances
perceived the happening simultaneously. What occurs is that
one vulture will see what has occurred and will start its
downward flight. This action will trigger off the remainder
of the flock, which will follow suit — an interesting example

6o THE SENSES OF ANIMALS

of what appears to be simultaneous spotting, but which is
really an instinctive reaction stimulated by the behaviour of
one vulture, or possibly more than one vulture. The fact that
this is a kind of follow-my-leader process does not in any way
detract from the visual acuity of vultures — quite the reverse.
Any one of the flock may have been the first spotter, and to do
this from hundreds of feet up is sharpness of sight with a
vengeance.

Perhaps the most interesting of birds to study from the
point of view of their eyesight are the owls. Our own species
are not all completely nocturnal, but all can hunt in light so
dim that one can scarcely credit that their eyes could per-
ceive anything. We have five species of owls in Britain : the
Tawny or Brown Owl, the Barn Owl, the Short-eared Owl,
the Long-eared Owl and the introduced Little Owl. Of these
the Long-eared Owl is the most nocturnal, followed closely
by the Tawny Owl. The Barn Owl is more nocturnal than
not, but it is often seen hunting in daylight — particularly in
winter months. The Short-eared Owl is mainly diurnal, but
can also hunt in dim light; while the Little Owl seems in-
different to the time of day, hunting at all hours depending
much upon the season of the year and the prey most easily
available at the time. (See Plate 3.)

These variations in hours of hunting naturally entail dif-
ferences in the internal structures of the eyes. Some references
to the size and nerve-cells of the eyes of owls have already
been made earlier on, and it is only necessary to remind the
student that whereas no owl can see in utter darkness, all of
them can see well enough in daylight to enable them to fly
to a tree and land on it safely. The Long-eared and Tawny
Owl will seem a little unsteady in flight if the day is very
bright ; and if closely observed, it will be noted that they do
not land with the precision which marks their activity at
night. No such uncertainty is seen when watching the Little
Owl or the Short-eared Owl — nor even the Barn Owl — when
these fly in daylight. In fact, it can be stated that the first
two of these three species are indifferent to the degree of
brightness prevailing. I have seen Short-eared Owls hunting
at midday in the height of summer. What must be remem
bered is that the relative number of rods and cones in th

SIGHT FIELDVVORK AND EXPERIMENTS 6l

eye structure of our owls will vary according to the habit of
the species in question.

As far as field observations are concerned, these are not
so difficult to undertake as when observing smaller birds.
A visit to good Short-eared Owl country will seldom go un-
rewarded ; for the owls will usually be in evidence, and they
are easy to watch as they go up and down their feeding terri-
tory. Voles are their most usual prey, and the number of
kills they are seen to make in a given time will, to some
extent, give an indication of the numbers of voles present.
The only difficulty that will be encountered is in deciding
whether a particular owl under observation is making cap-
tures by means of its eyes or its ears.

Should the type of country being covered by the owl be
rough ground with short grass, or a freshly mown meadow,
it may be reasonably deduced that kills are the result of
what has been seen. If, however, the country is a large area
of rough tussocky grass (which is particularly good for Short-
eared Owls) their sight will not play the main part in hunting
behaviour ; the hearing of these remarkable and beautiful
birds will be the sense used. It may be appropriate here to
mention that the ear-tufts of the Long and Short-eared Owls
have nothing to do with their hearing. They are probably
used in display.

Little Owls may frequently be seen perched on fences,
posts and the branches of trees, where they are keeping a
constant look-out for their varied prey. Attention will be
drawn to them on account of their habit of bobbing their
heads if they see that they themselves are under observation.

I have spent much profitable time watching Little Owls
in daylight, and have seen them swoop down on voles, lizards
and newly fledged birds, and twice I have seen one fly after
and catch an Emperor Moth as it flew over the heather —
probably a male moth on the trail of a waiting female.

Such incidents as I have just described will give plenty of
evidence of the sharpness of vision in daylight. All the same
it must never be forgotten that not only does the prey of
owls vary according to the season of the year, it also varies
with the time of day. Owls in general, but the Little Owl in
particular, are fond of beetles. Some beetles are about in the

62 THE SENSES OF ANIMALS

daytime and some at night. Cockchafers (May-bugs and
June-bugs) are night-flyers and then Little Owls will hunt
them ceaselessly. They are taken in flight and caught in the
claws of the owls. I am of the opinion that in spite of the
buzzing flight of these beetle pests, Little Owls mostly capture
them aided by sight, since I have noticed that on light nights
captures will be frequent while on dark nights far fewer
cockchafers seem to be taken. My views have also been re-
inforced by my experience with a tame Little Owl which
I had many years ago, and which I trained to hunt from my
fist like a hawk. I tested this Owl in two ways : I would
collect some cockchafers from the trees in which they rest
during the day and then toss them into the air in good light
when the owl seldom missed one. I would also take out the
owl on a warm May night, just before dark, when the cock-
chafers were beginning their normal flight period. The owl
would bob about on my hand, twisting its mobile head this
way and that, and then, having seen a cockchafer, would
silently launch itself from my hand and take its prey. Its
general attitude and method of behaviour both pointed to
sight being the sense used. However, I have no doubt that
when put to it, Littie Owls can detect the location of prey
by hearing as do their relatives.

Observations on owls at night provide the most interesting
results as far as studying the eyes is concerned, though for
obvious reasons experiments with captive owls produce the
best data. As it is their night vision which is being discussed
here, and as it will be obvious that the more activity on the
part of the owls the greater the chances of making good
observations, the breeding season should be chosen if possible.
This is because when there are young to be fed the parent
owls can be watched nightly as they bring rats, mice, voles,
and other prey to the owlets. When conditions are good for
hunting the student will be surprised at the number of food
items brought in; these often exceed the amount that one
would think necessary for a night's feeding, but surplus
corpses will be laid down near the "nest" and if the weather
is not too warm these are usually either eaten by the parents
or given as a feed early in the evening of the following day.

Once the breeding place of a pair of owls — Tawny Owls

SIGHT — FIELDWORK AND EXPERIMENTS 63

or Barn Owls are the most profitable to watch — has been
located, all that has to be done is to take up position at the
most favourable vantage point and be ready to devote some
hours to your vigil. A good pair of night glasses is a help, but
not absolutely necessary; but a position should be selected
so that as much light as possible is available to make your
task easier. Nearly any entrance hole, be it in a tree, barn
or other building, will have some angle from which it can be
viewed better on any night save a pitch dark one.

The object of watching owls at their breeding time is to
be able to gain some idea of the numbers of "kills" brought
in and then to relate them to the conditions of light at the
time and also to the question of wind and rain. I have already
referred to the difficulty of dealing with one sense without
some other sense being also taken into consideration ; and it
is in connection with weather and light conditions that one
must remember that owls have to adapt their feeding
methods to quite a considerable extent.

On moonlight nights or on reasonably light nights, without
appreciable wind, owls will hunt mainly by sight ; but if the
night is very dark indeed, or if there is much rain, or it is
very windy, owls will rely much on their ears to aid them in
hunting. On the whole a fair number of kills will be made
even when light conditions are bad so long as there is no
strong wind or heavy rain ; but on really unfavourable nights
the number of kills will fall off quite noticeably.

The watching of owls is perhaps one of the most promising
activities from the point of view of the student of animal
senses, since by careful notes of what is observed over a
period of some weeks and by paying equal attention to the
weather, information about the seeing abilities of owls and
about their hearing abilities may be acquired at one and the
same time.

While the period during which owls are feeding young is
certainly the most easy for watching, it is quite feasible to
watch owls at other times of the year, though this is never
so easy. First, one must have definite knowledge of the
presence of owls in some particular area or suitable habitat;
and then, having located them, much time and patience
must be devoted to watching for signs of hunting. With luck

64 THE SENSES OF ANIMALS

you may be able to study a pair of owls hunting in concert,
and this is not only most instructive but is a very thrilling
sight. It is true that more often than not a single owl may
be observed, and this will be more difficult to watch since
lone owls often cover more territory than when a pair hunt
together. What happens when a dual hunt is about to begin
or is in progress will be something after the following pattern
of behaviour.

These dual hunts seem to take place mostly in the autunm
and winter, possibly because prey is more difficult to come
by then. The male bird acts as if it were a spaniel at work
flushing game. He ffies along over a hedge or just inside a
piece of open woodland every now and then uttering the
hunting call usually written as "kewick". The female ffies
parallel with her mate and does the killing. I am not stating
that this can be observed frequently unless the student has
a great deal of time to spend at a known hunting area ; I have
only witnessed it myself three times, but on each of these
occasions the method of hunting has been exactly the same.
The nights in question were moonlit ones and therefore it is
reasonable to assume that the owls doing the killing were
guided by sight. Whether on dark nights the same process
is used, the owls being guided by hearing, I cannot say ; but
it would be well worth while carrying out some organized
observations on dark nights as well as light ones.

Experiments designed to test the sight of owls can also be
carried out with tame owls so long as these are really under
control and have reached the stage of pouncing on prey
themselves. They must be taught to do this by letting them
have flying room either in a large aviary or in a barn or
similar place. A dead mouse must be tied by the tail to a
long length of thread and drawn along the floor within the
owl's vision. Most tame young owls will soon react and will
swoop down and seize the mouse. Once it has grasped the
mouse the owl will spread its wings protectively over the kill,
and one must then approach quietly and gently with a pair
of scissors in gloved hands and snip off the thread a few
inches from the owl — the small piece of thread will do no
harm and will eventually be cast up with the pellet ejected
the next day.

SIGHT FIELDWORK AND EXPERIMENTS 65

Once this method has been successfully carried out the
owl will behave in the same way with a live mouse or young
rat which has been caught in a "live-trap". Much informa-
tion about the quickness of eye and the extent to which the
owl can adapt its sight to varying conditions of light will be
gained by carrying out such experiments.

It is, of course, essential that the light in the place where
the test is being made must be sufficient for you to see what
happens yourself, but it will be found that once one's own
eyes have become accustomed to the dim light of a small
lamp or torch it will be quite easy to follow what goes on.
In the next section which will deal with hearing I shall
describe rather similar experiments designed to show the
acuteness of an owl's ears.

Mammals

In dealing with the sight of mammals in general and with
field observations in particular, there is one pitfall which the
student must be careful not to fall into. It is all too easy to
ascribe to keen sight many displays of behaviour which in
fact have nothing to do with sight at all. Sight may be the
dominant sense in birds, but mammals for the most part live
in a world of sounds and smells. This is not to say that all
mammals have poor sight ; but compared with their fantastic
powers of scent and hearing, their eyes do not have anything
like so much importance as one is often tempted to think.

Many mammals of varying sizes have very short sight,
some have virtually none at all, and most of them see moving
objects far better than any object which is still. All field
workers know, or should know, that if one suddenly comes
across, say, a fox, or even a bear or a rhinoceros, so long as
the wind is not blowing towards the animal and the observer
stops moving and remains absolutely still, he has a good
chance of remaining undetected. He can then frequently see
some incident which is well worth while and which he may
not get the chance of seeing again. This is an exercise, if not
an experiment, that can be carried out as a form of training.
A field with a few cows or sheep in it will do for a trial. All
one has to do is to see that one approaches up wind, and at

66 THE SENSES OF ANIMALS

the first sign of interest from the beasts — stay completely
still.

I have been asked many times why, if one's initial move-
ments alert an animal or animals, these do not at once make
off whether the observer stays still or not. The answer is that
if one's approach has been normal and reasonably quiet the
animals will be alerted, but they will wait to find out what
ensues before running away. I have been able to watch deer
in fairly open country in this way — and deer of all species
are nervous creatures with very acute senses.

An example of this kind of thing happened to me not so
long ago when I was wandering through an open wood on
a bright sunlit afternoon. I rounded a bend in a path and
saw — not twenty paces from me — a stoat dragging along a
freshly killed rabbit. I "froze" immediately; and although
the stoat spotted me rounding the bend and regarded me
with its sharp black eyes, it seemed as if my instant im-
mobility deceived it, or else my outline was not easily seen
against the background of trees and dense undergrowth.
After about a quarter of a minute the stoat, dragging its prey,
moved across the path and entered a deep cleft at the base
of an old oak into which it disappeared. As soon as it had
gone out of my sight, I squatted down, but remained quite
still. I wanted to know if the stoat's traditional curiosity
would cause it to reappear to see if danger was near.

Nothing happened during a wait of some five minutes so
I concluded, rightly I think, that the stoat had a family to
feed. I left it to its domestic duties and walked quietly away.
This was an unimportant incident, but one which was of
some interest; and it proved how valuable it is to be able
to remain still and quiet at a moment's notice. Baden-Powell
in his instructions to Scouts put much emphasis on training
oneself to stay motionless for quite long periods. It is worth
following his advice though it takes some practice to do so
successfully. This kind of thing may seem a little juvenile,
but I have seen so many people — more than old enough to
know better — who were quite unable to remain motionless,
thus robbing themselves of opportunities for possibly unique
observations.

As I have already said, there are some mammals with

SIGHT FIELDWORK AND EXPERIMENTS 67

poorer sight than others, and some with virtually no effective
vision at all. Among the larger species the rhinoceros stands
out as having such short sight that until it is nearly on top
of some object — particularly a static one — it probably has
no precise picture. Its other senses, however, more than make
up for this deficiency. At the other end of the scale of size
we have such mammals as moles, shrews and bats which,
though they have eyes, do not rely on them any more than
as indicators of light and dark.

Moles and shrews can be easily tested in this respect ; and
even bats may be experimented with, though their feeding
in captivity is much more difficult than with moles and
shrews. Both the latter can be kept under captive conditions
for long enough to enable experiments to be made; but as
they consume something in the region of their own weight
per day in food, it is clear that keeping them for any length
of time is an arduous task which, for the purposes of testing
their eyesight, is not in any way necessary.

Both these insectivorous mammals can be housed in an
old aquarium tank for a day or two. A sprinkling of peat moss
litter on the floor and a flower-pot or artificial tunnel so that
they can hide away at will is all that is required, except for a
dish of water and an adequate supply of earthworms. The
idea of the experiment is to satisfy yourself that moles and
shrews can find their food in complete darkness.

Only one individual mole or shrew must be used at any
one time, and all that has to be done is to place in the tank
a known number of worms and then either cover the tank
in such a way that light is completely excluded, or do your
experiment at night in a pitch-dark room. The length of time
required for the completion of the test will, of course, depend
upon the food already taken before you start; but as these
creatures require to be almost continuously feeding, you
won't have to wait long.

An hour should be sufficient once you have placed the
worms in the tank and excluded all light. After this interval
inspect the tank when, unless you are most unlucky, you
will find that the worms have vanished. A word of warning
is necessary if you decide to adopt the method in which a
cover for the tank is used. Moles and shrews may be able

68 THE SENSES OF ANIMALS

to do without light but they can't do without oxygen ! So
see that your tank is not too small if you wish to cover it
with a light-proof box or with a blanket or other soft cover. The
reason why you must take this precaution is due to the fact
that these mammals use up their energy so quickly that
they must renew it by means of oxygen intake as well as
food.

As to colour sight, it has already been stated that with
the exception of the anthropoid apes and probably some of
the monkeys as well, mammals do not have colour sight.
This has been well substantiated by dissection and examina-
tion of the eyes of dead specimens, and it is really a waste
of time to carry out experiments to prove what is already
established. The truth is that mammals, with the exception
of the kinds mentioned, have no use for colour sight and
nature seldom, if ever, endows a creature with a sense which
is of no practical use to it.

IV

HEARING— GENERAL

At the risk of appearing repetitive it may be as well to
remind students that the senses of animals must not be
viewed anthropomorphically. We think of hearing in our-
selves as a means of perceiving and interpreting sounds we
hear in an intelligent way — once we have passed the infant
stage. The spoken word, which is the great barrier between
the human animal and the wild animal, is translated by our
brains in such a manner as to convey to us the meaning of
these sounds as language. Other sounds ranging from gunfire
to the songs of birds are also interpreted intelligently accord-
ing to the degree of intelligence with which age, education
and civilization endows us.

Among wild creatures the sounds they make are certainly
methods of communication with others of their own kind,
and are perceived as warnings of danger and so on. But this
does not mean the same thing as conversation which must
imply a degree of intelligence lacking even in the higher
groups of animals.

It may be argued that our dogs are capable of learning
and responding to many words and short phrases in the
course of their training; but of course no dog interprets
these words as would a human being : it is the general sound
of such words that make a dog respond and, even more
important, it is the tone of voice in which we utter these
words that conveys to the dog what it is required to do.

The scientific and technical side of hearing will be fully
treated in Part II by Dr. Harrison Matthews; but a brief
reference to the nature of sounds will not be out of place
here.

All sounds are caused by vibrations of widely differing

frequencies both through the air, the earth, and in water;

and the many types of animals are adapted, according to

69

70 THE SENSES OF ANIMALS

their requirements, to receive these vibrations. Some may
be able to receive and use those sound waves which are of
a very high frequency — bats for instance ; while others, like
some of the whales, respond to low frequencies. Song birds,
which on the whole live in a world of high frequency sounds,
will not be adapted to deal with those of very low frequen-
cies ; and our human ears are not sufficiently sensitive for us
to hear the very high-pitched squeaks which enable bats to
navigate and even catch food.

The sense of hearing, however, may at times be as vital,
or even more vital, to some groups of animals as is sight ;
and general observations on certain creatures when asleep
will satisfy anyone on this point. It is maintained that in
normal life very few birds or mammals sleep really deeply —
their hearing mechanisms are always subconsciously on the
alert. Were this not so, many would fall a prey to animal
or human enemies simply because their sleep might be too
deep for them to be awakened in time to take defensive or
avoiding action.

All the same it must not be forgotten that certain special
circumstances will show that human beings share this semi-
alertness during sleep with some wild animals. A mother
with an infant or small child may be well and truly asleep ;
but the slightest cry from the child will waken the mother
when a thunderstorm might not. The fortunate father is far
less likely to be awakened by his offspring's night cries, and
this seems to show that the female at such times either sleeps
less heavily — though she may be unconscious of this — or
else, deep down within her she is, at least temporarily,
extremely sensitive to high-frequency sounds. Of course, the
wild animal mother will also be equally super-sensitive re-
garding noises made by her young; but basically the wild
creature is at all times acutely alert, having more dangers
and threats about her than a human mother. In this con-
nection it is interesting to remember that on the whole
women are heavier sleepers than men, and it would be
instructive to know the differences in depth of sleep among
wild animals as between the two sexes.

In this book we are endeavouring to refer to as many
different kinds of animals in relation to their senses as is pos-

HEARING — GENERAL 7I

sible; and it is convenient to begin the chapters on each
sense by deahng with the lower organisms first and then
working up to the higher forms of animal life. It follows that
it is important to point out that just as primitive light-
receiving organs exist in primitive creatures, so do those
same levels of animal life have simplified ways of detecting
vibrations which can scarcely be called "hearing" in its true
meaning. Indeed it is not always easy to tell whether the
response to vibrations in some lower forms is more nearly
related to a sense of hearing or to the sense of touch or feeling.
In some cases it may be both senses that produce reactions
of one kind and another.

Many of the microscopic and near microscopic animals
are most susceptible to vibrations, and it is well known that
some of the colonial protozoa such as Vorticella, and some
of the more highly developed rotifers — particularly those
known as "tube-dwellers" — are so sensitive to vibrations that
the lightest touch on a jar or other vessel containing them
will immediately produce a kind of defensive action which
causes the former to retract their stalks, and the latter to
withdraw into their tubes. It would, in my opinion, be a
mistake to regard this as an elementary form of hearing and
such behaviour would seem to be nearer to the sense of
touch.

Hearing is most important to many insects, though their
auditory organs are often placed in some weird situations —
on their bodies and on their legs. Numerous insects can make
mechanical noises by rubbing wing-cases and legs in such a
manner as to produce sounds which are necessary to keep
colonies together and to assist in mating. This method of
making sounds is known as stridulation and is most common
in grasshoppers and crickets.

The humming noise made by bees, wasps, and beetles is
again mechanical, being brought about by the rapid move-
ment of the wings. It is of no significance to the insects them-
selves. Similarly, the so-called "squeak" of the Death's Head
Moth is not vocal, but is produced by air in the body being
forced out through the proboscis (feeding tube).

In the amphibians sounds play an important role in
certain groups. Frogs and toads, which have already been

72 THE SENSES OF ANIMALS

mentioned briefly, have voices which may be croaks, grunts,
trills and even bell-like notes. Newts are voiceless; and the
reason for this may puzzle a student who has not given
attention to the matter. It is during the breeding season that
the sounds made by frogs and toads are most frequent and
noticeable. This is because the croaking of the males and
the feeble answering calls made by the females help to keep
a colony together in order that mating and fertilization may
be completed satisfactorily. These sounds may also be re-
garded as a form of vocal display and in some species defence
of territory. Newts, on the other hand, are not colonial in
their spawning, and there is quite an elaborate courtship in
most species which takes the form of active movements by
the males which curve their tails, show off their seasonal
bright colours — mostly on the belly — and nudge and push
the desired females in order to stimulate both them and the
males as well. This kind of display is mainly visual, though
touch plays its part. It is therefore not surprising that newts
make no sounds and do not possess vocal cords. This being
so they have no need for hearing. The squeaks which emerge
from newts which are carelessly handled are mechanical, and
are caused by the air from within their bodies being forced
out.

The reptiles vary very much in regard to sound. Snakes
have no ears but they are very sensitive to vibrations from
the ground. The receptive mechanism to deal with this will
be fully referred to in Part II. Tortoises and terrapins can
hear well, though the sounds they utter are largely limited to
various kinds of hissing. Crocodiles and alligators can roar
quite loudly but they have watertight ear-flaps which, how-
ever, do not stick out from the head. They can hear sounds
both in water and on land.

The lizards have very sensitive ears, though here again
there are no external ear-flaps. The sounds made by some
lizards are confined to hisses which in some species are almost
loud enough and low enough in pitch to be designated
"roars". Sound plays little part in the courtship of lizards
which is mainly of a physically demonstrative nature ; but
their hearing is much used when hunting prey. Even our
own three lizards — the Common or Viviparous Lizard, the

HEARING — GENERAL 73

Sand Lizard, and the Slow Worm — can hear their prey
moving and are often directed to some insect or other food
item by the sounds made by them. Of course, as has already
been said, sight is the most regularly used sense employed
in feeding, but hearing undoubtedly acts as an additional
aid.

In the fishes we meet the difficult point as to where the
detection of vibrations of various kinds comes near to real
hearing. It is probably true to say that fishes do not "hear"
sounds in the way in which sounds are heard and reacted to
in higher animals ; but nevertheless fishes are most sensitive
to vibrations in the water which may act as alarms and as
guides to navigation in streams and rivers, and also in the
finding of food in certain species.

All birds have excellent hearing, though no bird has
external ears. Hearing is used in connection with song, alarm
calls, protection of young, and for the detection of food
items. The familiar thrushes and blackbirds and robins can
detect worms in their burrows by means of sound, and in
the next chapter the way in which this can be observed and
investigated will be explained. In the owls hearing plays a
special and most interesting part in their hunting behaviour,
and the particular way in which the ears of owls are adapted
for this purpose is one of the most fascinating aspects of the
sense of hearing in birds.

In many species of owls this special adaptation takes the
form where the right and left ears of the birds are different
in shape and structure — they are asymmetrical. This would
seem to show that these owls use their ears as direction-
finders when conditions are unsuitable for hunting by
sight.

Another interesting feature of the hearing abilities of birds
is the way in which many greatly varying species can hear
and respond to alarm calls — even when uttered by a dif-
ferent kind of bird from the one which calls. We see this
without difficulty in our gardens where the blackbirds seem
definitely to be the sentinels. The alarm note of a blackbird
when it sees a cat will not only act as a warning to its own
newly fledged young, it will also cause other birds — tits,
wrenSj hedge-sparrows and so on — to take cover or take

74 THE SENSES OF ANIMALS

flight. I am not suggesting that the blackbird has "philan-
thropic feelings" towards its fellow birds; the point I am
making is that the alarm call is understood, so to speak, by
the other species. This does not apply to the territorial song
of birds of differing kinds. A cock robin will warn off another
cock robin who seeks to invade his territory ; but within that
territory there may be tits or thrushes to whom the robin's
song has no significance at all.

The same sort of thing can be noted in country where
Redshanks are to be found. The redshank has truly been
called "the warden of the marshes", and its striking alarm
note will cause other meadow and marshland birds to freeze.
The redshank would seem to react very quickly indeed to
an approaching danger.

This appears to indicate some kind of discriminating
ability in hearing even if it be discrimination of an elemen-
tary and limited kind. Among mammals, however, discrimi-
nation between one sound and another is very marked ; and,
as we shall note later on, a mammal's ability to sort out
scents is even more marvellous.

Much further work on this kind of problem by field
naturalists would be well worth while, because although it is
obvious that a particular animal may only be able to detect
sounds within certain ranges of frequencies, and therefore
sounds beyond its own limits are not heard at all, there are
many animals which share ranges of sounds, even though
they themselves are unrelated.

Mammals are, on the whole, as much dependent upon
their hearing as on sight, and the way in which many of
them can pick out certain sounds from a jungle chorus, or
remember sounds and attach particular importance to them
is a most remarkable feature of their auditory abilities.

It will be shown in Part II that the external ears of
mammals give some indication of their sensitiveness of hear-
ing, the Elephant being an extreme example of this ; but
there are somewhat puzzling instances where it can be shown
that a mammal is very sensitive indeed to sounds although
its external ear is neither unduly large nor very mobile. This
can be observed in the Water Vole which can react to the
slightest sound ; but, as compared with other rodents such as

HEARING — GENERAL 75

the Wood Mouse and Brown Rat, the outer ear is normally
held flat against its head and has not got the trumpet shape
which is so clearly seen in the other two species referred to.
Whether mice and rats can hear fainter sounds than water
voles it is difficult to say, and it would be a most interesting
experiment to carry out tests designed to show their relative
acuteness of hearing.

Deer of all kinds, in my opinion, have hearing which is
equal to their sense of smell ; and a hunted deer, while using
its nose to obtain the maximum amount of protection from
and warning of danger, will use its hearing for the same
purpose with great advantage. It can discriminate between
the baying of hounds and the barking of farm dogs (of which
it will take no notice) over considerable distances ; and when
the wind is unfavourable for scenting purposes, I believe that
deer rely mainly on their hearing.

Hares and foxes, too, use their ears to a great extent, while
badgers have a most delicate sense of hearing, as anyone
who has tried their novice hand at badger-watching will soon
learn.

The peak of hearing ability in mammals is reached by the
bats which, as is now fairly generally known, navigate and
even locate food by means of their specially adapted auditory
mechanisms. The basis of this super-normal sensitivity lies in
their ability to utter high-pitched squeaks which, when as
sound waves they bounce back towards the bat, can be used
and interpreted to tell the bat how far away is the object
from which the squeaks have returned as echoes.

While it is always difficult to compare the degree to which
an animal uses one sense or another, it is safe to say that the
majority of animals utilize their acuteness of hearing far
more than the average person thinks.

V

HEARING— FIELDWORK AND EXPERIMENTS

Observations and experiments with the lower organisms
designed to inquire into their reaction to vibration have
already been mentioned and are comparatively easy to carry
out with the aid of even a low-power microscope.

Larger Invertebrates

With earthworms, their sensitivity to light is perhaps
exceeded by their reactions to vibrations. This can be
demonstrated by placing some worms on a piece of stiff
cardboard which has been moistened sufficiently to enable
the worms to move about freely. If a tuning-fork is struck and
then lightly brought into contact with the card, the worms
will retract in response.

The same kind of experiment can be carried out with web-
spiders. A tuning-fork held very near, but not even touching,
the outer threads of the web will often bring the spider forth
from its lair, the vibrations causing the spider to react as if
an insect had touched the web.

Caterpillars, too, will often behave in the same way if they
are feeding and the plant is touched gently. In fact, a very
wide variety of animals will show their response to vibrations,
though to us no audible noise is detected.

Many insects use their peculiar hearing organs to receive
sounds made by their mates ; and this can be observed with-
out much trouble by capturing some grasshoppers or crickets
and confining a male in a container — even a paper bag
will do. If the month of the year and the time of day and
temperature are correct, the male insect will "stridulate" ;
and if some females are then liberated in the vicinity of the

container, they will move towards it, thus showing not only

76

HEARING — FIELDWORK AND EXPERIMENTS 77

that they have received the sounds given out by the male,
but also that his courtship "song" has a particular signifi-
cance for them.

Amphibia

The use of vocal sounds among frogs and toads can be
observed any spring by first locating a pond used by one or
other amphibian. It is best to do this well before the normal
time when these creatures migrate to their ultimate spawning
places. The males do all the real croaking (though females
can utter sounds as well) and they are usually the first to
come out of hibernation and make their way to the ponds.
Incidentally, it has not yet been definitely established exactly
how toads, at least, locate their ponds with such precision
year after year ; but when this problem has been fully solved,
it is likely that it will be discovered that one of the normal
senses — possibly smell — is used and not any mysterious extra
sense.

Once at the ponds, the males start croaking and are soon
followed by the females, though it is most improbable that
croaking serves as any guide towards the water ; for if this
were so what would guide the first of the males? The frogs
or toads, having congregated in the pond, croaking is thought
by most of us who have studied these animals to be used for
keeping the colony together.

There seems to be little real rivalry among Common Frogs
or Toads ; but in the case of the Marsh Frog and the Edible
Frog there is evidence that the males take up special croak-
ing points or situations which they will defend against other
males.

I have spent a great deal of time observing these species —
especially the Marsh Frog in the Romney Marsh area in
Kent; and not only I, myself, but other naturalists and
zoologists with whom I have had the pleasure of working,
have seen this territorial defence behaviour going on. (See
Plate 4.)

One male will take up his chosen position and will com-
mence to utter his loud and very distinctive call. Often
another male, which has not yet secured a favoured site, will

78 THE SENSES OF ANIMALS

hear the croaking and will approach. This will stimulate the
first male into even louder croaking which is answered by
the second male. Should this intruder come near the holder
of that territory, it will be attacked in a very determined
manner. The attacker will jump at the other male and try
to bite at its vocal sacs, thes i. being inflated during croaking
so that they stand out on each side of the head, looking like
small grapes.

This very definite attack surely shows that these male frogs
instinctively try to damage the organs on which the rival
depends for his mating success. Similar attacks on vital
organs are known in other animals.

Reptiles

The hearing of snakes has already been referred to ; but it is
necessary for the student to remember that snakes have no
true ears; and the specialized bones, which allow them to
detect vibrations from the ground, are not "hearing"
mechanisms in the accepted sense of that word. I have tried
time and time again to produce response from snakes of
many species by playing to them on whistles and a clarinet,
and I have never succeeded in doing so. I have, however,
caused snakes to protrude their tongues by gently resting a
clarinet against their tanks — a sign that the vibrations are
detected by the snakes, because they are conducted via the
tank itself The action of the tongue means that at times
snakes may be aware of possible prey by reason of vibrations
made by some animal, in addition to the scent by which
snakes normally detect their victims.

Of course, different frequencies and degrees of vibration
mean diflferent things to snakes. A heavy footfall may spell
danger and will evoke either retreat or attack; but lighter
vibrations may well herald the presence of a meal.

The hearing of lizards can be easily tested in those species
which are often kept as pets. The British Sand Lizard is a
good one to use for this purpose since it becomes very tame
and soon learns not to react in alarm to slow and quiet move-
ments on the part of its owner. There are two ways of con-
ducting suitable experiments.

HEARING — FIELDWORK AND EXPERIMENTS 79

Let the tank in which you keep your Hzards have a good
carpet of dried leaves at one end. Once you have got your
Hzards tame (which only takes a few days), introduce some
mealworms into the place where the leaves are; they will
soon vanish from sight among the leaves. The lizards, if
hungry, will be seen to look alert and this means that they
have heard the movements of the mealworms. They will
approach the leaves giving every sign of listening by turning
their heads sideways and downwards. They will then move
the leaves with their snouts (sometimes with their front feet)
and, with their tongues going in and out to pick up scent
and their sharp eyes looking here and there, they will seize
and eat the mealworms they uncover. This is an interesting
demonstration of what might almost be called "team work"
of the senses, because the prey, being at first hidden, is
detected by hearing; then both scent and sight are used in
the final capture.

The very acute hearing in lizards may also be observed in
their habitat, though this demands some patience and good
luck. Two persons are required and the procedure is as
follows.

You must first know of a place where either our Common
(Viviparous) Lizard or its larger relative the Sand Lizard
is known to be present. If the site in question is familiar to
you, there will probably be special spots where lizards have
previously been seen sunning themselves, and you will, I
hope, have noted these carefully. Lizards are very conserva-
tive indeed regarding these basking spots, and if no predator
has taken the lizards in the interim, there is every reason
why you should be reasonably certain of finding them there
day after day, if the weather be warm and sunny. It should
not be too hot, for both snakes and lizards dislike undue
heat. The best times of day are between 9 a.m. and 1 1 a.m.,
and 5 p.m. to 7 p.m. These periods are, of course, variable
to some extent, depending on the temperature and precise
situation; but the point to be remembered is that the real
heat of a spring or summer's day will not be conducive to
finding lizards lying out.

Let us suppose that you have your area planned and that
the conditions are favourable. You and your companion

8o THE SENSES OF ANIMALS

will have different roles : one must be the observer and the
other the person who, at the right time, makes a noise by
clicking fingers or crumpling dry leaves or bracken.

The observer must walk slowly and silently towards some
known place; as soon as a lizard is seen basking he must
stop and remain quite still, keeping his eyes on the lizard —
a pair of binoculars can be most helpfiil in getting an early
sight of the quarry without disturbing it. As soon as the
observer stops moving, the assistant, who should try to be
in a position nearby, but so placed as to ensure that no
shadow falls across the lizard, must make the agreed noise.
If both persons have acted with caution and luck is good,
the observer should be able to see how the lizard either takes
up an alert posture or, if the noise be loud enough, will
scuttle to cover. A series of experiments of this kind, with
the noise being decreased each time, will again show the
high degree of hearing ability possessed by lizards.

My own most impressive test was to obtain a reaction
from a sand lizard when my colleague had merely "clicked"
his finger and thumb-nails together a good eight paces from
the lizard.

Birds

In considering ways and means of observing and testing the
hearing powers of birds, we come to a group of animals
where sounds play an important part in their lives. Defend-
ing territory, courtship, alarm, and food finding, all have
some connection with a bird's ability to hear and utter
sounds; but this aspect of our subject is complex, because
the sounds vary much as between species and species, and
also according to the purpose for which the sound is made.
Once again, it must be remembered that no animal has a
sense well developed unless there is some use for it; and in
relation to hearing, it follows that birds utter different notes
to suit certain conditions. They hear and respond to the songs
and calls of their own particular species, and they also can
hear sounds made by other animals or by mechanical means.
They, therefore, have the ability to take in vibrations of
greatly differing wave-lengths, e.g. the high twittering of the

HEARING — FIELDWORK AND EXPERIMENTS «I

tiny Goldcrest, or the loud bang of a gun or a stone hitting
a wooden fence.

The songs of birds are mainly for the purpose of proclaim-
ing and maintaining territory, and are not, as so many
sentimentalists think, uttered in order to charm hen birds
by their beauty. They are certainly not for the purpose of
pleasing us, however lovely they may be. But the song of
a cock bird will be heard by his mate and so assist in
stimulating the activity of the sex hormones which bring
about mating and egg-laying — as any canary breeder should
know.

There seems to me to be little point in trying to devise
experiments to show that birds have acute hearing in the
wild state — particularly in the case of what are popularly
known as song-birds — because their auditory acuity is well
established. At the same time, it is worth while to consider
one or two other aspects of hearing in birds.

I once had the good fortune to hand-rear a cuckoo from
the time it was a few days old until it migrated, and I was
struck by the great sensitivity of this bird's hearing powers.
While it was still being largely fed by hand, I used meal-
worms for the insect portion of its diet. These beetle larvae
(for that is what they are) were kept in a tin near the cage
in which the cuckoo was housed. I was very intrigued to
find that when I picked up the tin the bird would increase
the frequency and intensity of its hunger-calls. Though it
would have been tempting to attribute this to intelligence,
much experience in rearing young animals had taught me
that most — but not all — behaviour of this kind is due to the
high development of one sense or another. That I was right
was proved to me when this cuckoo was a little older and
had, at intervals, some measure of freedom within the room
where it was kept. I noticed that when hopping about it
would move towards the tin of mealworms and cock its
head sideways in what I can only term an attitude of listen-
ing. It would go right up to the tin and would peck at it.
When the bird was old enough to feed itself, and the flat
tin containing the mealworms was opened, the mealworms
would immediately be attacked and eaten — too many of
them, if I did not intervene.

82 THE SENSES OF ANIMALS

I naturally wished to eliminate any possibility of "associa-
tion" ; and so, in order to make some kind of test, I procured
an exactly similar tin but without mealworms in it, and this
was placed in the same spot as the usual tin while the bird
was resting and unable to see what was going on. At the
appropriate time I put the young cuckoo near to the tin and
awaited events — nothing ever happened. If the proper tin
were substituted for the empty one, there was an immediate
reaction.

Of course, even the human ear can detect the faint rustling
made by mealworms in a tin containing bran, but this cannot
be heard at a distance of — say — ten feet, but the cuckoo
would show signs of interest from a much greater distance —
at times it would fly right across the room to alight near
the tin.

Another aspect of hearing in birds may be noted in those
species (and there are many) whose members have powers
of mimicry. Parrots, budgerigars and mynahs, to say nothing
of crows and starlings, are well known to be able to imitate
the human voice and whistling, and also mechanical sounds.
To do this the birds in question must not only be able to
hear the greatly varying sounds clearly, they must be able
to remember them; and though the fascinating question of
memory in animals is not within the scope of this book, the
performances of good bird mimics do go to show the accuracy
with which the sounds are first heard and then learned and
stored away in that part of a bird's brain which is concerned
with memory.

The great capacity for memorizing sounds possessed by
those birds which are known to mimic is most striking. I,
myself, have had several parrots which had a repertoire of
well over fifty words, noises, and tunes, while an African
Grey of mine totalled seventy diflferent words. I have also
had a mynah which could say more than a dozen words.
I am certain that there are far more imposing records than
these, and it is worth mentioning that the popular budgerigar
is probably better at learning or memorizing words and noises
than any other bird.

Why some kinds of birds excel in this way and some do not
is, as far as I know, a mystery. There is little in common

HEARING — FIELDWORK AND EXPERIMENTS 83

between the tongues of parrots, starlings, and crows, yet all
can hear and store up and repeat very diverse sounds. The
syrinxes (voice-boxes) are the organs by means of which
birds utter their songs and cries; and these, too, differ in
their construction between group and group. It would be
interesting to know if the ears of the various mimicking birds
vary a great deal; for clearly the ability to mimic depends
on acute and selective hearing, because without such the
birds would not be able to hear and distinguish the sounds
they learn.

All owners of talking birds are unconsciously experiment-
ing with the hearing of their pets, and though many probably
do not realize this, and consider that they are only concerned
with words and whistling, it is a pity that more of them do
not study the whole of this matter more thoroughly. Dr.
W. H. Thorpe of Cambridge University is at present devot-
ing much of his time to investigating this side of bird be-
haviour and, in due course, when his work is complete, we
shall probably learn a great deal more about both the vocal
and auditory apparatus of birds and the interpreting of their
sounds.

Mammals

In considering the hearing of mammals, we are dealing with
a sense which is probably as highly developed as the sense
of smell, though the mysteries of scent present features that
are by no means fully understood. In addition, those demon-
strations of scenting powers which we can observe may seem
to be more wonderful than those of hearing. Nevertheless,
the sensitiveness of hearing manifested in mammals can
hardly fail to impress the student.

Mammals are the only creatures that have external ears,
or "ear-flaps" as they are sometimes crudely called. These
external ears are seen in great variety in different groups,
but all have the same basic function, which is that of con-
ducting sound to the inner ear and thence to the brain. The
external ears of some mammals are very mobile and are
capable of being turned this way and that so as to be able
to detect far-off sounds and to locate the origin of the sound

84 THE SENSES OF ANIMALS

with a very high degree of accuracy. This mobility, com-
bined with the extreme sensitivity of the structure of the
inner ear, is used for many purposes : to alert the mammal
to danger ; to find its way about, as in the bats ; and to aid
in the capture of prey, as in the bush-babies.

In order to appreciate the use of the external ear, observa-
tions of a very simple kind may be carried out just by visiting
a good zoo and noting carefully the variations in the size
and shape of the ears of the mammals to be seen. It does
not necessarily follow that because an elephant has huge
ear-flaps it is able to hear any better than — say — a harvest
mouse; but the elephant requires to hear sounds over
much greater distances than a mouse of any kind which
is largely concerned with sounds comparatively close at
hand.

Nearer to home, the way in which our own dogs and cats
twitch or cock their ears shows very well how much these
organs are used in daily life. A dog can recognize the sound
of its owner's motor car even when another car of similar
make is also moving in the vicinity. It can also separate out
the footsteps of each member of the household and will react
differently towards one member or another. Similarly, a dog
which will respond to its owner's footsteps with a welcoming
bark will stiffen its tail if the footsteps of a stranger can
also be heard at the same time.

Try a small experiment for yourselves. Make sure your
dog knows that you have left the house, having first shut it
up in a room where it cannot see your eventual return, but
where you can hear it when it barks. After a while, start
walking up your path or drive with your normal pace and
gait. The dog will almost certainly give its welcoming bark.
Stop still for a moment and then continue your progress
but alter your step — perhaps shuffling your feet. The dog's
tone will change at once. This will demonstrate how very
acute and discriminating a dog's hearing is.

A cat's ears are peculiarly sensitive, and have most delicate
adaptations of the inner ear. There is no doubt that a
domestic cat uses its ears to aid in hunting, whether the prey
be a young bird in undergrowth or a mouse or vole in
amongst thick vegetation. Good nocturnal vision is, of

HEARING — FIELDWORK AND EXPERIMENTS 85

course, present in cats of all kinds, but it cannot be too
strongly stressed that even the best endowed mammal is, in
this respect, at a disadvantage under conditions of pitch
darkness. An additional sense to aid in catching live food is
clearly necessary, and hearing is what assists a cat to locate
victims at some distance, after which its stealthy movements
will bring it close enough for a catch to be made.

It is scarcely necessary to suggest special tests in order to
prove the delicacy of hearing in the feline family nor, indeed,
is much more required for dogs in the same context. How-
ever, there are two points worth mentioning. The first is
the use of so-called "silent dog-whistles". These are whistles
in which the construction is such that no noise audible to
us is produced. Dogs can easily and quickly be trained to
respond to the very high-pitch vibrations of these whistles,
and this ready response is surely proof enough of their
ability to hear very high frequencies.

The second point is in connection with the limitations as
regards the location of sounds in dogs, and to some degree
in cats too. Dogs will seldom be able to locate a sound which
comes from a point well above them ; and this can be shown
by calling or whistling a dog from some elevated position —
an upstairs window or some similar spot. A dog will hear the
sound all right, but will not be able to tell from which
quarter it originates.

Deer are endowed with an acute sense of hearing ; and
those who live in districts where our Red, Roe, and Fallow
Deer — particularly the two former species — are to be found
should have no difficulty in proving to themselves the value
of good hearing to deer as a means of escaping danger. Any
deer-watcher, stalker or hunter knows that as much caution
must be observed in respect of quiet and careful movements
as in respect of the necessity to keep down wind of these
animals and so avoid being scented by them.

Horses, cattle, sheep, hares and rabbits (where the latter
still exist) all offer opportunities for observations on their
hearing abilities. No student of badger or fox life will be
successful if, having established himself at a "set" or "earth"
for the purpose of keeping watch, he moves about or allows
his clothing to brush against bushes and undergrowth; he

86 THE SENSES OF ANIMALS

will have no success be he ever so careful about wind direc-
tion and scent. (See Plate 5.)

I have been asked frequently which mammal has the most
sensitive ears and which one is the easiest to experiment
with. The first question is easily answered; the bats must
take pride of place on account of their "built-in" sonar
equipment about which Dr. Harrison Matthews will have
much to say in Part II. As far as ease of experiment is con-
cerned, I, personally, consider the galagos (bush-babies) offer
any student fine chances for testing their hearing. I have kept
many of these delightful creatures, and feel I must devote a
little space to them here.

Bush-babies, which belong to the lemur family, are fairly
commonly kept as pets and they do not require any descrip-
tion. All species have large and very mobile external ears
which can be moved in many directions, giving a high
degree of powers of location. These powers are obviously of
value to the animal in detecting the presence and where-
abouts of enemies ; but it is not so generally known how
useful are these finely adapted ears in locating the flying
insects on which they feed to a considerable extent.

I have carried out many experiments in this respect, but
before going further into them, I must hark back to sight
for a moment. Bush-babies are, in nature, exclusively crepus-
cular and nocturnal creatures, and they possess the huge
eyes which are associated with nocturnal vision. These eyes
are obviously used for catching food where conditions allow,
but as I have stressed before, no animal can see when it is
nearly or completely pitch dark. However, food must be
obtained even when conditions are most unfavourable for
hunting by sight. It is then that the large ears come into
their own. Moths and many other flying insects create sound
waves as they fly — anyone who has had a cockchafer in their
bedroom knows that — but it is one thing for us to be able
to hear the wing-flutter of such insects, and quite another
to catch one in the dark ; a bush-baby will make light of this
task.

Let us suppose that we have a bush-baby in a roomy cage
placed in a room where the light can be conveniently
switched on and off. My own cages for bush-babies are

HEARING — FIELDWORK AND EXPERIMENTS 87

ventilated from the top, but have a glass front and a door
at the side for the introduction of food.

The next thing to do is to catch some large moths (or
cockchafers in the right season) . Enter the room where your
cage is situated without switching on the light, guiding your
own way to the cage with a very dim torch. Having got to
the cage, have your tin or jar containing your flying insects
ready, and hold this with the lid only just in place. Put out
your torch as soon as you have your other hand on the
cage-door fastening. Wait for some moments, and while
you are waiting, dispose of the torch where you can regain
it quickly. Take up the jar with your now free hand and
with the other quietly open the cage door sufficiently for you
to insert the jar (with the lid loose). Slide the lid to one side
and shake the jar inside the cage. Your insects will then be
freed. Shut the door and listen. You may be able to hear
your moths or beetles fluttering around, and you will without
doubt soon hear the plopping noise made by the bush-baby
as it leaps to and fro. Wait for some ten minutes, and then
either switch on the main room light, or take up your own
torch again.

Inspection will usually show you that most, if not all your
insects, have now vanished. If you feel that further evidence
of their capture is necessary, you will probably have this
provided for you by the bush-baby itself, which may not
have finished its meal and will still be crunching away with
occasional licking of the lips. Not too many insects should
be given at one time — four or five will do. If this precaution
is not taken, you may find that some — moths in particular —
have settled soon after release, and these, of course, will be
caught by visual means as soon as the light comes on again.

Another experiment on exactly the same lines as that
which I recounted in connection with my tame cuckoo, may
also be carried out. A bush-baby will hear mealworms
crawling about in a tin or paper bag and will at once pay
attention to the receptacle. This test can, of course, be car-
ried out in a lighted room so long as it is not too brilliant,
and so long as you take care not to let the bush-baby see
what you are preparing. Your subject may have nocturnal
sight, but it can see quite well in fighter conditions once it

88 THE SENSES OF ANIMALS

has had time to allow its eyes to accommodate themselves
accordingly.

No doubt readers will be able to devise further experi-
ments to test the hearing of common mammals, and it will
be found that in most instances their hearing will be shown
to be only a little less acute than their sense of smell ; the
trouble is that to compare the one sense with the other raises
difficulties which are, for the ordinary student, far from being
simple to overcome. After all, in investigating the senses of
animals as a whole, we must constantly remind ourselves
that we are not "wild" animals, and it is in a world of senses
so much more acute than our own that most animals live.

VI

SMELL— GENERAL

Most naturalists and students of animal life have their
favourite groups of creatures, and this applies also to many
who study animals' senses. This aspect of the make-up of
living things — be they birds, insects, mammals or any other
kind, is absorbing taken as a whole; but some particular
sense may interest individual workers more than others.

The power of scent, with which so many widely differing
animals are richly endowed, seems to me to be the supremely
interesting one. There is so much concerning scent about
which we still have a great deal to learn, and there is the
additional point that some animals have the ability to give
off as well as detect scents.

In the mammals, scent is used for so many different
reasons. It may be used in trailing and capturing prey; it
can be the means by which an animal is warned of the
approach of enemies; scent is also used to recognize or
attract mates, and it is of great importance to some kinds
of mammals for marking and establishing territory.

Smell is what is known as a chemical sense (as is taste)
and thus differs from sight and hearing. The use of the
word "chemical" means that particles of certain chemical
substances in solution are carried by the wind or diffused
in water, and these minute particles are detected in various
ways by the animals which have powers of scent. These
smells are naturally of many kinds : they may emanate from
a prowling foe ; they may come from plants or from animals
that serve as food ; they may be given off by an animal in
the breeding season in order to stimulate courtship and
subsequent mating. Whatever form they take they have to
be detected and received by olfactory organs which may
differ in structure and situation as widely as the smells them-
selves differ.

89

go THE SENSES OF ANIMALS

Dr. Harrison Matthews will be dealing with these delicate
structures in Part II and it is not necessary to go into their
complexities here. It is, however, very relevant to fieldwork
and experiments to refer to the factors which may affect the
scents given off and perceived by animals. The weather has
a great effect on scent : wind, quite obviously, has a bearing
on the transport of scents ; and humidity and temperature
also affect the degree of scent present and also the persistence
of scent on the ground, in the air, and in water.

Though the sense of taste will be covered in the next
chapter but one, it must be appreciated that the two senses
of smell and taste are closely linked. There is much work
to be done by specialists in this connection, and one aspect
of the relationship between smell and taste about which we
appear to know little is where certain creatures are thought
to have meagre powers of scenting — birds for instance — yet
these same creatures appear to be able to taste well, as is
evidenced by their quick rejection of food which seems to be
unpalatable.

It is interesting that scenting powers do not necessarily
increase as one goes up the ladder of animal life. It is impos-
sible to say with certainty that a mammal can smell better
than an insect, and comparisons of this kind lead us no-
where. It is clear, however, that an animal will have the
sense of smell developed to the degree necessary for it to
survive. Its olfactory abilities may be acute under the circum-
stances in which it lives, but outside those circumstances
this sense becomes useless. One can say that some moths
use smell as a means of locating a mate or a source of flower
nectar, but outside these uses scenting ability is not needed.
On the other hand, a mammal may require to use scent for
more diverse purposes and therefore it may seem to have
"better" smelling powers. But this does not mean that a
mammal detects scent more accurately or more delicately
than an insect. It is more true to say that a mammal has a
wider use for scenting abilities than an insect.

The vast world of insects contains many examples of deli-
cate and specialized scent detection. The organs of scent are
found in the antennae or in the palps, and these are usually
referred to as chemo-tactic organs, that is to say organs which

SMELL — GENERAL 9I

combine the ability to receive chemical particles with the
delicate sense of touch.

In the reptiles — particularly snakes and lizards — the
tongue acts as a detector of smells in conjunction with the
organ known as Jacobson's organ. This is situated in the
roof of the mouth in the form of sensitized pits to which scent
particles are conveyed by the tongue and then translated
into messages of scent in the brain. (See Plate 7.)

In the amphibians the sense of smell is present and is
closely related to taste. Investigations into the scenting
powers of amphibians — notably those of Dr. Maxwell Savage
— tend to show the possibility that frogs and toads find their
way to their spawning places by means of the smell coming
from the waters used for breeding, and which is caused by
the particular scent of minute plants in the ponds. The
scent is diffused into the air and so to the amphibians them-
selves and, aided by the air currents close to the ground,
can enable the frogs and toads to select by scent those ponds
which contain the right plants for the support of the tadpoles
which will hatch from the eggs in due course. In addition,
the actual chemical content of the water will play some part,
as this will, to a considerable degree, affect the plant growth
in it.

Many other aquatic animals, including fish, have powers
of scent which, in the main, are used for the detection of
certain kinds of food. This applies to marine animals as well
as fresh-water species.

Fish have a highly developed sense of smell, and in a
great number of species this is the way in which they locate
their food. Of course, predatory fish, such as pike and perch,
hunt by sight, but not all carnivorous fish rely on their eyes.
Sharks, for instance, can smell blood or flesh from great
distances and may well be the best scenters among the
fishes.

Many fishes take water, with its dissolved chemical scent
particles, into their nasal cavities where it comes in contact
with specialized cells capable of appreciating the scents so
conveyed.

Anglers will be familiar with other kinds of fishes which
have on their lower jaws fleshy finger-like processes known

92 THE SENSES OF ANIMALS

as barbels which are themselves sensitive to dissolved chemi-
cals. The Barbel, a noteworthy fish to be found in many
of our British rivers, derives its name from these, and under
very muddy water conditions these fish rootle among the
debris on the bed of the streams using these organs to find
food.

Birds have already been mentioned in connection with
their ability, or lack of ability, to smell ; and this poses a
pretty problem. It is generally considered that the power
of scent is confined to geese, ducks, and some sea birds; and
that the majority of birds, being well endowed with keen
hearing and sight, use these senses in their search for and
taking in of food. It can be argued that they do not require
a sense of smell because they have no use for it. Then againt
there are birds such as snipe and woodcock and curlew that
feed on invertebrate animals which live out of sight in mud
and boggy places. These birds, however, locate food items
by means of a localized sense of touch which will be referred
to later on.

There is an old idea, which dies very hard, that vultures
can scent their prey (carrion) from great heights in the air.
This has no foundation in fact for, as pointed out earlier,
these great birds have the most acute vision and can
with ease see a prospective meal from hundreds of feet
up.

I, myself, have always been interested in parrots; and
there have been few periods in my life when there was not
a parrot in the house. I have more than once wondered
whether parrots possess some fair degree of the sense of
smell. Though cheese is not a food to be given frequently
to parrots, a little now and then does no harm. Now it is
obvious that if some item of food is offered to a parrot by
hand it can see it well, and usually the bird will take it into
its beak at once ; but before nibbling at it, the item will be
carefully tested with the tongue, thus showing the ability
to taste. However, what has puzzled me — and still does —
is how a parrot recognizes cheese in the mass when it is on
the table. It is easy to see how a small piece of cheese or
rind can be recognized when held close to the bird, but
such a morsel must look very different from a pound of cheese

SMELL GENERAL 93

on a dish ; yet many of my parrots, including my present
African Grey, show every sign of interest as soon as cheese
appears on the table.

This parrot, almost without fail, betrays its wish for
some special kind of food by saying, "What do you want?"
And should a cheese of any kind be at hand, this utterance
is given forth with special vigour. Could this be due to a
well-developed sense of smell? I just do not know. What I
do know is that the more smelly the cheese, the more quickly
does the bird take an interest and say its piece.

Puzzling and intriguing as are the scents and scenting
powers of the animals dealt with up to date, it is in the
mammals that scent becomes a very dominant feature of
their daily and seasonal life. Much has been discovered
about mammalian scents and their uses, but it is a fair bet
that much more remains to be found out.

In mammals the uses to which their own scent is put,
and the accuracy with which they can smell the odours of
other animals are rightly regarded as wonderful ; but what-
ever complexities there may be in connection with scent, it is
basic to any investigation that the conditions under which
scent is diffused are all important. A mammal has its various
ways of using scent but unless wind, temperature, and
humidity are at their best for scenting, the mammal is handi-
capped, whether it be a stoat trailing a rabbit or a foxhound
casting around to pick up the scent of a fox.

Under cold conditions — frost and cold winds — but other-
wise fine, the atmosphere will be dry and scent will be poor.
Should the weather be rather damp and dull, the ground
warmer than the air and the wind from a warmer quarter,
then scent will be good. Of course, there are many modifica-
tions of these simplified weather conditions, and these will
affect scenting for good or bad. It is necessary, too, to
remember that true ground scent is the result of contact be-
tween the feet, paws, or hoofs of mammals and the soil
beneath them. Body scent is different, and may be the
product of special glands in a mammal's body ; or it may
be solely sexual in its function and therefore not continually
present, while normal excretory matter — particularly urine
— can also be regarded as a component of body scent. At

94 THE SENSES OF ANIMALS

times, of course, all these scents may be combined in what
is commonly referred to as "body scent".

Whether the odour given off by a mammal is on the
ground, or in the air — due to the mere passage of the beast —
it is the degree of moisture and temperature that controls
the diffusion of scent particles, plus the effect of the wind
currents. All "scent" is vapour, and it is the intake of this
vapour and its consequent warming up as it travels along
the nasal passages of a mammal that finally guides or
stimulates that mammal in one kind of activity or another.
It is not possible to go into all the aspects of scenting con-
ditions here, but some reference to what are known as
micro-climates must be made. A micro-climate may, perhaps,
be described non-technically as the conditions of damp, heat
and cold, and wind currents prevailing in a small area of
ground within a larger area. For instance, we may be inspect-
ing a piece of waste land of uneven character with a ditch
or two, perhaps some patches of scrub and so on. Now the
general weather conditions over the whole of that area may
well be changed in one way or another due to shelter from
the wind, the presence of water, or some conformation of
the ground ; and in certain spots the degrees of moisture or
warmth or cold, and even the direction of wind currents
will differ again from those of the whole area. These micro-
climates will affect scent to some degree and will confuse
the observer's conclusions if not taken into consideration.

It is not only humans who may be deceived by conditions
in micro-climates, some wild animals may themselves be
handicapped — at least momentarily — in this way. Naturally
in these small areas where wetness or dryness and wind
currents may differ from the general weather prevailing it is
the part of the area nearer to the ground that is most
affected.

All the points mentioned will, I hope, be made clearer
when various groups of animals are discussed in the next
chapter.

VII

SMELL— FIELDWORK AND EXPERIMENTS

Readers may have already noticed that what are usually
termed "laboratory experiments" have hardly been men-
tioned. This is because I feel very strongly that nearly as
much can be learned by observation in the field, and also
by tests in captivity where the natural conditions of the
animals concerned are reproduced as faithfully as possible.
I fully realize that laboratory experiments are necessary for
certain kinds of research, but none the less very valuable
information can be gained without specially contrived cages
and apparatus, so long as the student is well versed in the
general natural history of the species being studied.

With all due respect to those who deal mostly with
laboratory work, I think that wrong conclusions may at
times be reached due to a lack of knowledge of the animals
in the wild. Many scientists have stated that work on captive
animals is suspect because the living conditions of such
animals are so different from those in nature. This is true
to some extent, but if the investigator knows the basic habits,
requirements and behaviour of creatures in their natural
habitats, there will be far less risk of error when dealing
with animals in captivity. On the whole, captive animals
will display their normal basic behaviour, but it is vital that
this basic behaviour be at least roughly known to the experi-
menter. If this is not so, then the interpretation of the behaviour
of a captive animal will be at fault. It is not so much the
fact that an animal in a cage will behave in a markedly
different way ; it may be that certain actions are carried out
in abnormal surroundings in such a manner that they are
not recognized as being normal due to the fact that limita-
tions of space or accommodation force the animal to perform
a perfectly natural function or piece of behaviour in an

95

gS THE SENSES OF ANIMALS

unusual way. The action performed may be just what is done
in the wild ; but changed conditions bring about a kind of
"substitute behaviour" that seems wrong or misleading,
although it is really perfectly normal if observed with
some background information regarding basic wild behav-
iour.

As an example of what I mean, let me refer to experiments
in the intelligence of rats, one test of which is to construct
a maze through which the rat has to find its way accurately
and repeatedly in order to obtain a "reward" in the shape
of food, the time taken to learn the correct route being con-
sidered indicative of the rat's intelligence.

I well remember discussing this "test" with an experi-
menter. I asked him if the mazes were used frequently on
the same rat, and I was told that this often had to be done
due to lack of space in the laboratory and also expense — for
a number of mazes would take up valuable room and cost
more money. I then asked whether the cages were cleaned
out each time an experiment was made in order to exclude
the possibility of scent being the real cause of the rat's success.
The reply was that not only were the cages cleaned out, but
that they were scrubbed with hot water.

This, I fear, did not convince me, because much experience
has taught me that it is difficult in the extreme to eliminate
traces of mammal scent however much one scrubs. Consider
the domestic dog: few dog owners have not had the ex-
perience of "accidents" occurring in their houses — either
when their own dog is in the process of being trained or
when a visiting dog (not yet house trained) has performed
a natural function on the carpet or against a convenient
chair ! Now, no matter how well the carpet or chair cover
is cleaned, it is a fair bet that, even months after the accident,
some dog will come to the house, make straight for the
original spot and will mark its approval — or disapproval —
in the usual way. This is good proof of the persistence of
concentrated scent.

Let us now consider some field observations and experi-
ments in connection with scent in some varying kinds of
animals.

smell — fieldwork and experiments 97

Insects

In insects, the organs or parts of the body capable of per-
ceiving scent are diverse and curious. Many kinds of male
moths detect the scent of females with their antennae, but
both sexes use these organs in seeking flower nectar on which
they feed. This is fairly obvious in night-flying species, when
the colour of the flower is no guide. However, the antennae
are not the only parts of an insect's anatomy which can detect
scents : the palps can do so, and also the specialized hairs on
the feet of some flies.

With the aid of a strong electric torch, it is simple to
observe some night-flying moths visiting the flowers of
tobacco plants, valerian, stocks, and so on. The Elephant
Hawk Moths are suitable species. The moths will be seen
flying around the flower-beds, every now and then hovering
before a particular bloom or flower cluster; then the moth
will drop nearer, almost touching the flower. With care in
the use of the torch, the long tube-like proboscis can just be
seen as it is inserted into the flower to suck up the nectar to
which the moth has been attracted by the scent emanating
from it.

If carefully collected, live specimens of moths can be
liberated into a large cage previously provided with some
suitable flowers among which are placed a number of well-
made artificial flowers in imitation of the live ones. If the
live blooms are very lightly sprayed with water an hour or
so before the experiment is carried out and the room is
warm, the moths will often fly ; and it will be seen that the
artificial flowers are approached, but no attempt will be
made to feed from them. The moths will then try another
bloom where there is a supply of sweet-smelling nectar.

It is also possible to train butterflies to take in sugar-water,
and this kind of test can demonstrate taste as well as smell
if a drop of perfume, to match the natural scent of a flower
known to be favoured by the butterfly in question, is mixed
with the sugar-water. If two dishes are used for the feeding
liquor and one is scented and the other is not, the butter-
flies will tend to feed more readily from the dish which is
perfumed.

gS THE SENSES OF ANIMALS

Other tests can be made with some of the day-flying moths
which mate with the aid of scent particles Kberated by the
females in order to bring the males to them.

Moths, such as the Emperor and Oak Eggar, are very
suitable for this. April to mid-May is the period for tests
with the Emperor Moth ; and July to mid-August for the
Oak Eggar. When the females first emerge from the pupa
they give off scent from glands near the vent, and this scent
is carried by the breeze over quite a wide area. In nature
males will very shortly come up wind and find the resting
females. Pairing results, after which the females cease to give
off scent.

A very interesting and dramatic experiment can be carried
out with a captive female. One which is known to be newly
emerged should be placed in a box with a muslin top, or
in a container made from perforated zinc. If the receptacle
is placed out of doors, it will not be long before a male or
males will arrive. These will settle on the box containing the
female. Once this has happened, the male may be allowed
access to the female by opening the box ; mating will occur
and later the female will lay eggs which, if the student so
desires, can be kept with the object of rearing the caterpillars
in due course.

To prove beyond doubt that scent is the reason for the
males locating their mates a further experiment may be
performed. To do this, leave the female in her box for, say,
an hour; after this take her out and put her in a cage well
away from the spot where the box was situated. The males
will arrive and make straight for the box and show every
sign of interest in it; I have carried out both these tests in
my own garden which is situated not less than a mile from
the nearest area known to have a population of the moths
in question. This particular kind of behaviour is known
among entomologists as "assembling" — a very apt word.

Ants are very sensitive to odours and they, too, can locate
a source of food some distance from their nests. An ingenious
student can easily devise tests to demonstrate this.

The so-called burying beetles are, perhaps, the most re-
markable insects in respect of their scenting powers. These
beetles seek out the dead bodies of animals — mice, toads,

SMELL FIELDWORK AND EXPERIMENTS 99

birds, and so on. The corpses are used as food stores for the
larvae which will hatch from the eggs which are laid on or
near the body; but before depositing the eggs, the beetles
— often in numbers — excavate a "grave" by scraping away
the soil from underneath the corpse. This being done, a pair
will usually take possession and lay their eggs. However, it
is with the location of the carrion that we are concerned.
The beetles themselves will find a suitable dead animal
from a long way off. At times they will use their delicate
powers of scent when on the wing, or they may travel over-
land. In either case, their smelling abilities must be most
efficient.

As I have already stated, scents are used by some insects
for defensive purposes, and good examples of this can be
observed in the ladybirds and those plant pests known as
"shield bugs". These both excrete an offensive smelling
liquid (which even our own noses can detect) and this serves
as a protective warning to predators.

Fishes

In fishes the sense of smell in some families is marked ; and
these fish are either attracted or repelled by odours diffused
through the water. Anglers have known this for hundreds
of years, and even today anglers who go in for coarse-fishing
often scent their bread paste bait with aniseed in the hope
of attracting fish more easily. Many put far too much aniseed
or other sweet-smelling substances in their bait and so defeat
their own object. Only a very little is required to make a
scented bait more attractive than unscented. Aquarium
fishes may be used to test this, and if small pellets of plain
paste are fed to the fish in addition to some that are scented,
the fish will go first for those which smell. Care must be taken
in such experiments to see that the unscented pellets are
dropped into the tank well away from the place where the
scented ones are put. The plain pellets must be made up
before those which have the scented substances added, or it
will be found that one's hands — even after washing — will
carry some scent, thus spoiling the experiment.

Sharks are, perhaps, the fishes that can smell most keenly,

100 THE SENSES OF ANIMALS

and it is well known that blood from a wounded animal
will be detected from afar by these voracious fishes. It is
unlikely that the average student will be able to see this for
himself, but the fact is well established. Those who go in for
shark fishing pour some blood from an animal into the sea
in order to attract their quarry, and anyone lucky enough to
witness the way in which the sharks arrive will not be likely
to forget the experience.

Reptiles

The sense of smell is most marked in snakes and lizards,
though tortoises, turtles, and crocodiles also use their olfac-
tory organs. If you are a lucky field worker — and believe
me luck plays a great part in outdoor observations — you
may be able to see a snake following a scent trail, or a lizard
protruding its tongue before seizing some item of food.

I remember one incident I witnessed not so many years
ago which showed how an adder could follow the scent trail
left by a field vole. The vole had been struck by the snake,
but not so effectively as to immobilize it completely. As I
was leading a natural history society's outing at the time,
several members were able to share in this interesting occur-
rence. We were walking along a path through some open
woodland, when I saw a sudden movement in a small clear-
ing not more than six yards from where we were. An adder
had just struck at a field vole and it still had it in its mouth,
when I drew the attention of the others to it.

Our approach disturbed the adder, which made off
quickly, leaving the vole where it was. The vole, after a
second or two, crawled slowly into some undergrowth in the
opposite direction from that taken by the snake. I told my
companions to remain still for a while and then sit quietly
awaiting the adder's return, which I was sure would be
before long. In due course, the adder appeared, and with
its tongue flickering in and out very swiftly, it literally cast
around until it picked up the scent of the vole when it took
exactly the same route into the undergrowth.

On another similar occasion, I saw an adder crawling
along, quite clearly following scent. As I watched, its

SMELL — FIELDWORK AND EXPERIMENTS lOI

tongue quickened its movements and I saw, just ahead of
it, what at first looked Hke a dead field vole. The adder
moved right up to it and was just about to seize it, when I
made a movement and firightened the snake away. I picked
up the vole which was still warm, and I could see the tiny
punctures where the adder's fangs had originally pierced the
skin just behind the head. The vole died in my hands and
I still have its body preserved in my collection as a souvenir
of a most interesting observation.

It may be of use if I explain what probably happens on
these occasions. It must be remembered that snakes do not
always strike home accurately, and they are not always fully
venomed. In any case, a mouse or a vole, if not instantly
seized or if not paralysed at once, will bite at an aggressor.
Therefore an adder first of all disables the victim and does
not at once try to swallow it. If the venom is at its peak and
the victim lies helpless and still, the snake will stay where it
is and will only take the prey into its mouth when there is
no marked movement. In the first instance I have described,
the adder would almost certainly have located the vole
again — even if it had gone down a hole ; in the second case,
had I not intervened in order to obtain a unique specimen,
the vole would have been quickly taken and then swallowed.

Experiments of a similar type may be carried out with
captive snakes and lizards. As is well known, our Grass
Snake lives on frogs, toads, newts, and small fish; and this
species of snake feeds readily in captivity once it has settled
down. If such a snake is fed regularly for a time, and is then
deprived of food for, say, a week (this will not harm the
snake) it will be keen to feed again and the hunger induced
by its fast will cause its senses to be sharpened.

Remove the snake from its tank or vivarium and place it
where it cannot see what is happening in its usual quarters.
Then get a frog or toad — a toad is best, since it leaves more
scent — and grasping it firmly but gently, rub it in the sand
or earth on the bottom of the tank. You can also let it move
around a little on its own, but take care to note the route
taken. Remove the toad and put it away out of sight. Re-
place the grass snake which, if tame, will not take long to
calm down again.

I02 THE SENSES OF ANIMALS

Very soon you will note that the snake will flicker its
tongue rapidly and will quickly pick up the scent trail. It
will follow this faithfully, and if you have accurately noted
where the scent was laid, you will obtain excellent evidence
of the way a snake can track down prey that is not in
sight.

I have had a film taken of one of my Smooth Snakes
following the scent track left by a lizard ; and this film made
me realize that unless the creature which a snake ultimately
eats is moving when first encountered — in which case the
snake will be stimulated by sight — a victim is often tracked
down by scent until it is finally located and seized.

Captive lizards too may be tested in this way by laying
your trail with a crushed mealworm or dead moth. Most
lizards will follow such a trail just as accurately as a snake.
Our legless lizard, the Slow Worm, will do the same with
a slug. It will scent the slime trail left by the slug and follow
it up with great precision.

Amphibians

Frogs, toads, and particularly newts, all possess the sense of
smell, though in all of them sight is the principal sense used
in catching food. As already mentioned in the previous
chapter, there is some reason to think that toads may be
able to scent the ponds to which they go to spawn. Frogs
are less likely to behave in this way since they do not often
travel overland. They ^depend more on ditches and small
streams which lead to their breeding places ; in many cases
the frogs may breed in the ponds where they have spent the
winter in hibernation amongst the mud and debris at the
bottom. All the same, we cannot claim to know just how
frogs do find their way along ditches and streams, and it
may well be that smell also plays some part in this migratory
behaviour.

Our Natterjack Toad gives off a scent from its skin when
irritated and this undoubtedly is a means of protection. I
have never known a grass snake attempt to swallow a natter-
jack, though the tongue will be used to "test" this little
toad. The odour given off by natterjacks is said to resemble

SMELL — FIELDWORK AND EXPERIMENTS IO3

that of burnt rubber, and I think this is as good a simile as
any.

Our common toad is reluctant to eat the yellow- and red-
banded worms known commonly as "brandhngs". These
live chiefly in manure heaps and have a smell which is easily
detected by our own noses. Curiously enough, fish of several
species do not hesitate to take these worms, and this is a
good example of how a smell may be offensive to one kind
of animal and attractive to another.

Newts can smell well, and if a tank containing them is
placed in darkness, they will find and eat worms without
difficulty. This is an easy experiment to carry out, and it
may be done with newts in water or when they are in their
terrestrial state.

Birds

Most books on the habits of birds are lacking in references
to their degrees or variations in smelling ability ; and this is
probably because they are generally considered to be defi-
cient in this sense except for the species already referred to.
I know that my co-author considers that some birds — notably
petrels and albatrosses — can smell as well as we can ; but
that is not necessarily a great achievement when we realize
how feeble are our own noses, both in delicacy and dis-
crimination.

I am sure that a series of experiments designed to test
various birds in respect of their powers of scenting would be
well worth while.

Mammals

It is when we come to the subject of testing and observing
the uses of scent in mammals that we are up against a very
complex aspect of the study of senses. As mammals all have
their own natural scent specific to their particular kind, and
as in addition some have scent glands which are used for
marking out territory and to aid in mating, we have to be
careful, when discussing mammalian scent, that we are
certain what kind of scent we are referring to.

Probably the best way to observe mammals following

104 THE SENSES OF ANIMALS

scent trails left by other mammals is to go out and watch —
and if possible follow — a good pack of hounds when they
are hunting. The hounds will show you how strong scent is
on a particular day ; and at the start of a hunt when hounds
are seeking it is interesting to watch how they spread out
in order that their chances of picking up the desired scent
may be increased. Once scent has been found and the pack
follow, the voices of the hounds will tell an observer how
good or bad scent is. When in full cry it is obvious that
scent is good, and when hounds "speak" less or stop speak-
ing altogether, it will mean that scent has been lost or has
much decreased, or that some other scent has crossed the
line or otherwise confused hounds. It is then that the wonder-
ful discriminatory powers of hounds will be demonstrated.
Unless conditions have altered completely, good hounds —
often individual ones in the pack — will cast around and, if
the original scent is detectable, will start off again on the
line.

Whether or not you approve of hunting, much will be
learned from watching the way in which these members of
the great family of dogs set about using their finely developed
powers of smell. It must always be borne in mind that
hounds will have to learn to discriminate between what is,
to them, a generally attractive and stimulating scent given
off, or left by mammals other than their proper quarry, and
that of a deer, fox or hare. When puppies, hounds have to
be warned off such scents otherwise they will, later on, dash
after rabbits or some other unsuitable creature; and this
will lead to greater difficulties when they are put to more
serious work. Admittedly hound puppies will often chase by
sight animals which they may disturb when being exercised
and this, too, must be curbed ; but even so, they can also
get "fixed" on the wrong kind of scent, which can be a great
nuisance to all concerned. Once they have learned that
nothing other than their true quarry is to be hunted, they
will store its scent in their smelling memory and should
thereafter stick to it and be able to ignore other scents —
another example of the discrimination which is such a
marked and wonderful feature of the smelling behaviour in
dogs.

SMELL — FIELDWORK AND EXPERIMENTS IO5

Since we are dealing with dogs, it is as well to continue
with them here before going on to the uses of scent and
smell in other mammals.

Sporting dogs — retrievers, spaniels, terriers, pointers and
setters — all have, or should have, keen noses which are used
in various ways in field sports. Retrievers, as their name
implies, are used for finding and bringing to hand shot game
and, more important still, to trace and bring back a bird
which may have been wounded and incapable of flight.
Spaniels are double-purpose dogs and have to flush (put up)
game and then, having done so, remain steady until the
bird or hare has been killed, when it must locate and retrieve
it successfully. Pointers and setters are both used for the
same purpose, which is to work up wind and "point" when
a bird has been scented. To do this, they must not only
have fine noses but they, too, when in training, must be
broken from hares and rabbits and so ignore them and their
scent. Terriers are used for a variety of purposes, but hunt
terriers, which are put into the earths of foxes with the
object of bolting a fox which has gone to ground, must be
good at scenting, since foxes will, if they can get down the
holes and tunnels, go to ground in rabbit warrens or even
in drains, and some of the latter may have branching pipes.
In such cases, the nose of the dog must be good, and it must
be able to tell the terrier as quickly as possible just where
the fox is lying up.

We cannot leave sporting dogs without referring to the
subject of game birds which are sitting on eggs and which
are often said to be able to "shut off" their scent, so to speak,
in order to protect their eggs. Most country people who own
a gun-dog will have had the experience of being out with
their dog during the nesting season and of seeing their dog
walk right by a sitting pheasant or partridge of which the
dog takes no notice.

This is unlikely to be because the bird has some method
of retaining scent, it is much more likely that her body scent
is there all right, but sitting close and still as she does when
incubating, her scent is not diffused by any movements of
body or plumage.

Much more could be written about sporting dogs and

I06 THE SENSES OF ANIMALS

their highly developed powers of smell, but it will be suffi-
cient to say that if the student can get permission to go out
with a good gamekeeper or as a favoured spectator at a
shoot, he will learn a great deal about the way in which
gun-dogs use their noses.

There is often much confusion over the question of grey-
hounds. The fact that they hunt by sight does not mean that
they are without any scenting powers ; they are well equipped
with long noses and nasal passages, but their eyesight is very
keen compared with other dogs which are not bred for
hunting by sight and this is the sense on which they rely for
their success.

Poachers' dogs are often "lurchers" — a word used to de-
scribed a greyhound cross — often a cross between a greyhound
and a retriever of some kind. These lurchers are genuine all-
round dogs which combine the speed of the greyhound with
the smelling powers inherited from the retriever side. Such
dogs can find game, follow it, and retrieve it too. To make
friends with a real poacher is to learn more about animals
and their ways than most keepers can teach you. An old-
fashioned poacher is a field naturalist of a high order and
must not be compared with modern poaching gangs abroad
at times with machine-guns when poaching deer. These
scoundrels are merely ruffians who oflTend against the laws
of man and the animal they steal, and no genuine poacher
would think of associating with such riff-raflf who should, by
rights, be punished with the maximum severity that our
game laws allow.

Before going on to consider the other powers of scent used
by mammals, it must be remembered that all our predatory,
species can follow a trail or find their victims by means of
their noses. Badgers will find rabbits below ground, not >
only by hearing the squeaking of the young in a breeding
chamber, but by scenting them too. Foxes will track down
poultry and game by the same means, and stoats and weasels,
though using their keen sight as well, are capable of follow-
ing scent accurately and over quite long distances.

In smaller mammals such as voles, mice, and squirrels,
smell is used to distinguish between one kind of food and
another and to tell whether a nut, for instance, is sound or

SMELL — FIELDWORK AND EXPERIMENTS IO7

rotten. This can be shown by keeping a tame dormouse or
squirrel and offering it nuts, some of which must be known
to have rotted kernels. The rodent will sniff the nut and
will invariably discard one which has gone bad. I have
never known any of my pet squirrels or dormice to bother
nibbling a nut unless it was a sound one.

Hedgehogs are very discriminating in their sense of smell
and will ignore ground beetles which have an unpleasant
odour, while eating greedily those beetles and other insects
which have not. Carrion, too, will be scented from afar by
hedgehogs as will a broken bird's-egg or the milk from the
leaking udder of a cow. Incidentally, hedgehogs do not suck
the cows' teats, they simply detect the smell of milk that
may at times be dribbUng out on to the ground in a meadow.

I have already made reference to the scents from glands
in their own bodies that are used to convey "messages" and
mark territories by many mammals. Deer are among these,
and in this country the Roe Deer will, in the rutting season,
give off scent from glands in its forehead. At the same time,
the buck will rub itself against herbage and thrash to and
fro with its antlers, thus dispersing the scent.

Many other mammals "set" scent for marking out their
own territories — badgers, mongooses, lemurs, all do this;
and, of course, our domestic dogs leave trails by urination
and depositing faeces. These are nearly always attended to
by other dogs which, in turn, leave their marks in similar
fashion.

In captivity, the marking out of territory can be easily
observed by anyone who keeps a bush-baby as a pet. As
soon as it is put into a new cage it will mark the limits of
the cage by urinating in the corners of it. This is not only
a territorial affair, it is also connected with breeding, for I
have noticed that breeding seldom takes place unless a cage
has been well and truly "marked".

The scents given off by female mammals when in season
are naturally a very necessary preliminary to mating. Such
scent is powerful and persistent; and any dog-owner will
know full well the distances from which amorous male dogs
will come in order to seek out a bitch on heat. This will prove
a great nuisance to owners of dogs and bitches alike, unless

I08 THE SENSES OF ANIMALS

the owners of bitches use the excellent products available to
avoid the dispersal of scent. Why more people don't take
advantage of anti-smell tablets and anointing liquids, passes
my comprehension. If used regularly and as directed, these
are very efficient, and not only save dog-owners from end-
less trouble, but also prevent the risks of male dogs being
run over on our roads when, ignoring all else, they go out
— often in company — to follow what is probably the most
powerfully attractive scent of all.

VIII

TASTE

Like smell, taste is a chemical sense, and these two senses
are closely related and seem in many cases to overlap. In
man, it may appear almost impossible to separate tasting
from smelling; for as every young child knows, one good
way of dealing with a nasty medicine is to pinch the nose
and keep pinching it until the unpleasant dose has been
swallowed — in other words : if we suppress our smelling
powers, we deaden those of taste. This simple way of avoid-
ing being nauseated by medicines is satisfactory as far as it
goes ; but while pinching the nose will prevent the liquid or
powder from affecting us as we swallow them, as soon as
we release the pressure, we take in breath and have an
instinctive desire to touch the palate and the roof of the
mouth with the tongue. At once, we get the taste of the
substance we have swallowed. It is true that "after-taste" is
not so nasty as it would have been without the nose-pinching,
but this does show in a marked way the close connection
between smell and taste.

I well remember Dr. Harrison Matthews, during a broad-
cast we were doing together, pointing out that as human
beings could only detect four basic "tastes" with their
tongues, all other tastes were flavours. The types of things
we can taste are salt, sweet, bitter and sour; and on our
tongues are what are known as taste-bulbs which are situated
in different areas of the tongue and are there to deal with
these basic tastes.

In other animals very different parts of their bodies con-
tain their tasting organs. In some insects — flies for instance —
there are special hairs that serve as tongues and enable the
insects in question to tell whether a substance is palatable
or not. Other insects use their antennae as a kind of com-
bined smelling and tasting organ.

109

no THE SENSES OF ANIMALS

Earlier on, I made reference to the strong possibility that
frogs, toads and newts find their way to breeding ponds by
means of smell ; and in water-living animals this naturally
brings up the question as to whether such creatures can taste
in water. There is evidence that they can do so ; but it must
not be forgotten that in fish and terrapins and many other
aquatic animals, the sense of smell also plays a strong part
in tasting — the water itself may well taste, and this could be
one reason why some ponds are well populated with animal
life and some are not.

Scent particles are carried well by water, and substances
with a strong taste can, of course, be detected when dissolved
in water. However, the difficulties of drawing any clear line
between tasting and smelling are great and very hard to put
into words.

The probability that only a few types of birds have a good
sense of smell has already been referred to, but it is quite
certain that birds can taste and have some powers of dis-
crimination in this respect. We can, therefore, say it would
seem that at least one group of animals can taste well even
though smell is not a sense which is present in all families
within that group. Does the converse apply in other groups?
Are there any animals among those of high degree that can
smell but not taste? It seems unlikely; but much work is
being carried out on this subject and, like many other puzzles
in the world of living things, the solution to this problem
may be fully explained before long.

Insects discriminate between one kind of taste and another
— salt from sweet for instance. Some caterpillars will feed on
a number of different food plants, while others will only eat
one particular plant. It seems reasonable to assume that
taste — in addition to other factors — may play a part in this
marked selectivity. (See Plate 8.)

Fish can taste, and newts can taste ; tortoises and terrapins
can taste, though it is worth while noting that these creatures
appear to test certain food items before taking them into
their mouths ; this testing is carried out by smell, and if the
smell of something is repugnant, the substance will not be
eaten.

Crabs and other similar animals which are feeders on

TASTE III

small particles will reject certain of them ; snails and slugs
show preferences and some are very conservative indeed in
their choice of food.

The mammals — most of which have a delicate sense of
smell — also demonstrate the ability to taste ; but in monkeys
and apes which do not rely so much on their noses as, say,
wolves or foxes, it is still true that, given an unfamiliar kind
of food, this will be held near their noses before being put
into their mouths. This shows that even if their sense of
smell is not so finely developed as it is in other animals, their
noses are used first of all in deciding whether some food is
palatable or not. Wherever we go in our endeavours to
separate smell from taste we find that some overlapping
exists. Thus the chemical senses are the most complex and
difficult to define and understand.

It is, however, important not to succumb to the tempta-
tion to interpret what a mammal may like in the way of
tastes and flavours by our own similar senses. Many mam-
mals — carnivores in particular — are attracted by smells
which to us seem revolting, and they will eat carrion with
every sign of relish. This does not mean that they have any
less tasting power, or degree of discrimination.

Mention has been made of shrews and their musty odour
and unpleasant taste, and there are few mammals that will
normally eat a shrew. Watch your cat when it has been out
hunting and brings you home the trophies of its chase.
Shrews will often be caught, but I have never known a case
of one being eaten by a cat. Dogs, of course, are less finicky
than cats, yet they will also leave shrews uneaten, though they
will, if hungry, eat mice and voles. Foxes will do the same.

A very interesting aspect of this taste/smell behaviour is
the way in which many carnivores deal with pregnant female
prey. I have noted this many a time with my dogs, cats,
tame foxes, and mongooses. All these creatures eat mice,
voles, rats and rabbits; and on occasions they will catch
pregnant females. A dog, if the size of a victim will allow,
will usually give a nip, a crunch and swallow the animal
whole and so will foxes, unless a vixen is breaking up food
for newly weaned young. Mongooses tear up their prey
before swallowing it.

112 THE SENSES OF ANIMALS

Now if the creature captured is a pregnant female, the
unborn young will either be expelled during attack or they
will be exposed in tearing at the body. I have never yet seen
any of the mammals I have named eat the foetuses. I have
seen a terrier of mine swallow some unborn mice in its excite-
ment, but these have immediately been vomited up again.

Incidentally, while on this interesting if gruesome subject,
it is well worth mentioning that mongooses (and I have no
doubt other small carnivores as well) will eat every scrap of
a mouse or rat so long as it is not pregnant — with the
exception of the gall bladder. The way in which a mongoose
can seize, and dismember the body of a rodent and yet leave
the gall bladder is a matter for wonder. A great friend of
mine, who was a surgeon of high repute and also a first-class
naturalist, the late F. J. F. Barrington, was amazed at this.
More than once he saw my mongoose perform this delicate
piece of dissection, and in his characteristic way said : "It
can do that job a damned sight better than I can !"

Whether this avoidance of the gall bladder is evidence of
actual taste — cum-smell — or whether it is an instinctive
characteristic handed down through thousands of genera-
tions from some remote ancestors which had learned not to
puncture this bitter-tasting organ, I cannot say.

The avoidance of what would seem to be unpalatable
items is also shown when rats, stoats and others plunder
breeding frogs and toads when they are engaged in spawning.
The unfortunate amphibians, peacefully engaged in their
amorous behaviour, are dragged ashore from the margins of
ponds and streams and are opened up. The legs only seem
to be eaten, and males are more popular than females. The
really interesting point is that the unshed spawn of the
females is left severely alone. Any student who may have the
luck — and it will be luck — to discover a place where rats
and stoats have been at work can see this neat discrimination
for himself. I admit that it is difficult to say whether taste
or smell is responsible for this choosey behaviour, but it is a
most interesting matter none the less. The bodies of the
hapless frogs and toads will be more or less scattered at
random if the predator is a rat ; while if a stoat has been at
work, the bodies are usually left in a tidy pile on the bank.

TASTE 113

In monkeys, tasting can frequently be seen when some
unfamiliar piece of food is offered. The monkey will first
sniff at the food, holding it close to its nose ; and it will then,
very cautiously, lick it with its tongue. If the food is not
attractive, it will be thrown away in seeming disgust. I think
that this behaviour is interesting because though monkeys
— and especially apes — are at the top in the animal scale,
there would appear to be signs that scent only is not relied
upon.

Notwithstanding the overlapping of the chemical senses of
smell and taste, these few examples of what can be observed
in respect of taste may, perhaps, stimulate some student to
think up and carry out further tests. I do not think that any
practical purpose would be served by writing an additional
chapter on field observations and experiments as I have done
in dealing with the other senses. The examples given here
should be sufficient to show how taste is used by different
groups of animals.

IX

TOUCH— GENERAL

There is a jocular saying that no beer is bad, but some is
better than others ! Similarly, it might be said that all senses
are complex but some are more complex than others. Each
sense has its own kind of complications and that of touch is
no exception.

We are apt to think of touch solely in connection with
our fingers, hands, toes and feet, but of course it goes much
further than that. Touch and feeling go hand in hand ; and
certain sensations which we and other animals have in
common are related to the tactile part of our nervous system.
Animals can "feel" pain according to their degree of develop-
ment ; they are responsive to heat and cold ; and these sensa-
tions are in addition to the sense of touch as usually under-
stood.

Humans can feel the shape of things with their fingers
and can manipulate machines and tools very largely by
means of the tactile sense. Other widely diverse animals can
also use a direct sense of touch, and the apes and monkeys
probably use this sense in a similar way to ourselves. An
elephant's trunk, in addition to being a very enlarged nasal
organ, is extremely sensitive at the tip, and the "lips" at
the trunk's extremity are capable of great delicacy of
touch. (See Plate 9.)

The tips of the bills of certain birds — snipe, woodcock,
curlew, ducks and so on, have an area which is covered with
what look, when magnified, like tiny pits, and this area is
served by nerves which give the birds in question a sense of
touch. This is so fine that the birds can locate and pick up
small invertebrates which live below the surface of marshy
ground and which are, of course, out of sight.

The tongues of the parrot family are thick and fleshy, and
are sufficiently tactile for the bird to shell a nut or seed held

TOUCH — GENERAL II5

in the beak; this is turned round and shifted about by
means of the sense of touch centred in the tongue. So dehcate
is this sense that the huge massive beaks of macaws are able
to deal with very small seeds — even canary seed — from
which they can remove the husk. This is an operation that
would defy the comparatively clumsy hands of a human
being.

Woodpeckers, too, with their long tubular tongues, barbed
at the tip, can "feel" the grubs moving in the tunnels which
they have made in trees. Once located in this way, the flexi-
bility and the barbs on the tongue enable the woodpeckers
to extract the grubs safely and accurately.

Those puzzling bristles at the sides of the mouths of night-
jars are thought, by some workers, to be organs of touch
even though they may also serve to catch up moth scales
from victims caught on the wing. The nightjar also has a
claw-comb which it uses to keep these bristles clean after a
meal. Such combs are found on other birds, and although
they are modified claws, they are certainly capable of some
degree of sensitiveness to touch. The whiskers with which
many mammals are provided are also very sensitive tactile
features.

As far as sensations are concerned, we come across these
in many cold-blooded animals. The fact that these types of
creature are dependent on the temperature of the surround-
ings for activity, shows that they must have specialized nerves
which can convey impressions of warmth or cold all over the
body.

The "pit vipers", represented so well by the rattlesnakes,
are so called on account of the pits (which look like holes)
on each side of the head and situated between the eyes and
the nostrils. These are now thought to be sense organs which
can register temperature, and are used as additional detectors
of warm-blooded prey.

Fish, as has already been mentioned, are very sensitive to
vibrations in the water, and the delicate nervous system
connected with the lateral line is considered to be the means
by which fishes can feel currents and other vibrations —
another example of tactile sense.

In the insects, certain hairs on the bodies and limbs are

Il6 THE SENSES OF ANIMALS

organs of touch ; while the antennae, in addition to their
chemical-sense functions, are also organs of touch.

It will be appreciated from this outline that the sense of
touch in animals is no less interesting than the other senses
which have already been dealt with.

X

TOUCH— FIELDWORK AND EXPERIMENTS

Just as there are close affinities between the senses of smell
and taste, there are similar affinities in respect of touch and
hearing. Sound waves can be felt as well as heard, and in
observing animals in order to find out whether, and to what
extent, they respond to touch, this must be taken into con-
sideration. For instance, fishes and snakes cannot "hear" in
the normal way in which we use that word, but both are
most responsive to vibrations in water or on land, and in
carrying out any experiments aimed at demonstrating the
sense of touch (or feeling), care must be taken not to draw
conclusions which are not accurate.

Many of the lower organisms — even unicellular ones — are
sensitive to touch; and this can be shown by arranging
under a microscope a slide on which a drop of water has
been placed containing a colony of the organism vorticella,
which has been referred to in a previous chapter. It will be
remembered that vorticellae respond to a tap on the slide
containing some of these primitive animals and, as stated
before, the vibrations set in motion will cause the organisms
to retract their stalks. Now, if it is desired to produce evidence
of response to a direct touch, it will be necessary to set up
your colony in a "trough" rather than on a simple slide. It
will also be necessary to have at hand either a very fine
piece of platinum wire, or the finest of bristles with which
you can provide yourself. Then you must wait until the
vorticellae are in a state of maximum stretch and leave them
undisturbed for a few seconds while you pick up your wire
or bristle. Moving with the greatest care, gently insert the
wire into the trough and very slowly move the wire until it is
brought into direct contact with one of the vorticella. As
soon as this happens, the individual touched will retract
and this reaction will be communicated to others in the

colony which will do likewise.

117

Il8 THESENSESOFANIMALS

The same behaviour can be demonstrated with the larger,
multicellular Hydra; the touch should be directed to the
tentacles which will come together and turn inwards to-
wards the "mouth", retracting slightly as they do so. This
is what happens in nature when a water-flea comes within
range of the hydra's tentacles and minute poisoned darts.
The tentacles seize the prey and convey it to the mouth.

Spiders are singularly responsive to touch both on their
bodies or by means of a light touch on their webs — even
the vibrations caused by holding a struck tuning-fork near
the anchor strands of the webs will produce a reaction on the
part of the spider.

Among the insects, a very fine demonstration of the extent
to which touch is used may be seen by the behaviour of
those species of ichneumon flies which have long antennae
and long ovipositors. The females, having mated, seek out
a tree trunk in which the larvae of other insects dwell at
the ends of the tunnels they have bored out in the wood.
The object of these particular ichneumons is to locate a
living larva and then insert their long ovipositors in order to
lay an egg from which their own larva will, in due course,
emerge to feed on the body juices of the larva so parasitized.

I well remember watching this whole process on one occa-
sion in company with the late L. C. Bushby, one-time curator
of insects at the London Zoo. We were on a "bug-hunting"
expedition at Frensham Pond in Surrey when we saw a
female ichneumon on a rotten wood post. It was clearly
searching for an occupied tunnel among the dozens to be
seen in the post.

First of all the ichneumon used its antennae with great
care, touching every now and then the outer edges of the
entrances of the tunnels. After some minutes, the ichneumon
paused, paid extra attention to one particular hole and then,
moving forward a trifle, it arched its body and bent its
abdomen forward so as to bring its ovipositor into place;
after which this long egg-laying tube was thrust into the
tunnel and an egg laid. It was a wonderful sight and the
whole operation took about seven minutes — it was a fine
example of the employment of the sense of touch in which
the antennae and the ovipositor each played its tactile part.

TOUCH — FIELDWORK AND EXPERIMENTS II9

Earthworms, slugs, snails, starfish, sea anemones are also
responsive to touch, not only on the most sensitive parts of
their bodies, but on their bodies as a whole. Frogs, toads
and newts, too, are sensitive to touch over the whole body
surface.

The response to touch in amphibians and reptiles is easily
tested in captivity by using a long stem of grass and touching
the bodies in various places. Even the scaly bodies of lizards
and snakes are sensitive enough to show that a light touch
is felt and reacted to by escape movement. Tortoises and
terrapins are well known to be sensitive to touch; in fact
one golden rule when buying one of these creatures for a
pet is to test the response to touch in order to find out if
the specimen being tested is in good health — a quick response
shows good condition and lack of it means ill health.

I have mentioned the delicate sense of touch located in
the tips of the bills of certain birds which feed mostly on
soft-bodied creatures hidden from sight on damp or marshy
ground. It is not easy to test this ; but observation in the
field, aided by a good pair of binoculars, will show clearly
that sight plays no part in these feeding activities below the
surface of the earth ; for as the bills of these birds probe here
and there, and every now and then emerge with an item of
food, it will be clear that the invertebrate prey are located
by feeling their movements and also that a very sensitive
tactile sense is employed in aiding in their capture once they
have been found.

Anyone who possesses a parrot or other seed-eating bird
can watch for himself and see the way in which seeds are
shelled ; and it can only be a sense of touch which enables
such a delicate operation to be performed. I have hand-
reared woodpeckers and have noticed with what care these
birds explore crannies and holes, using their long tongues
to tell them whether an insect or seed is hidden from sight.
In fact, it is my opinion that any unfamiliar item of food is
tested by touch; and even inanimate objects are touched
with the tongue as though in curiosity, though it is more
likely that the real motive is to ascertain by touch (and
possibly by taste) whether the object is edible or not.

In the mammals, a sense of touch manifests itself in many

120 THE SENSES OF ANIMALS

ways. Moles have flexible noses, and even though they are
endowed with a good sense of smell, both moles and shrews
use their snouts as a fifth limb and when searching for food
will push aside stones or other small movable obstacles ; this
they could not do if their elongated noses possessed no tactile
sense.

The same applies to some extent to badgers and hedge-
hogs. Badgers do a deal of their searching for grubs and
worms by using their noses to push aside soft earth or to
supplement the digging done with their claws. Hedgehogs
will lift up loose stones with their snouts to get at the insects
or worms lying underneath ; and they are continually grub-
bing about with their snouts as they seek for food. This
shows that their snouts are not only strong, but that they
have nerves which convey information by means of the sense
of touch.

Cats and many other mammals have stiff and extensive
whiskers, and these are not there for adornment. They are
instruments of touch; and the old saying that if a fully
whiskered cat can get its head through a hole it can get its
body through, is true. This would not be so if the whiskers
were not sensitive to touch to a very high degree.

It is well known that racoons have the curious habit of
washing in water (or at least going through the motions of
washing) pieces of food. The exact significance of this is not
fully agreed ; but one thing is certain : a sense of touch comes
into this behaviour, since the object being washed is held
firmly.

The hands of monkeys and apes and, to a lesser extent the
feet as well, are much used : in climbing ; in picking off nuts
and fruits and leaves ; in manipulating these when plucked ;
and, in some anthropoid apes, in making sleeping and rest-
ing platforms in trees. These are constructed with branches
and have to be arranged.

Apes, such as chimpanzees, orang-outangs, and gorillas,
can use their hands when in captivity for purposes which
they would not encounter in the wild. Chimpanzees in par-
ticular have been recorded many times as users, and even
makers, of tools. They have learned to pull back bolts and
other fastenings of their cages, and there are reliable reports

i«/

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■:?#;•'

1 . In this remarkable picture of a dragonfly just emerged from its nymphal case, the huge
compound eyes are easily seen (Page 33)

2. Here we see (right) a south American giant toad just about to finish swallowing an earth-
worm while the female (left) is attracted visually by the movements of another worm just
out of the picture. The male is also using its eyeballs to help push the worm down its
throat. (Page 24)

3- This close-up of the head of a Tawny Owl — taken in sunlight — shows the third eyelid in
use over the eye nearest the sun while the other eye gives an idea of the large size of the
eyeball necessary for nocturnal vision. (Page 60)

4. The tympanum, or external ear-drum, can be plainly seen in this study of a Marsh
Frog. (Page 77)

5- These fox-cubs, ears pricked, are listening to the sound of one of the parents returning to
the earth with food. (Page 86)

6. Close-up of male Emperor Moth showing the large feathery antennae. Each tip has a
nerve ending capable of detecting scent molecules given off by the females — often from a
distance of a mile. (Page 17)

X

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s^^m^

7. A magnificent example of a Grass Snake's tongue — its organ of smell. (Page 91)

8. A combination of taste and scent. This Convolvulus Hawk Moth has been attracted to
the tobacco-flower by scent : it then protrudes its long proboscis (feeding-tube) and draws
in the sweet nectar. (Page 1 10)

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g. The Heshy tongue of the frog touches the worm and thus sets ofl the swallowing action.
(Page 114)

lo. A Ringed Plover beside its nest on a beach. The eggs are camouflaged so that it is very
difficult to find them. (Page 135)

1 1. Quin scallops swimming away fiom a starfish that has disturbed them. Their numerous
eyes can be seen as dark dots along the edge of the mantle just inside the shell. (Page 160)

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12. A Mouse-cared Bat in flight emitting sonar pulses through the open mouth and receiving
the echoes with the ears. (Page 185)

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13. Close view of the face of a Greater Horse-shoe Bat showing the nose-leaf between the
eyes and surrounding the nostrils. In this species the sonar pulses are emitted through the
nose, and the leaf helps direct them as a beam. (Page 186)

14. A tame dolphin towing a girl on a suil-boaid. (Page if

15. A whale tragedy : a male Pilot whale gently lifts the dead body of a female to the surface
in an effort to get her to breathe again. (Page 190)

16. A mother Grey Seal coming out of the sea to suckle her pup, and identifying it by
sniffing at it. (Page 198)

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17. x\. llashlight photograph of the rock wall in the Niah Cave, Sarawak, showing Cave
Swiftlets on their nests. The birds find their way about in the complete darkness of the
caves by echo-location. (Page 193)

18. Cave Swiftlet nests in the Niah Cave. The nests are made from the birds' saliva, and are
the raw material of birds'-nest soup. The nest in the centre contains a young Swiftlet,
that on the left an egg, and an adult perches on that on the right. (Page 193)

ig. A Western Diamond-back Rattlesnake, one of the Pit- Vipers, showing the sensory pit in
front of and below the eye. The pit contains receptor organs sensitive to the infra-red
radiation given off by the bodies of the small mammals on which it preys. (Page 216)

20. The Manx Shearwater that homed over the Atlantic from America to Wales in 12 days,
12 hours, and 31 minutes, across at least 3,200 miles of ocean. It was released hundreds of
miles out of its normal range in a place it had never seen before. (Page 226)

TOUCH — FIELDWORK AND EXPERIMENTS 121

of their using a piece of wire to pick locks. Several chimps
at the London Zoo (and, I have no doubt, at other Zoos)
have learned to select the right key from a bunch with
which to open the padlock on the doors of their den ; and
though all these instances are proofs of their high intelligence,
the manipulative skill required is evidence of a very delicate
sense of touch as well. Touch also enters into one of the
regular tests of intelligence which are presented to chimpan-
zees by research workers. I refer to the test where a banana,
or other favourite food, is placed well out of reach, and the
chimp being tested is given two or even three sticks which
fit into one another so as to form a longer one, after the
fashion of a fishing-rod. These, when successfully fitted to-
gether, will enable the ape to knock down the desired fruit,
Here, again, some considerable degree of skill is employed,
depending on a high degree of sensitivity to touch and an
appreciation of shape by means of feeling.

I should like to finish my portion of this book by recalling
an exhibition of delicacy of touch by an animal that, from
its size and apparent clumsiness, would not, perhaps, be
thought to have such tactile ability.

Not so very many years ago, there was an elephant at the
London Zoo which performed what its keeper called its
"conjuring trick". This little show was given every now and
then for the amusement and instruction of both children and
adult visitors.

The trick was done with one of the old silver threepenny
pieces. After having allowed some of the audience to feed
this elephant with hard biscuit — about twice the size of a
big dog-biscuit — the keeper would make the elephant take
a piece of food and then give it back again. He would then
tell those watching that this great beast would perform a
conjuring trick. A silver threepenny piece was then produced
by the keeper who offered it to the elephant. The elephant
would take this in the lips of its trunk — no mean exhibition
of dexterity in itself; having done this, the threepenny piece
would appear to be put into the huge mouth and on the
word of command, "Give it back", the coin would be handed
to the keeper again. This was only a preliminary to the trick
itself. The coin would again be offered to the elephant,

122 THE SENSES OF ANIMALS

apparently as before; the trunk would be bent and placed
in the mouth but when this had seemingly been done, the
trunk would be stretched out empty. The keeper, playing
up to his audience, would say, "Where is it?" Eventually,
after some of this by-play, the threepenny piece would be
shown to have been held by the elephant between the lower
lip of the trunk and the trunk itself. Apart from the funny
side of the "trick", the fact that this great animal, with its
powerful and enormous trunk, was capable oi feeling our
smallest coin, and of holding it in the way described, is surely
an amazing example of the sense of touch.

PART II

HOW DO THE SENSES WORK?
by L. Harrison Matthews

XI

SENSES AND NERVES

All animals have a skin of some sort that keeps the things
outside their bodies separate from the things inside — that
divides the external environment from the internal environ-
ment. The sense organs and nervous system keep the two in
touch so that the animal can respond to changes in the
external environment, a response that is one of the chief
signs of being alive, for if an animal no longer responds to
such changes it is generally dead.

Every animal has to keep alive by finding and eating food,
and by avoiding being eaten by other animals; secondly it
must reproduce its kind so that further generations shall take
its place when it dies, for with few exceptions no animals are
potentially immortal. The sense organs play an essential
part in co-ordinating these fundamental life processes
successfully.

There are some extremely small animals which, although
they have no visible sense organs, find their food, breed, and
sometimes avoid their enemies. Such animals consist of a
single speck of living jelly, similar to that which forms the
cells of all the other animals, including ourselves, in which
the body is built of millions of cells, each a minute unit
separated from its neighbours by a very thin membrane.

The largest giants among the animals that are not made
of a mass of cells stuck together (the Protozoa) are no larger
than a pin-head ; most of the others can be seen only by the
aid of a microscope. The Amoeba is a well-known example
of the Protozoa which can be found everywhere in stagnant
water, where it crawls about on the bottom or on the surface
of submerged objects. It has no definite shape and its outline
continually changes as it moves about and pushes out irregu-
lar lobes from different parts of its surface. The animal
crawls by pushing out a lobe ahead into which the rest of

126 THE SENSES OF ANIMALS

the body flows ; when it meets with a minute plant or frag-
ment of debris fit for food it pushes out two lobes to surround
the particle, and the lobes then flow together so that the
food is engulfed in the body where it is digested. Amoeba
does not blunder blindly about its environment; when it
touches an object it avoids it if it is inedible, but engulfs it
if it is suitable food ; it moves towards a smell or taste dif-
fused into the water by food, and away from the taint of
anything harmful. It moves from darkness into light but
retreats if the light is too bright, it moves from temperatures^
that are too hot or too cold, and it reacts to gravity. Thus
although Amoeba has no sense organs as we know them —
no eyes, ears, nose, or tongue — it reacts to stimuli from the
environment in a way similar to that of more complex
animals that have those organs. The ability to make these
responses must therefore be a property of the living matter,
the protoplasm, that makes up the body of Amoeba — and of
all other animals.

The reactions of Amoeba, however, do not imply that it
sees, tastes, or feels consciously as we do, any more than a
cell in one of our muscles knows that it has been stimulated
when it contracts in response to stimulation — we may or
may not know that the muscle is stimulated, but the cells
of which it consists certainly do not. The single blob of
protoplasm that makes up the Amoeba is able to react to
many different sorts of stimuli, but in the more complex
animals, that consist of numerous cells, more precise reactions
are possible where the cells specialize on different single
functions — each has its own job to do and is not concerned
with anything else. Even in the Protozoa there is an analogous
specialization.

Amoeba is structurally a simple type, but many of the other
Protozoa are much more elaborate and have a definite shape,
front and rear ends, and gullets for taking in food particles ;
they swim by the lashing of innumerable hair-like cilia all
over the surface of the body, or by means of a whip-like^a^^/-
lum at one end. The soft bodies of some are supported by
minute skeletons of silica, or by calcareous shells, which are
objects of great beauty under the microscope. There is, too, a
dark-coloured spot, usually deep red, in a definite position in

SENSES AND NERVES I27

the body of some Protozoa that swim by means of a flagellum ;
it is generally found in those kinds that possess one or more
granules containing chlorophyll. The chlorophyll enables
them to absorb the energy from sunlight for the manufacture
of food from carbon dioxide dissolved in the water, in the
same way that plants carry on their photosynthesis. The
substances in the spot of red colour are changed chemically
by the action of light, and as a result the animal swims to-
wards the light which will give the greatest amount of
energy for the food manufacturing process. The speck of
colour, or eye-spot, is the first example of a special sense
organelle found in the least complex of the animal kingdom.
It does not enable the animal to see, for it forms no image,
and even if it did the animal has no brain with which it
could interpret the image, but it is a part of the animal
specialized to react to light, a photoreceptor.

The more complex animals are usually larger, and differ
from the Protozoa in that their bodies do not consist of a
single unit of living matter. The protoplasm is divided into
thousands or millions of separate units, each in many ways
similar to an Amoeba, stuck together so as to build up the
body. Such animals with many-celled bodies are distin-
guished from the Protozoa by the name Metazoa,

In the Metazoa each cell, while still performing the basic
functions, is additionally specialized for one or more par-
ticular functions. The comparison with people living in a
village is obvious ; the butcher does not bake his own bread,
nor the baker make his own clothes, and each inhabitant
has his own special trade from which all benefit.

In the animal body, however, the cells depend upon each
other much more closely, and are merely components of the
whole, for they have no individual independent existence.
Cells of one kind are generally massed together to form
special tissues such as muscle, fat, glands, or nerves. Com-
binations of different tissues as functional units form the
various organs. In Metazoa, too, only the cells near the
surface can receive stimuli from the outside, and some of
them are specialized to form sense organs whose sole function
is to receive those stimuli.

The Sea-anemones and their relations, such as jelly-fish and

I2b THE SENSES OF ANIMALS

the little fresh-water i/y^ra, are among the least complex of the
Metazoa, and have few special sense organs. A sea-anemone
is built of a vast number of cells, yet the animal functions
as a whole, and the actions of the individual cells are co-
ordinated. The co-ordination is effected because some of the
cells are specialized as nerve cells, each of which is drawn out
into a number of threads called axons, which nearly touch the
threads from neighbouring nerve cells to that a nerve net-
work extends throughout the body. The ends of the threads
from different nerve cells do not actually join, but lie very
close to each other forming a synapse. Special sensory cells are
packed in between the cells forming the skin that lines both
the outer and inner surface of the body, and axons from the
nerve network lie almost in contact with their inner ends. The
inner ends of some of the cells of the outer skin can contract
when they are suitably stimulated and thus function as
muscular tissue; they too are in near-contact with the
threads of the nerve-net.

The combination of sensory cells connected by the nerve
network to the muscle cells enables the sea-anemone's in-
numerable cells to act in concert so that the animal behaves
as an individual. The cells are an orderly army, and not a
disorganized rabble, through the mediation of the nerve
network.

One of the well-known sea-anemones of the British coasts
is Taelia felina, often several inches in diameter, and
coloured with streaks and flecks of green, red, white, brown,
and grey. The stalk below the disc of tentacles is covered
with warty lumps which led the Victorian naturalist P. H.
Gosse (the father in Edmund Gosse's Father and Son) to give
it the English name of Dahlia Wartlet. He also remarked
that the colours "are very sportive, and scarcely two speci-
mens can be found alike". In i860 Gosse published the first
book on British sea-anemones, a classic work illustrated with
beautiful coloured plates made from his own water-colours
of the living animals. There was a widespread interest at
that time in the "wonders of the shore", mainly as a result
of Gosse's numerous books on natural history. His book on
anemones, although a scientific treatise, was also intended
to help the amateur naturalist to identify the anemones he

SENSES AND NERVES I29

came across during his rambles. Gosse therefore invented
an Enghsh name for each kind because until he popularized
them they had only scientific ones in Latin. He made the
names descriptive, some quite straightforward such as the
Rosy, Scarlet-fringed and Snake-locked anemones; but he
also produced a series of diminutives, and without a smile
dubbed these beautiful creatures the Warted Corklet, the
Eyelet, Opelet, Trumplet, Beadlet, the Glaucous Pimplet,
the Arrow Muzzlet, and others.

If you come across a Dahlia Wartlet in a rock pool on
the beach at low tide expanded in all its glory, with brightly
coloured tentacles swollen and translucent from the pressure
of the fluid inside, and you prod it with the end of a stick,
it is almost instantly transformed. The tentacles fold in over
the disc and disappear inside as the stalk contracts, so that
the animal-flower of a moment ago becomes a rounded
conical lump of jelly with a puckered hole at the summit.
The stimulus received by the sensory cells on the surface
has been sent through the conducting paths of the nerve net-
work to all the muscle cells in the body which have responded
by contracting, and pulling the animal into a shape and size
least vulnerable to danger. But even the diffuse nerve net-
work of the Wartlet does not trigger a random or simul-
taneous contraction of all the muscular tissue ; the folding up
of the anemone is an orderly process — first the tentacles
contract and fold inwards, then they are drawn down with
the disc while the stalk contracts. Although it all happens
very quickly the movements are co-ordinated so that the
body does not, for example, shut the tentacles out by con-
tracting before they have folded inwards.

Sea-anemones are sensitive to other stimuli besides the
simple one of touch. Like the Amoeba, they respond to
chemical stimuli, that is to substances dissolved in the water
and producing what we should call tastes or odours. They
also react to light ; Gosse tells us that "it is under the veil
of night that the anemones in general expand most readily
and fully. While the glare of day is upon them, they are
often chary of displaying their blossomed beauties ; but an
hour of darkness will often suffice to overcome the reluctance
of the coyest." They are, however, not equally sensitive to

130 THE SENSES OF ANIMALS

light of different colours, and some kinds are relatively in-
sensitive to light of longer wave-length at the red end of the
spectrum. At the aquarium in the London Zoo advantage
has been taken of this "blindness" to red light to trick
Plumose anemones into staying fully expanded all day. The
Plumose is a particularly beautiful anemone; it is a large
species, as much as six inches high, and the stalk is crowned
with a feathery fringe of innumerable tentacles which are
well likened to a head-dress of ostrich plumes. It is plentiful
round the British coasts, in deep water rather than the tidal
zone, and lives at depths where the intensity of daylight is
reduced by twenty fathoms or more of sea-water. Only
small and presumably young examples are found in the
region between the tides, and there they are almost always
confined to the gloom of a sea-cave or the water channels
under a huge boulder. In colour the Plumose anemone
ranges from olive-brown through pale orange, salmon-pink,
and cream, to pure white. Gosse calls it the noblest of our
native sea-anemones; "when we see a full grown specimen
of some of the more delicately coloured varieties, — the pale
orange, the flesh-coloured, or the clear white, — rising erect
from its broad base like the stem of a massive tree, crowned
with its expansive disc of myriad tentacles, we cannot but
consider it a most noble, as well as a most lovely object."
Gosse goes on to say that although the species is not very
shy of daylight, "if you would make sure of seeing it in all
the gorgeousness of its magnificent bloom, visit your tank
with a candle an hour or two after nightfall." He apparently
did not know that red light is as good as darkness to the
creature, and he would have been delighted to see the dis-
play that it makes in the aquarium when brightly lit with
many-candle-power of red light.

The anemone responds to changes in the intensity of light
because when light falls on certain of the sense cells in the
skin it produces a chemical change in them. As a result a
stimulus is sent into the nerve network, which causes the
muscular cells to contract or relax when it reaches them.
The nerve network is so important in regulating the responses
of the animal to the stimuli reaching it from outside that we
must look at it more closely. The network consists of cells

SENSES AND NERVES I3I

which are speciaHzed in their structure and function as
nerve cells. Broadly speaking, the function of nerve cells is
to transmit messages from one part of the body to another.
Like all cells, a nerve cell consists of a minute unit of proto-
plasm surrounded by a very thin membrane; in the proto-
plasm there lies a central body, the nucleus, which differs
chemically from the rest of the protoplasm and is in effect
in control of all its activities, for without it the cell cannot
live. But the nerve cell differs from other cells in having its
body drawn out into a number of threads, some long and
some short, radiating in different directions. The longer
threads are the nerve fibres which when gathered together
like the wires in a telephone cable form a nerve. In higher
animals there is a single long fibre which serves for carrying
messages to the nerve cell from a sense organ, or from the
nerve cell to a muscle or other executive structure. There
are generally several shorter fibres, and their function is to
make contact with the fibres of other nerve cells.

In the sea-anemone the nerve cells and their fibres make
a diffuse network, but in higher animals the network is infi-
nitely more complicated because the nerve cell-bodies are
concentrated in a central nervous system, such as the brain
and spinal cord in the animals with backbones, or the nerve
chain in the under part of the body in those without. The
long fibres are bunched together and run to all parts of the
body as nerves. The network, whether simple or complicated,
is not strictly like a man-made net, although it may be
roughly likened to one, the knots representing the nerve cells
and the joining strings the fibres. It differs fundamentally
from an artificial net because the threads radiating from
each nerve cell are not actually joined to the threads of the
other cells ; the ends of the threads lie very close to each
other but they are not in complete contact, though they are
often branched so that the twigs interdigitate like the fingers
of two interlocked hands.

The end of the long fibre of a sensory cell lies in similar
near-contact with an end-organ, the nature of which we shall
examine later on ; when it receives a stimulus from the end-
organ it sends a message back to the body of the cell. The
stimuluscauses a physico-chemical change — a re-arrangement

132 THE SENSES OF ANIMALS

of ions — at the end of the nerve fibre which causes a similar
change in the nearest part of the fibre ; that in turn causes a
change in the next bit, and so on, with the result that the
change runs along the fibre to the body of the cell, from which
it is sent along the shorter fibres. The physico-chemical
change is repeated at the ends of the short fibres and so is
passed over the gap to the fibres of other nerve cells.

Any chemical change is accompanied by electrical changes,
and consequently the passage of a message in a nerve can
conveniently be recorded in terms of the electrical change
that takes place; the potential of each part of the fibre
changes successively, and the "action potential" is easily

MV

100-

90-
80-
70-
60-
50-
40-
30-
20-
10-
0-

MICRO-SECONDS

Fig. I. Tracing recorded by an oscillograph of the "action potential" when the
impulse travelling along a nerve fibre passes a pair of closely spaced electrodes
in contact with the fibre. The potential first rises and then falls to a lower value
and returns gradually to its original level.

recorded with an oscillograph. The impulse or message
travelling along the nerve is not like one of those ingenious
boxes that brings you your change when it is blown along a
pipe by compressed air in a large shop; nothing discrete
travels along the nerve. The process is much more like the im-
pulse that travels along a goods train when the engine starts
and all the trucks one after the other are jerked forward
with a noisy bumping of buffers until, an appreciable time
after the engine has started, the guard's van at the end is
jerked into motion. Nothing tangible has passed along the

SENSES AND NERVES

133

train, but you can see and hear the effect ot tlie impulse
as it runs from one end to the other.

In the sea-anemone impulses received from the sensory
cells in the skin are passed to other parts of the body by the
nerve network in this manner, and the animal responds to
different stimuli although the sensory cells are not clumped
into end organs specially adapted to pick up specific stimuli.
In some of the sea-anemone's near relations such as the
jelly-fish, however, there is greater specialization. Amelia is
a small jelly-fish less than a foot across that is common
round British coasts in the summer; it is practically trans-
parent and apart from four purplish horseshoe-shaped roes
is not adorned with the brilliant colours that make some of
the larger jelly-fish so beautiful. Eight equally spaced notches
round the edge of the disc or umbrella mark the position

Fig. 2. The jelly-fish Aurelia. The edge of the umbrella bears numerous short
tentacles. The four horseshoe-shaped roes are seen through the semi-transparent
body and two of the four long "arms" surrounding the mouth project beyond
the edge. Some of the eight equally spaced notches round the edge of the um-
brella are seen ; they mark the position of the sense organs.

of the sense organs, which consist of eye-spots, other cells that
respond to smells or tastes, and small pockets of limey

134 THE SENSES OF ANIMALS

particles that act as balancing organs. Nerve cells are con-
centrated at the sense organs and convey impulses to the
rest of the nerve network. The sense organs are particularly
concerned with sending stimuli to the muscle cells that cause
the pulsations of the umbrella so that the jelly-fish swims
and avoids sinking in the water. In other kinds of jelly-fish
the nerve network is more highly modified than in Aurelia,
and in addition to the diffuse network some nerve cells are
concentrated into a definite nerve ring round the edge of the
umbrella. Jelly-fish such as these show a simple form
of the central nervous system characteristic of the higher
metazoa in which a concentration of nerve cells controls the
activities of the whole animal.

Whether an animal has a nervous system consisting of a
diffuse network or of a central nervous system connected
with the different parts of the body by bundles of nerve
fibres, the stimuli from outside are transmitted to the nervous
system in essentially the same way by the sense organs, simple
or specialized. The nervous impulse is basically a physico-
chemical change, and must be started by a similar change at
the place where the nerve leaves the sense organ. The sense
organs are thus devices for translating stimuli from outside
into physico-chemical changes that can act upon the nerve
ending and start the change in the nerve fibre that trans-
mits the message. The organs of touch, sight, hearing, smell
and taste all depend upon similar actions for starting the
transmission of their messages to the rest of the animal
body; all the complicated special sense organs such as eyes
and ears are adapted for translating received stimuli of
light, sound, and so on into the necessary physico-chemical
changes. The sense of touch seems to be the simplest of the
senses, for pressure on a sensory cell is sufficient, probably
by the distortion it produces, to cause a minute change
sufficient to stimulate the nerve ending in contact with it.

The stimuli received from the sense organs cause an
animal to react appropriately to changes in the environment
— to approach and take food, or to avoid an enemy, or to
move away from too great cold or heat and so on. Although
our senses are the same as some of those of other animals
we must not think that all animals are consciously aware

SENSES AND NERVES I35

of the Stimuli produced by their sense organs in the same
way that we are. The reaction to a stimulus may be almost
automatic, as when we jump if we sit on a drawing-pin — it
is only after reacting that we look to see what has hurt us.
On the other hand, when your dog spots you coming down
the street a hundred yards away he obviously sees you in
much the same way that you see him. As we heard in
Chapter I, frogs and similar creatures will not seize their
food unless it is moving ; yet frogs have eyes of complicated
structure that throw a perfectly good image of what they
are looking at on the screen at the back of the eye. Why does
the image of food not produce any reaction until it moves?
We must look for the reason not in the frog's eye, in the
sense organ, but in its brain where the messages from the
eye are sent. If the brain is not capable of building up and
interpreting a mental image of what the sense organ tells it,
no signal for action is sent on to the muscles. Compared
with the brain of a man the brain of a frog is much less
complex; in particular it lacks the enormous expansion in
front that makes up the largest part of the human brain
and gives the facility of building up complicated mental
images. So although a frog may look at a fly it does not see
it until there is movement, and the difference is not the fault
of the sense organ but of the brain to which it is reporting.
Similar things happen in the human brain in spite of its
enormous advantages over less complicated ones. The whole
art of camouflage depends upon getting the eye to send
messages to the brain that will build up a meaningless mental
image — to make the beholder look, but to prevent him
seeing. A similar process is very common in nature ; we all
know how difficult it is to find the nests of some ground-
nesting birds such as lapwings, ringed plover, or oyster-
catchers, even when the eggs are completely exposed — we
can look straight at them less than a yard away without
seeing them. The mental image we build up from the message
sent from the eyes means nothing — or at least it does not
mean "eggs" — although the image of the eggs is perfectly
clear at the back of the eye we are no better off' than the
frog looking at a stationary fly. It is strange that we, and
other animals, should have such nearly perfect sense organs,

136 THE SENSES OF ANIMALS

and yet that we should not have learnt to use them to the
limit of their capabilities. Thus it is the use that animals can
make of their sense organs by means of their brains that is
all important, just as a telescope is a completely useless set
of brass tubes and pieces of glass unless someone is looking
through it. (See Plate 10.)

The brains of men, dogs, frogs and other creatures all the
way down the scale of complexity to insects, worms, and even
jelly-fishes, receive messages from their sense organs, but the
responses to the messages depend not only on the sensitivity
of the sense organs but also on the use that the brain is able to
make of them. In ascending the scale from the simpler types
of sense organs and brains to the more complicated, the
awareness of the animal of the changes in its surroundings, as
signalled by the sense organs, seems to increase. It is probably
greatest in the warm-blooded animals and possibly reaches
its culmination in man, but the differences are those of
degree and not of kind.

XII

SIGHT

As we have seen, the simplest sense organs respond to touch ;
when they are pressed so that work is done upon them and
some of the energy from the stimulus passes into them, a
physico-chemical change takes place. Similarly the more com-
plex sense organs respond only when they receive energy
from outside. In the sense of sight the energy in the light does
the work ; the energy of the photons or the electro-magnetic
waves produces the change, just as it does in the silver salts
in the emulsion of a photographic film.

Thus it comes about that many animals, such as the earth-
worms that Maxwell Knight told us about in Chapter II,
are sensitive to light and darkness although they have no
eyes and cannot see. When light falls upon the light-sensitive
sense organs in the skin a change takes place which gives rise
to changes in the nerves. The associated nerve impulses are
transmitted to the muscles and cause the animals to pop
down their burrows into the dark.

Sight, however, needs much more complicated apparatus
to make possible the formation of a mental image that gives
a picture of an animal's surroundings. Although there is
more than one way of doing this the fundamental principle
is dictated by the physical properties of light, which is
refracted when passing obliquely between media of different
densities, as between air and water — or glass. A piece of
glass, for example, with curved surfaces forms a lens which,
if its surfaces are convex, throws a picture of what is in
front of it on to a screen behind it. The lens of a camera
throws a picture on to the sensitive film which is changed
chemically by the light according to its intensity so that the
picture is recorded. The eye is no more than a little box
camera with a lens in front of a sensitive screen that is
changed chemically according to the intensity of the light

138

THE SENSES OF ANIMALS

in the different parts of the picture thrown upon it. But the
eye is full of gadgets much more cunning than any in a
man-made camera.

We need not go deeply into the details of the eye's struc-
ture but it will be helpful to look briefly at some of its more
important features. In all the higher animals, that is those
with backbones, the basic plan of the eye is the same. The
box is more or less globular and is made of tough, fairly
rigid tissue so that it is not easily distorted. It lies in a
socket of the skull which gives it protection from accidental
damage and it can be rolled round in the socket by six
small muscles so that it can point in any direction.

The front of the eye globe is, naturally, transparent to
let in the light, and it is important that this window should
be kept clean. There is no problem for the animals that
live permanently in water, such as the fishes, for it is being
washed all the time. But vertebrates that live on dry land
have to do the washing themselves ; a gland beside the globe

IRIS
(Transparent) CORNEA
PUPIL

AQUEOUS
HUMOUR

VITREOUS HUMOUR
RETINA

FOVEA CENTRALIS
OPTIC NERVe

PLIND SPOT

Fig. 3. A human eye cut in half to show the internal structure.

pours out a continual drip-flow of tears, and there are
movable eyelids that constantly wipe the moistened window
to sweep away minute particles of dust. The arrangement
is like the windscreen wipers and washers on a motor car,
but is even better because there is a small drain pipe at the
inner corner of the lower eyelid that carries away the used
tears into the nose so that they do not overflow and run down
the face. Everyone knows how uncomfortable such an over-
flow can be when one has a "cold in the eye" so that the
pipe is blocked and in a bad attack the tears may cause
inflammation and soreness of the skin. A few mammals, for

SIGHT 139

example the seals, have no naso-lachryma] ducts — they spend
so much of their lives in the water that they hardly need
them. Consequently when seals come out to sleep on the
land their tears overflow on to their faces and when their
coats are dry the wet patches round their eyes make them
appear to be weeping copiously.

The snakes differ from most of the land vertebrates in
having no movable eyelids ; each eye is covered by a single
transparent scale. It is difficult to suggest a plausible reason
for this arrangement, for many of the lizards which have a
very similar way of life, have normal eyelids. Yet there must
be some advantage to these reptiles in dispensing with eye-
lids because some of the lizards have done so too, either
partly or completely. In some lizards the lower eyelid is a
transparent scale or "spectacle" so that the animal can see
even when the eye is shut ; in yet others the eyelids have
grown together so that the spectacle is permanently in place.
There is little doubt the snakes have lost their eyelids by a
similar evolutionary process.

We are so used to having only two eyelids that it seems
peculiar that some animals should have three, yet a third
eyelid is the rule rather than the exception in the land verte-
brates. It can easily be seen in birds, and most people must
have noticed it in action in their household cat. It is a
semi-transparent membrane that folds away at the inner
corner of the eye inside the paired lids. It is used for clean-
ing the front of the eye globe and works sideways, unlike
the outer lids that work up and down. We ourselves carry
a degenerate and functionless third eyelid in the little pink
speck at the junction of the eyelids beside the nose. It is
peculiar that when a mammal shuts its eyes the upper lids
move down but when most birds do so the lower lids move

up.

In addition to their cleaning function the outer eyelids
protect the eye from injury, shut out disturbing visual stimuli
during sleep, and allow the light-sensitive structures at the
back of the eye to rest when they are not required to be in
action. The eye is further protected from injury, and shaded
from strong light, by the fringe of eyelashes on the edge of
the eyelids. The eyelashes vary in their development but

140 THE SENSES OF ANIMALS

are nearly always longer and thicker on the upper lid ; they
are inconspicuous in most birds although in some, such as the
ostrich and the hornbills, they are so luxuriant and sweeping
that they might well be the envy of any film star.

We saw that the globe of the eye is housed in a socket
of the skull; in many amphibians, however, the socket is
incomplete and has no floor of bone. The eyes are com-
paratively large and protuberant, and they protrude not
only upwards but downwards so that if we look into a frog's
mouth we see two bulges in the roof where the eyeballs lie.
As a result of this arrangement frogs and toads use their
eyes in a very peculiar way for a purpose that has nothing
to do with sight. When a toad eats a large worm it takes
one end into its mouth and cleans off the slime and crumbs
of earth sticking to it by running the worm through its
fingers. At each gulp it shuts its eyes because it pulls them
down in their sockets so that they bulge farther than usual
into the roof of the mouth and act as ramrods to push the
worm down its throat.

The optical system of the eye is comparatively simple for
it consists of a single lens suspended towards the front of
the globe. It divides the globe into two parts, or chambers,
a small one in front filled with watery fluid, the aqueous
humour, and a larger one behind filled with soft jelly, the
vitreous humour. The humours in the anterior and posterior
chambers help to hold the lens in place so that the sling in
which it is suspended does not need to be rigid and im-
mobile — indeed it is essential that it should not be. The
lens is comparatively hard and gristly, but it is not so hard
and fixed in shape as a man-made lens of glass ; it is elastic
so that it can be pulled into different shapes. And herein
lies the beauty of its mechanism, at least in the higher verte-
brates ; when the muscle fibres contract the supporting sling
is slackened so that the curvature of the surface of the lens
is altered, and with the alteration of curvature the focal
length is altered. When the fibres relax the sling tautens
and pulls the lens back to its previous shape. The lens, un-
like that in a camera, thus has an infinitely variable focus
and assumes the right focal length for producing a sharp
image of an object at any distance ; it does not have to be

SIGHT 141

moved towards or away from the screen in focusing, as in
the fishes. In fishes focusing is done by moving the lens to
and fro, and not by altering its curvature. There are limits,
however; when the muscle fibres are relaxed the lens is
focused on infinity and all distant objects are in focus. The
muscles contract to keep an object in focus as it comes nearer
but when the sling is quite slack the curvature of the lens
can increase no more and the object then goes out of focus.
The normal distance at which this accommodation for near
objects is comfortable for us is about ten or eleven inches —
the usual reading distance — but by making a special effort
things can be focused nearer to the eye for a short time
although this action soon fatigues the muscles. If we want
to look at things so small that they must be brought closer
to the eye than the distance at which we can accommodate
we have to help the eye with a hand-lens or the combination
of lenses in a microscope.

As we grow old the elasticity of the lens decreases so that
it becomes more and more difficult for the muscle fibres to
alter its shape to accommodate for near vision — we get
"long-sighted" and have to help our sight by wearing
spectacles for seeing near objects or reading. "There's noth-
ing wrong with my eyes," the patient said to his doctor, "but
my arms are too short to hold the book far enough away to
read the print." On the other hand, people who are short-
sighted in youth find they can discard their spectacles when
the increasing long-sightedness of more mature years brings
compensation.

The edges of a simple lens focus the light slightly in front
of the image given by the central part, so that the picture
is a little blurred. The sharpness of the picture is increased
if we prevent this by using only the centre of the lens, but
of course its brightness is diminished for less light comes
through. In a camera the edge of the lens is blanked out by
using a diaphragm, and in the eye the coloured iris serves
the same purpose. The muscles in the iris make the pupil in
the centre larger or smaller according to the brightness of
the light and thus there is a compromise between sharpness
and brightness — when the light is poor the eye has to put
up with a slightly fuzzy image.

14^ THE SENSES OF ANIMALS

Another defect in all simple lenses is due to white light
being a mixture of different wave-lengths. The light of each
wave-length is focused a little in front of or behind that of
the others, so that only one of the overlapping images is in
sharp focus. As a result the image is slightly blurred and the
outlines of objects are edged with coloured fringes, especially
with red and purple which have the longest and shortest
wave-lengths. Here again the use of a diaphragm or an iris
so that only the centre of the lens is used decreases the
"chromatic aberration".

In day animals the variation in size of the pupil is not
nearly as great as in nocturnal ones where it opens very
wide in poor light. In nocturnal animals the pupil when
contracted is often a vertical slit, as in the cat, fox and
adder, but there is no general rule and in many others it
stays circular and may shut down to no more than a pin-
hole during the day. The larger the pupil when open the
more difficult it is to shut it down to a pin-hole ; closing to
a slit probably makes a neater job and avoids puckering
the edge, although it cuts out some of the width of the field
of vision. A vertical pupil is found in animals, generally
predators, that look more or less ahead to concentrate on
their prey; their potential victims, such as the hoofed
animals, on the other hand, which need to keep a sharp
look-out over a wide field, have horizontal pupils.

In birds the pupil can be closed, but it is nearly always
kept fully open, even in bright light, because birds have
another way of cutting out the blurred image produced by
the edge of the lens, as we shall soon see. Penguins are an
exception, for their pupils are very small in bright light and
in some kinds they pucker into a square or many-sided
shape ; no doubt the iris opens widely when the penguin is
under water in dim light. The iris is often brightly coloured
in birds, reptiles, amphibians and fishes, but in the mammals
it is duller, usually some shade of brown or yellow. In birds
with brightly coloured irises the pupil is sometimes opened
and shut rapidly as an expression of emotion, or to accen-
tuate the eyes as a threatening display in defence or aggres-
sion.

The image made by the lens falls on a screen at the back

SIGHT 143

of the eye, the retina, which contains the nerve cells that
react to light. These are of two kinds, narrow cylindrical
rods and shorter dumpy cones. Many thousands of them are
closely packed into a layer at the back of the retina, some-
times mixed, and sometimes in separate patches of one sort.
When light falls upon them a physico-chemical change is pro-
duced which starts an impulse running along the nerve
fibres joining the cells to the brain. The change can actually
be seen in the rods because they produce a coloured sub-
stance, visual purple or rhodopsin, which is bleached to
become colourless when light falls on it.

Behind the rods and cones there is, in many animals, a
layer of velvety black colour which absorbs any light that
passes on through the retina, and prevents it being reflected
back to give a second confusing ghost image, or a scattering
of light. It is exactly parallel to the backing put on photo-
graphic plates to prevent halation. In many nocturnal
animals the black backing is replaced by a reflecting mirror,
the tapetum, which reflects any light that goes through the
retina. Evidently when there is little light it must all be
used for stimulating the rods, and the risk of halation in
day vision must be accepted — as someone has said, the
tapetum "gives the retina a second chance". Reflection from
the tapetum makes the eyes of cats and other animals shine
in the dark when a bright light is directed on them. A rather
similar but much less bright reflection can sometimes be
seen in the eyes of animals that have a black backing and
no tapetum, sometimes even in man. This kind of shine,
which is pink and not greenish, has a different origin and is
caused when a bright ray falls into the eye at just the right
angle to be reflected to the observer from the layer of
minute blood vessels on the outer surface of the retina —
it is light that has not penetrated the retina at all, and its
pink colour is the colour of the layer of blood that reflects
it.

Besides the rods and cones the retina contains several
layers of nerve cells and a network of small blood vessels
that bring nourishment to it. One would expect the rods
and cones to be on the surface of the retina so that the
image could fall directly upon them, but they are not ; they

144 'I'HE SENSES OF ANIMALS

lie at the back, and the hght shines through the layers of
cells and vessels before it reaches them. In practice this makes
little difference for in life the nerve cells are transparent and
the blood vessels are very small ; nevertheless they do inter-
fere with the very sharpest sight and part of the retina is
arranged to overcome this defect.

The rods are very sensitive to light but they do not
distinguish colours so that the picture they send to the brain
is like a black-and-white photograph. Moreover the com-
bined messages from many rods travel along single nerve
fibres so that the details of the picture sent to the brain are
not very sharp. The cones are much less sensitive — it needs
about a thousand times more light to stimulate them than
the rods — but they are sensitive to colours, and each has its
own nerve fibre running to the brain so that the detail of
the mental picture is much clearer. The rods are thus more
useful when the light is dim and the cones when it is bright.
Consequently the retina of nocturnal animals consists mostly
of rods, and that of many day animals contains more cones ;
in some birds there are practically no rods at all.

Although the cones need much more light to make them
work they give the clearest and most detailed mental picture,
and in eyes such as our own which have a mixture of rods
and cones there is one place in the retina which contains
nothing but cones. This is the. fovea, and when we look at
any object we move the eye so that the part of the image
we are attending to falls on it. At the fovea the cones are
tightly packed, and their nerve fibres run out radially like
the spokes of a wheel; the other nerve layers are pushed out
to the side so that there is nothing to interfere with the
image. Although we are unaware of it, our eyes are con-
tinually making slight movements scanning our surround-
ings and bringing different parts of the image on to the
fovea; when we look steadily at anything we have a less
definite picture of the things round it — we see them only out
of the "corner of the eye". The cones give acuity, the sharp-
ness of sight, whereas the rods are concerned with sensitivity
to the intensity of light, a very different thing.

Owing to the peculiar inversion which makes the nerve
fibres run on what we might call the "wrong side" of the

SIGHT 145

retina they have to pass through it at one spot in order to
get outside the eye and form the optic nerve running to the
brain. At this spot there cannot of course be any rods or
cones and it is consequently bhnd. The existence of the
bhnd spot is easily confirmed by the well-known trick with a
spot and a cross on a piece of paper. The blind spot lies
towards one side of the eye and the part of the image that
falls on it in the right eye is different from that falling on it in
the left. Consequently in combining the pictures obtained
from the two eyes there is an overlap which covers up the
hole in each separate picture. All the vertebrate animals have
a blind spot, but among the invertebrates the octopuses and
squids are without this defect. In many ways their eyes are
very much like those of the higher animals but the retina is
not inverted; the rods and cones are on the "right side" of
the retina and so all the nerve fibres are at the back and do
not have to pass through it to get outside the eye.

XIII

WHAT DO ANIMALS SEE?

The eyes of most animals are at the sides of their heads,
a position that gives them a very wide field of view, so wide
that in many grazing mammals which have to keep a sharp
look-out for enemies it extends all round, and the animal
can see behind it as well as in front. Such animals need less
to concentrate on any particular part of the image than to
detect any movement round them that might be made by
a predator. Consequently their retinas consist almost entirely
of rods, which give great sensitivity but less acuity and no
colour vision. Thus most mammals are practically colour-
blind, and once they have detected a suspicious movement
they bring their other senses, especially hearing and smell,
into play to help locate and examine whatever has alerted
them.

In animals whose eyes are directed forwards the field of
view is restricted, but in compensation for the loss of all-
round vision they gain stereoscopic sight in which the pic-
tures sent to the brain from each eye overlap and are com-
bined into one, and not just joined to each other at the edges.
Stereoscopic sight gives depth to the picture, so that distances
can be judged much more accurately, and the appreciation
of the three-dimensional picture thus obtained is increased
by the acuity given by the preponderance of the cones over
the rods. The preponderance of cones also gives colour vision
which itself aids acuity by giving an additional means of
separating the components of the picture.

In very dim light the cones give no response and animals
with mixed retinas then make use of the rods. At night our
foveas, which contain no rods, are practically blind and con-
sequently if we wish to see any particular object we must not
look at it, but just beside it; if we look at it and bring the

image on to the fovea it disappears, but if we bring the

146

WHAT DO ANIMALS SEE? I47

image on to the outer part of the retina, rich in rods, we are
able to see it at once. Most birds are in a worse case than
we are, for their retinas consist almost entirely of cones and
as a result they are nearly blind at night. For this reason
most birds go to roost before darkness has completely fallen,
and if disturbed during the night they blunder about
clumsily. Poachers have been known to take advantage of
the night-blindness of game birds.

In daylight, however, most birds enjoy an acuity of vision
much better than ours — they can see small objects four or
five times further away than we can. The cones at the fovea are
very numerous and closely packed, and the pupil of the eye
is relatively large. The large size of the pupil tends to de-
crease the sharpness of the vision because it emphasizes the
colour fringes at the outlines of objects, but this defect is
corrected by the presence of coloured oil droplets in the cones
which filter out the unwanted fringes without abolishing
colour vision. The central fovea in birds lies in a compara-
tively deep depression in the retina, and it has been suggested
that the sides of this pit act as a lens to magnify the size of
the part of the image falling upon it. This is probably true
to a limited extent, but the advantage of the deep fovea is
more likely to be in emphasizing the movement of the image
of an object crossing the line of sight so that "fixation" is
enhanced. This makes it easier both to pick up a moving
object and to avoid losing it when once picked up, especially
against a featureless background such as the sky.

Fishes and reptiles also have a deep central fovea and they
too are quick to respond to movement, but do not react to
a static field of vision. It is well known that snakes generally
will not eat dead prey which lacks the movement of the
living, but they can be induced to do so in captivity by
waggling the offered food before them. One may speculate
too whether the to-and-fro movement of the head that is
characteristic of many birds when they walk may move the
visual picture across the retina and so bring different parts
of it in succession on to the fovea. This might help grain-
eating birds to find the seeds and other small objects on
which they feed.

Birds that chase moving prey in the air have eyes that are

148 THE SENSES OF ANIMALS

directed more forwards and less to the side than usual. They
thus sacrifice the power of all-round sight in exchange for
stereoscopic vision which helps in judging distance and speed.
They also have two foveas in each eye; in addition to the
central fovea there is a temporal one which is used in looking
ahead in stereoscopic vision. The sharpness of sight in birds
is also increased by the absence of a network of blood vessels
such as covers the retina of mammals. The blood vessels are

IRIS

PUPIL
LENS

Fig. 4. The eye of a bird cut in half to show the internal structure, especially
the pecten.

carried in a thin membrane, the pecten, that projects as a
fold into the posterior chamber from the blind spot, and the
cells of the retina get their nourishment by diffusion from
these blood vessels through the vitreous humour. Although
the pecten is very transparent it is an obstruction, and one
would think that it must impair the vision. In fact it helps
vision because it casts a slight irregular shadow on the retina ;
this, by varying the mean illumination of the retina, has the
effect of increasing contrast when a small object crosses the
field of view against a uniform background such as the sky.
It is significant that the pecten is especially prominent in the
eyes of hawks and eagles. On the other hand, in owls, the
nocturnal birds of prey, it is small, but in these birds acuity
is low for other reasons though sensitivity is great. In owls
the retina consists predominantly of rods, there is no fovea,
the aperture of the iris is comparatively large and there is
little power of accommodation. The result of these peculiari-
ties is that an owl has to put up with a rather blurred picture
— but that it can get a picture when we should get none at

WHAT DO ANIMALS SEE? I49

all, for the sensitivity of its retina is a hundred times greater
than ours. In spite of this the owl's powers of night vision do
no more than enable it to find its way and avoid colliding
with things ; it certainly does not hunt its prey by sight alone.
As we shall see in a later chapter, the owl catches a mouse on
a dark night by using its extraordinarily efficient sense of
hearing.

Accommodation has another use besides focusing the lens
to give a sharp picture ; the amount of tension produced by
the muscles in accommodating helps in judging the distance
of an object. This comes about not through any fixed scale
of tensions but is learned through experience of focusing on
objects near and far. Judgement of distance is also helped by
the amount of convergence necessary in examining an object,
and this, too, has to be learnt by experience. When an
animal with forwardly directed eyes looks at any object it
adjusts the eyes so that the part of the image required falls
on the fovea of each. Consequently the nearer the object the
more the eyes have to be turned inwards towards each other
until, when it is very close, there is a distinct squint.

Judging distance by these means, however, is not the only
thing about using the eyes that has to be learnt by experience.
The eyes of a newly-born animal are little more use to it than
the light-sensitive skin of the earthworm, they enable it to
distinguish light from darkness but the images falling on the
retina mean little or nothing to it — how could they, for it
has never seen anything before? By exploring its surround-
ings mainly by touch and smell as well as by sight, it
gradually learns the physical properties of its environment
and co-ordinates what it is told by the different senses so that
it learns the meaning of the images focused on the backs of
its eyes. Eyes, without experience, are of comparatively little
use for telling animals what is going on around them.

Sight is one of the most acute of our senses and conse-
quently we attach great importance to it, but even our eyes
are most fallible organs and are very easily deceived, as is
well shown by the many optical illusions that are made to
trick the eyes for amusement or instruction. We betray the
unreliability of our sight if anyone shows us something we
have never seen before. "Whatever is that !" we exclaim,

150 THE SENSES OF ANIMALS

and then add "Let me see it" as we hold out our hand for
it, not in fact to see it — we can do that quite well already —
but in order to hold it and to find out by touch what it
really is. When we have done that we can recognize it by
sight alone on a future occasion.

I well remember driving a car on a long straight road ; as
I topped the crest of a hill I could see across the valley ahead
to the far hillside where a white goose was standing in the
middle of the road. I drove on down into the valley and as
I started climbing the far rise the goose was still there. It
stood quite still until I was about a hundred yards away
when it suddenly turned into a wet patch that had been
reflecting the sun. If I had turned off at the cross-road in the
valley I should have been ready to take any oath that there
was a white goose in the road, whereas the correct inference
would have been that a farmer had driven his herd of cows
across the road shortly before. Appearances certainly are
deceptive and sight, much as we prize it, is easily deceived !

It is impossible to know whether the other animals see
things as we do. Undoubtedly the images that fall on their
retinas are similar to those that fall on ours but, as we have
seen, eyes without experience are neither reliable nor in-
formative. Seeing is more than having an image in the eye,
it becomes useful only when there is an image in the brain,
and the building up of that image is so complicated a process
that we know little about it even in ourselves — and who
knows what goes on in an animal's brain? Nevertheless we
can, from a study of the structure of eyes, form some idea of
what the vertebrate animals probably see. For a start, most
of the mammals except the monkeys are more or less colour-
blind because their retinas are poor in cones but rich in rods.
The mammals that have eyes at the sides of their heads have
a much wider field of vision than we do, and some can see
almost all round them, but they have only limited stereo-
scopic vision because the fields of each eye do not overlap
very much. Although the picture they see is rather indistinct
and the detail is not very clear they have great sensitivity to
any movement that may occur in the field of vision.

Most birds are not colour-blind — their brightly coloured
plumage used in mating display would lead us to infer this

WHAT DO ANIMALS SEE? I5I

even if we did not know that their retinas consist mostly of
cones — and they have great visual acuity so that they can
see minute detail several times further away than we can.
Although their eyes are not telescopic in the sense that they
have a system of magnifying lenses they are telescopic in the
literal meaning of "seeing far". Sight is necessarily an im-
portant sense in animals that fly rapidly by day, and in
practically all birds it is the dominant sense. Hence the
comparatively enormous size of their eyes, in which most of
the globe is covered by skin and the part we see between
the eyelids is only a small window. The quickness of response
to the picture seen by the eye is also very important, for a
sparrow flying to a tree and alighting on a twig must be
equivalent to a jet pilot suddenly folding his plane's wings
and perching on a church tower.

In night birds, such as owls and some others, as we have
seen, the picture is not nearly so clear, but there is a picture
of sorts in light so dim that day birds could see nothing.
Furthermore owls have little power of accommodation or
convergence — they cannot alter focus nor move their eyes
in their sockets, and for the latter reason they have to turn
their heads to see anything out of the direct line of vision.
They can turn their heads through more than i8o degrees
so that they can look more than directly backwards. Hence
the old Yankee yarn about how to catch an owl : if you find
one sitting on a post you have only to walk round it three
times and it will follow you round with its eyes until it
wrings its own neck !

The fishes, too, are not colour-blind and this again we
could infer from the part that bright colours play in the
mating displays of many of them that live in shallow water.
On the other hand, the colours of fishes that live in more
than very moderate depths of the sea are very different down
there from their appearance at the surface. Although water
is transparent it does absorb light, starting with the red end
of the spectrum, so that in descending below the surface the
light gets bluer as it fades out to the total darkness of great
depths. Consequently as soon as the red component in the
light is eliminated, red animals appear black because there
is no red light to be reflected from them : beyond the critical

152 THE SENSES OF ANIMALS

depth there is no colour difference between a boiled lobster
and a live one — both appear to be black.

A fish's-eye view, if the fish is near the surface, must be
very peculiar. In deep water it is probably similar to what
we should see if we were at the same depth with an aqua-
lung. But near the surface, as in a river or pond, a fish can
sometimes see above the water as well as below. It can see
into the air above it and to its sides as far out as a line from
its eye would make an angle of 45 degrees with the surface.
There is thus a dome above it within this limit in which it
sees objects in the air. But owing to the bending of light-
rays when they enter the water from the air, the base of the
dome is in effect much wider and the fish may thus be able
to see things beyond the edge of the bank. There is a further
complication; beyond the 45-degree angle light is totally
reflected from the surface so that to the fish it appears as a
bright mirror, and in that mirror it can see the reflection of
things on the bottom, to its side and below it. The fish's-eye
view must therefore be a complicated picture and it is un-
likely that the fish can pay attention to more than a part
of it at any moment. We know from the structure of the
retina that the acuity of the fish's vision is good, and it is
probable that the fish ignores most of the picture but is very
quick in responding to any movement that occurs in its
visual field. All this can happen only when the surface of
the water is still — the moment it is ruffled by wind or wave
the surface appears only as a hammered glass mirror would
to us, a bright and shiny irregular surface through which
nothing can be seen and which gives no sharp reflection of
the objects below it.

It is convenient for fish that the bending of light rays in
passing from water through the surface of the eye is less than
in passing from air into an eye. Consequently the surface of
a fish's eye can be much flatter than that of an air-living
animal, and thus blend in well with the general streamlined
shape of the fish without the necessity of a vulnerable
bulge.

The most extraordinary eye found in fishes, or among all
the vertebrates for that matter, is that of a little tropical
fish Anableps, the "Four-eyed Fish". The iris extends across

WHAT DO ANIMALS SEE? I53

the eye as a horizontal bar dividing the pupil into upper
and lower parts with different optical properties ; when the
fish lies at the surface the upper part of the eye is out of
water and sees in air while the lower part sees in water. It
is difficult for us to imagine what picture of its surroundings
Anableps obtains ; perhaps the fish is mainly concerned in
detecting movement above or below it and looks at, or
attends to, objects in one medium at a time. Another puzzling
fish is Toxotes, the Archer fish, which catches insects by spit-
ting a drop of water at them as they fly overhead and knock-
ing them down on to the water. People have wondered how
this fish could allow for diffraction in making its shot, but
recent observations have shown that it avoids having to
solve this problem by coming very close to the surface and
shooting only when the insect is almost directly overhead
when diffraction is at its least. Furthermore the blob of
water that it shoots quickly breaks up into smaller drop-
lets so that in effect the fish is not shooting a single bullet but
a charge of shot. Even so, Toxotes' accuracy as a marksman
is astonishing. The action is stereotyped; Professor Hediger
had a specimen with a deformed mouth so that the shot
went to one side of the target, but the fish never learned to
allow for this and always scored a miss.

Even in some aquatic mammals, such as seals and whales,
the surface of the eye resembles that of a fish in being un-
usually flat, and for the same reason. Seals seem to be able
to accommodate to some extent when they come out of the
water but it is unlikely that they can do so enough to give
a completely sharp image and they are probably then rather
long sighted. It is improbable that whales can accommodate
for vision in air because they are much more completely
aquatic ; nevertheless they do sometimes poke their heads
out of the water to look at strange objects such as people in
a boat, but they probably do this because the water is ruffled
so that they cannot see through the surface. A peculiarity
of the whale's eye is the enormous thickness of the outer
fibrous coat which must serve to protect the eye in some
way. It cannot be to prevent the shape of the eye being
distorted by the pressure of the water when the whale dives
in pursuit of its food, because the whale's tissues, consisting

154 THE SENSES OF ANIMALS

mostly of water, are virtually incompressible. The eye-coat
is so massive that whalers often make a "souvenir" of a
whale's eye by scooping out the lens and retina and cutting
the back off so that it has a flat surface. They then dry it
and use it for an ash-tray — it looks like half a dried turnip
with a cup-shaped hollow at the top.

We have now looked at some of the ways in which the
eye works in the vertebrate animals and have tried to under-
stand what those animals see. It is fairly certain that although
they may look at the same things as we do they do not neces-
sarily see the same picture. Most of the mammals see patterns
rather than colours, whereas the birds, reptiles and fishes,
or many of them, certainly see colours as we do. We will
finish this chapter with a brief look at the complexities of
colour vision.

Several theories have been made to explain colour vision
but none of them exactly fits all the known facts, so that our
understanding of the process is by no means complete. It is
probable that the cones of the retina contain substances that
are chemically changed by light of different colours, that is
of different wave-lengths. According to one theory there are
three substances sensitive to red, green and violet respec-
tively. According to another there are also three substances
but the first is broken down by red and built up by green,
the second is broken down by yellow but built up by blue
and the third is broken down by white light and built up in
darkness. Some of the facts of colour vision fit one theory,
some the other, but neither theory fits all the facts, and
perhaps eventually some combination of the two will be
worked out.

The great difficulty in studying colour vision is that colours
differ in appearance according to the circumstances under
which they are seen. If you put a piece of grey paper on a
red sheet, and place a piece of tissue paper over the whole,
the grey appears to be green ; if the sheet is yellow the grey
appears blue ; and vice versa. If you look steadily at a bright
red patch for a few moments and then look at a white sheet
you will see a patch of green, and so on. Furthermore in
everyday life where the shapes of things are irregular and
intricate their colours depend upon factors very different

WHAT DO ANIMALS SEE? I55

from those affecting simple tests such as matching strips of
coloured paper.

A very striking experiment has recently been devised to
illustrate this : two black-and-white photographs, not
coloured ones, are taken of, say, a garden full of flowers.
One photograph is taken through a red filter and the other
through a green one. Two exactly similar black-and-white
lantern slides are made from the negatives and each is placed
in a separate lantern for projection on to the same screen.
The lanterns are adjusted so that the two pictures fit each
other exactly and form one. A red filter is placed over the
lens of the lantern containing the slide made with a red
filter; if its light is switched on alone a red picture appears
on the screen. The lantern containing the slide made with a
green filter has no filter placed over its lens; when it is
switched on alone a black-and-white picture appears on the
screen. But if you switch on both lanterns together so that
the red and the black-and-white pictures fall superimposed
on the screen, the picture appears in full colours — the red
flowers are red, the leaves and grass are green, the yellow
flowers are yellow, the blue ones blue and the white ones
white. This astonishing result certainly takes a lot of explain-
ing ! Although it may confirm some parts of the old theories
of colour vision it seems probable that they will have to be
revised or perhaps superseded by a new one. One thing
seems certain : that the appearance of colours depends not
only on the surrounding field, but on the way the message
sent from the cones of the retina is coded in the nerve cells
and sent to the brain. It depends, moreover, on the way the
message is interpreted by the brain when it is received there.
Colour, like beauty, lies not only in the eye of the beholder,
but also in the brain : it is subjective as well as objective.

XIV

SIGHT IN INVERTEBRATES

In the last chapter we considered the structure of the eye
in the vertebrate animals and saw something of what is
known about its working. We, being vertebrates, have eyes
built to the basic design common to them all and conse-
quently we can, with due caution, interpret the sight of
others by our own subjective experience. But what of the
sense of sight in the invertebrates, whose kinds greatly out-
number those of the vertebrates? Their brains are so dif-
ferent from ours that it is difficult to appreciate how the
world may appear to them. Yet different as their brains may
be, the same principle is found in all organs of vision more
complex than simple light-sensitive spots of pigment — a
camera consisting of a box with a lens at one end and a
light-sensitive screen at the other.

We have seen that the protoplasm of even the simplest
animals whose bodies are not divided up into separate cells
responds to light, but evidently early in the process of evolu-
tion it became an advantage to possess a particular part of
the body specialized for responding to the stimulus of light.
Hence even in the protozoa there are species which contain
a speck of pigment that, like the silver bromide in a photo-
graphic film, is very easily changed chemically when light
falls upon it. Eye-spots of this kind are found not only in the
protozoa but in many animals of more complex structure
whose bodies consist of great numbers of cells of different
sorts segregated into special tissues for different functions.

The evolution of eyes from simple eye-spots consisting of
light-sensitive substances was almost inevitable. In a many-
celled animal the cells containing such pigment will generally
lie at the surface of the body, and their pigment will be at
the inner end of the cells, as close as possible to the under-
lying nerve fibres. The transparent protoplasm of the cell

156

SIGHT IN INVERTEBRATES I57

body causes the surface membrane of the cell to bulge out-
wards slightly so that, especially in non-aquatic animals,
light falling upon it is refracted and concentrated upon the
pigment. Such simple eyes, like eye-spots, are light-gather-
ing organs and do not form images, but the basic structures
are present for the development of image-forming organs
by further stages of evolution. In the first stage, still a light-
gatherer and not an image-former, the density of the
refracting part of the cell is increased, thereby producing
a very simple lens. In the next stage the cell is divided into
two, so that a lens-cell lies above a retinular-cell containing
the pigment. This simplest form of eye containing an optical
system is found in the young stages of some Ascidians or
Sea-squirts, animals that swim freely in the sea while they
are minute larvae but then settle on the bottom and become
superficially more like vegetables in appearance when adult.
Once a lens-cell and a retinular-cell are separated the further
stages of evolution to produce an image-forming eye of great
efficiency are merely those of an increased differentiation of
cell structure, an enormous increase in cell numbers (the
human eye is said to contain 137,000,000 nerve endings) and
a general increase in complexity.

A series of eyes ascending from the simplest to those
probably as efficient as our own, perhaps even more so,
can be traced in the molluscs, the shell-fish that include
the snail, limpet and periwinkle, the oyster, clam and cockle,
the squids and octopuses — very different from those other
shell-fish, the Crustacea, which include crabs, lobsters and
shrimps.

The spiral-shelled molluscs, which creep about by means
of the "foot" on the under-surface of the body and have a
definite head-end, usually have a pair of eyes, although
many species that burrow in mud and sand, or live in very
deep water where it is dark, have no eyes ; in some, such as
the snail, the eyes lie at the ends of a pair of tentacles. In
the limpet the eyes are merely little pits in the skin; they
are lined with pigment and retina-cells, and are open to
the sea-water which bathes the surface of the lining cells —
the lens and all the other usual eye components are missing.
Such an eye is obviously no more than a light-gatherer.

150 THE SENSES OF ANIMALS

From this simple type of eye a host of progressively more
complicated types can be found in other species. In a slightly
more advanced type the pit is a partly closed cup filled
with jelly which is in contact with the water at the opening
through which light enters; in the next stage the opening
is closed by transparent skin ; in another, such as the snail's,
the jelly is much firmer and forms a lens separated from
contact with the retina. Judging from the structure of the
more complex types of eyes we should expect the animals
possessing them to have some considerable power of sight.
Experiment, however, has failed to show such vision and it
is very doubtful if the eyes function as more than light-
gatherers; if there is any vision the species that enjoy it
must be extremely short sighted. On the other hand, the
whole surface of the body of some of these creatures is sensi-
tive to light, and animals deprived of their eyes react to a
shadow falling upon them.

In the cephalopods — the octopuses, squids and cuttles —
there is a similar increase in complexity of the eye from a
very simple type. In the nautilus, one of the very few of
these animals that has an outside shell, the eye, like that of
the limpet, is an open cup lined with a retina but without
a lens. In the most complex types, such as the octopus and
squid, the eye is remarkably like that of a mammal, and
has lens and iris, focusing arrangements, anterior and
posterior chambers, and a well-developed retina in which
the nerve fibres are not spread over the surface of the light
receptors but run below them. These eyes are very efficient
image-formers and the behaviour of the animals shows that
they have very good eyesight indeed. Their brains, how-
ever, are very different in structure and comparative size
from those of mammals, and it is probable that the picture
of the surroundings built up in the brain is by no means
similar to that which we should build up from the same
images on our retinas.

In the other molluscs — those with two shells, such as
cockles and mussels — the eyes are very different. In the
first place these animals have no head and their range of
locomotion is comparatively restricted; indeed some are
permanently fixed to their surroundings and cannot shift.

SIGHT IN INVERTEBRATES I59

Most species feed on minute floating plants and animals
which they capture by drawing a current of water between
the shells and filtering their food out of it by means of the
gills. When the shells are opened a short way for this pur-
pose in many species the edges of the body lining each shell
move towards the opening so that light falls upon them. In
other species, which live buried in sand, part of the body-
edge is drawn out into a tube that can be pushed up to the
surface for the current of water to be drawn in through it.
All these animals are provided with eye-spots, often no more
than cells containing a speck of light-absorbing pigment,
though sometimes more complex.

In the species with water-tubes the eye-spots are concen-
trated at the outer end of the tube, but in the others they
are scattered along the edge of the body just inside the
shells. Although these eye-spots are only light-gatherers and
can give no sight, they are extremely sensitive to changes in
the intensity of the light falling on them, and the faintest
shadow crossing them causes the animal to withdraw into
the shells and close them tightly.

The eyes of the oyster are of this type, but those of another
favourite edible mollusc, the scallop, are very different. Un-
like the sluggish oyster the scallop often shifts its position
and swims rapidly about by flapping its shells. It does not,
however, swim by simple jet-propulsion with the hinge of
the shells leading and the closing of the shells giving the
power stroke, as one would expect. It swims with the hinge
trailing, and appears to be taking bites out of the water in
front — the water is taken in at the shell edges and is expelled
on each side of the hinge. This is, of course, a form of jet
propulsion, for the water squirted out beside the hinge is
concentrated into jets, and the method is thus more efficient
than what, at first sight, appears to be the obvious one. An
animal that swims about actively presumably needs some
power of vision, and it is therefore not surprising to find
that the eye-spots of the scallop are very prominent. There
may be over a hundred of them arranged round the body
edge immediately inside the shells. They are comparatively
large dark spots which shine like jewels with the light re-
flected from them; each contains a lens and retina and is

l6o THE SENSES OF ANIMALS

connected to an optic nerve. Nevertheless, they do not
apparently form images but are merely highly developed
light-gatherers, so that the power of vision in the scallop is
little better than in the oyster. In a few other species of
mollusc the eyes are gathered into bunches so that compound
eyes are formed, but here again they appear to be no
more than highly organized light-gatherers. (See Plate

Lastly, in some of the "Coat-of-mail" shells, slug-shaped
marine molluscs in which the shell consists of a number of
jointed convex plates on the back, we find the greatest pro-
fusion of eyes among the molluscs. Although the eyes are
minute some of them contain a crystalline lens ; they are
lodged in small cavities all over the shell-plates of the back
and a nerve from each passes through a minute hole be-
neath. In some species they are scattered irregularly, but in
others they are arranged in rows in definite patterns ; they
may number as many as 12,000 in one animal.

In one division of the invertebrates we find eyes which,
although based on the principle of the lens and retina, have
evolved in a very different way to form compound eyes.
These animals are the arthropods, the animals in which
the hard skeleton is outside and all the muscles, viscera
and other soft parts are inside. The most numerous of the
arthropods are the insects, but the division includes the
Crustacea, spiders, scorpions, millipedes and many others.
Not all of these possess compound eyes — many have only
simple eyes or ocelli, and yet others have both simple and
compound eyes.

Ocelli are called simple eyes to distinguish them from
compound eyes, but their structure is often anything but
simple. The lens is formed from thickened layers of the outer
cuticle or from large transparent cells derived from the
cuticle-forming cells ; a light-sensitive retina consisting of
pigment and nerve endings lies behind the lens. There is
no chamber filled with semi-fluid jelly as in the vertebrate
eye, but a layer of transparent supporting cells separates
lens and retina. Ocelli are fixed and immovable, and there
seems to be no mechanism for adjusting the focus for near
or distant objects. Ocelli of simple structure appear to be

SIGHT IN INVERTEBRATES

i6i

no more than light-gatherers, but those that are more com-
plex are almost certainly organs of sight that give a clear
image, although perhaps only at certain distances. In the
ocelli of some insects there are even two layers of retinal cells
— on the outer layer distant objects may be focused and on
the inner layer nearer ones.

Most spiders have eight eyes, each an ocellus, but some
have only six, and yet others that live in the darkness of
caves have none. The eyes are placed at the front end of
the "head" part of the cephalothorax and are grouped in a
great diversity of patterns so different from one another that
their arrangement is one of the most important characters

Fig. 5. The cephalothorax in three different sorts of spiders to show the ocelli
in differing patterns at the "head" end. The views are looking down on the
spiders' backs and the head is at the top. The bases of the eight legs are indi-
cated and the abdomens would join the centres of the lower edges.

used in the classification of spiders. Some of the eyes look
upwards, some sideways and some forwards, and sometimes
they are mounted on turrets that give a wide field of view ;
those directed forwards are usually the largest. It is difficult
to imagine how the world appears to a spider provided with
so many eyes. The images received by the different eyes
may be combined in the brain of the animal to form a

l62 THE SENSES OF ANIMALS

single broad picture, or the creature may pay attention
only to one set, top, side or front, at any one moment.
Those species with very large front eyes probably use them
in the second way and disregard the images in the others
when looking at something close ahead. It is possible, too,
that different pairs of eyes have different focal lengths so
that some are used for distant vision, others for near.

In some spiders some of the eyes are dark in colour, others
pearly white. It has been suggested that the dark eyes are
for day vision, the others for seeing at night or when the
light is dim. In the jumping spiders a peculiar flickering
and change of colour in the large front eyes is seen when
the animal is stalking its prey. This is probably caused by
a movement of the retina to alter focus, or by a slight move-
ment of the base of the eye-cup to give convergence. What-
ever may be the explanation it is certain that the eyes of
these spiders are highly efficient ; they evidently give a clear
image for these animals carefully stalk their prey and then
suddenly jump upon it from a distance of several inches.
Most of us have experience of the quickness of sight in
spiders. If a large house-spider is cautiously walking along
the base of the skirting-board in the room where you are
sitting quietly reading and you catch the movement out of
the corner of your eye (the image on the rods at the side
of the retina, not on the fovea) and you look at it (bring
the image on to the fovea) the spider at once notices your
slight movement and starts running for shelter. It seems
almost uncanny that the spider knows you are going to rise
and swat it — if you are one of those people that must swat
man-eating spiders — and it appears to have much too much
perception and intelligence for an invertebrate. But if you
can forget your emotions you realize that the unfortunate
creature is merely reacting to your own involuntary startled
reaction, though it is only a movement of your eyes, through
its acute powers of sight.

Although the eyes of some spiders are such excellent
organs of vision, the ocelli in many arthropods are less
efficient. The caterpillars of butterflies and moths have no
compound eyes but are provided with ocelli which resemble
the eyes of spiders in fundamental structure. Yet they are

SIGHT IN INVERTEBRATES

163

OCELLI

JAW

Fig. 6. Front view of the head of the caterpillar of a Privet Hawk Moth.
The ocelli are placed low down on each side.

little more than light-gatherers and certainly give no sharp
image as can be seen from the way in which a caterpillar
fumbles about in selecting a suitable place to start eating a
leaf of its food-plant.

Although most adult insects have complicated compound
eyes, most of them have ocelli as well, often reduced in

OCELLI

COMPOUND

Fig. 7. Front view of the head of a Honey Bee showing the three ocelli and the
two compound eyes. The antennae and the jaws and other mouth parts are
seen.

164 THE SENSES OF ANIMALS

number to one looking upwards and a pair looking side-
ways. In some insects such as dragonflies their structure leads
us to suppose that they give a sharp image and are used for
sight, but in others their function is probably different. In
higher insects the ocelli may be concerned with perception
of the plane in which light is polarized — in the honey-bee
orientation and direction-finding are certainly connected
with this ability. The exact function of the ocelli in most
insects is, however, not fully understood.

Most insects and most of the higher Crustacea are charac-
terized by the possession of a pair of compound eyes each
of which consists of a very large number of minute tubular
cameras stuck tightly together and arranged so that lenses
form part of the surface of a hemisphere. The details of the
structure of the tubular cameras, or ommatidia, are com-
plicated, and differ widely in different insects, but essentially
each consists of a lens at the outer end and light-sensitive
cells connected to nerve fibres at the inner end. Each tube
is surrounded by pigment cells so that light cannot pass
sideways from one to another. In some insects a fine net-
work of air tubes also surrounds each tube and acts as a

Fig, 8. A section of part of the compound eye of a Honey Bee showing the
closely packed ommatidia (highly magnified).

reflector, much as does the tapetum in the eyes of some
mammals ; it is the reflection from this network that makes
the eyes of moths glow when a light is shone upon them at
night.

SIGHT IN INVERTEBRATES 165

The pigment and the reflecting wrappings round each
ommatidium ensure that only Hght that passes straight down
the centre of the tube reaches the Hght-sensitive cells at the
bottom — any rays coming in from the sides are cut off. Thus
no image is formed at the bottom of the tube, and the cells
there register only the intensity of the light that reaches
them. The image of the surroundings is therefore built up
from an enormous number of separate dots of differing
intensity, like a photograph printed in a newspaper by the

-CRYSTALLINE LENS

-CRYSTALLINE CONE
PI6MENTCELL

-RETINAL CELL
— PI5MENTCELL

Fig. 9. A single ommatidium from the eye of a Honey Bee highly magnified.

"half- tone" process. A picture built up like this resembles
a mosaic, and consequently this kind of eye produces mosaic
vision, as explained in Chapter II.

It has sometimes been thought that each ommatidium
produces an image, and that an insect sees by "mentally"
fusing into one the thousands of separate pictures thus

l66 THE SENSES OF ANIMALS

received. This mistake has come about because it is possible
to strip the outer layer containing the lenses from the surface
of the compound eyes of certain insects. If this layer is
examined with a microscope it can be arranged so that each
lens gives a separate image of an object at a suitable dis-
tance, but that does not justify an assumption that the com-
pound eye works that way in life. The assumption overlooks
the function of the tubular part of the ommatidium which
ensures that only the central ray gets to the bottom. It is
very difficult to prepare an insect eye so that, under a
microscope, you can look through it from the back, but
it has been done successfully with the eyes of certain kinds.
When, in such a preparation, you look through the eye
from the bottoms of the tubes you find that a single image
of an object is formed and that it is built up from the minute
dots of greater or lesser intensity from each ommatidium.

A simple experiment can be made to show how the com-
pound eye works. Get a gross or so of lemonade drinking
straws, paint them black, and stick them together side by
side so that the ends are level and you have a solid chunk
of straw with a large number of long parallel holes running
through it. If you fix a piece of ground glass or thin paper
against one end of the tubes and point the other at a brightly
lit scene an image of what is in front appears on the screen
— and furthermore the image appears sharper if anything
is moving or if you move the tubes across the scene.

That is not, however, the whole story of the compound
eye — which is not only compound, but complex. The tubular
system which uses only the direct ray coming down the
centre is efficient enough in bright light, but it is very
wasteful. It wastes much of the light because the tubes are
so arranged that all the oblique rays are absorbed by the
pigment surrounding them and cannot get into nearby tubes
and thus produce a blurred image. But when the light is
dim so much of it may be wasted in this way that the
animal is practically blind since so little light goes down
the centre. For vision in dim light therefore the efficiency
of the tube system is abandoned, and light is allowed to
spread from one tube to another. The pigment cells sur-
rounding the tubes can very quickly expand and contract —

SIGHT IN INVERTEBRATES 167

when the light is bright they spread out and absorb the un-
wanted rays, but when it is dim they contract so that the
obKque Hght falhng on the sides of the tubes can pass
through. This enables the animal to see, but the picture it
gets is not nearly so sharp, and when the pigment cells are
fully contracted it must be very blurred. Nevertheless the
pigment cells are very quick in reacting and instantly adjust
themselves so that the image is kept as sharp as possible
while retaining no more than the necessary brightness.

The compound eyes of many insects and Crustacea are
capable of colour vision. Experiments have shown that some
kinds of butterflies visit flowers of certain colours more than
others, particularly the blues and reds. The late Professor
Eltringham made another experiment which showed that
Small Tortoiseshell butterflies are sensitive to red. He painted
their eyes with a red dye which allowed only red light to
enter the eyes. If the butterflies were colour-blind to red
they would have been able to see nothing, but when they
were liberated they flew about with no apparent incon-
venience. It is a pity that he did not repeat his experiment
with other dyes to establish the range of colour vision enjoyed
by these insects. Later experiments have established that the
upper part of the compound eyes in many insects is sensitive
to colour but that the rest is colour-blind. In addition to
these experiments with live animals it has been found that
different colour-sensitive pigments can be extracted from
the compound eyes of both insects and crustaceans and that
they are associated with different types of light-sensitive
cells.

Before leaving the subject of sight it is interesting to note
that very few animals have eyes that make use of another
method of forming an image, namely a pin-hole instead of
a lens. A small hole in a thin sheet of anything opaque forms
an image on a screen behind it, not by refraction as does a
glass lens, but by diffracdon. A pin-hole lens has no focal
length so that objects at all distances in front of it are in
focus on the screen behind ; the greater the distance between
pin-hole and screen the greater the magnification but the
lesser the brightness. The cup-shaped open eye of the limpet
is, as we have seen, probably incapable of forming an image,

l68 THE SENSES OF ANIMALS

for the diameter of the opening relative to the overall size
is too great to act as a pin-hole lens, but the more complex
open eye of the nautilus is one of the very few examples of
an eye which probably does form an image with a pin-hole
and not with a refracting lens.

A pin-hole can be a very useful improvised seeing-aid to
a long-sighted person who has mislaid his spectacles ; if he
makes a pin-hole in a thick piece of paper or thin card and
holds it close to his eye he can read the smallest print through
it perfectly well, provided the light is bright. Mr. Pepys,
who had to give up keeping his diary because of failing sight,
used what he called his "tubes" for reading and writing —
no doubt these were tubes with pin-hole lenses at the eye-
ends, for the use of a tube with a pin-hole increases the
apparent sharpness of the image by excluding stray light
from the sides.

The disadvantage of the pin-hole lens is that it transmits
only a small quantity of light ; if, as we believe, the evolution
of eyes started from light-gathering organs that had at first
no visual function it is not surprising that the refracting lens
has almost universally been adopted for image forming racher
than the diffracting pin-hole.

XV

HEARING

Hearing, like sight, is one of the distant-receptor senses and
is concerned with the perception of vibrations, though of a
very different kind. Unhke the electromagnetic waves of
light, which do not need any physical medium for their
transmission, the vibrations that produce sounds can travel
only in things that can be made to vibrate with them, solid,
liquid or gas. The vibrations can vary in frequency from very
low to very high but it is only certain of them, in the range
from about i6 a second to 30,000 a second at the utmojt,
that produce what we call sounds. They are generally, but
not necessarily, brought to our ears by vibrations in the air.
Vibrations of a higher frequency than our upper limit are
not only inaudible to us but imperceptible unless their inten-
sity is very much greater than normally occurs in nature.
On the other hand we can perceive vibrations of a frequency
below our lower limit, not as sounds, but by generalized
feeling. The vibrations, if strong enough, make our bodies
vibrate too and we become aware of them through our
"proprioceptive" senses which are discussed in a later
chapter.

The ear is a device for converting the sound waves that
reach us into messages that can be sent to the brain, and
for distinguishing between sounds of different wave-length.
The sound waves do not directly stimulate the nerve endings,
but are first converted into touch stimuli which produce the
physico-chemical changes needed to start the message along
the nerves. As may be imagined, a complicated mechanism is
required to carry out these processes. As in dealing with
sight we shall first consider the ear in ourselves and those
vertebrates in which it most nearly resembles ours before
turning to the more different forms in other animals.

The ear in mammals consists of much more than the

169

lyO THE SENSES OF ANIMALS

outside flap or pinna that the word "ear" means in ordinary
talk. The flaps serve to protect the deUcate inner structures
and help to prevent foreign bodies entering the ear-tube.
They also serve as hearing trumpets for concentrating the
sound waves received, and for directing them down the tube.
In many mammals they are also highly movable and help
in locating the source of a sound when they are moved in
different directions. Our own ear-flaps are comparatively
small and immovable, and consequently we have to supple-
ment them with a cupped hand behind when we listen
attentively to faint sounds.

Sound waves pass into the ear-tube from the pinna and
meet the ear-drum, a piece of thin skin that closes the tube

MIDDLE EAR

SEMICIRCUUR
CANALS

COCHLEA

Fig. 10. A diagram of the outer, middle and inner ear in man to show the
chief component parts.

at its inner end. They make the ear-drum vibrate at the
same speed as their frequency, and are passed on and ampli-
fied by the structures on the other side. The inner side of
the drum forms part of the outer wall of a small cavity sur-
rounded by bone known as the middle ear. The cavity is full

HEARING 171

of air which reaches it through a small tube, the Eustachian
tube, connecting it to the back of the mouth. This tube
serves to keep the air pressure the same on each side of the
ear-drum ; a wide difference in pressure would damage the
drum, and even a small difference distorts it sufficiently to
be painful. The tube automatically opens every time we
swallow and that is why we equalize the pressures by swallow-
ing repeatedly when going up or down in a fast-travelling
lift, or rapidly climbing in a non-pressurized aeroplane.
Similarly, if you don't keep your mouth open when near
artillery in action the sudden change of pressure caused by
firing may actually burst the ear-drums.

A chain of three very small bones stretches from the drum
across the middle ear. They are named from their shapes,
the hammer-, anvil- and stirrup-bones, and they form what
Professor Pumphrey has aptly called an impedance-matching
transformer incorporating automatic volume control. They
are joined together m such a way that movement of the
drum caused by the pressure of a sound wave is transmitted
by the hammer, which is attached to the drum by the end
of its handle, through the anvil to the stirrup. The leverage
is such that the stirrup moves through a distance decreased
by one-third, but produces a pressure increased by two-thirds
on the membrane to which it is attached on the inner side
of the middle ear cavity. Further, the joint between the
hammer and anvil is arranged so that it slips apart if there
is excessive movement of the drum, and the inner ear is
protected from damage. Finally two small muscles control
the movements of the bones to some extent, and take up any
backlash between them.

The inner ear in mammals is embedded in bone, and con-
tains the nerve endings that are stimulated by the pressure
waves sent to them from the drum. The hearing part of the
inner ear — there is another part which we shall consider
later — consists of a tube filled with fluid ; in mammals it is
coiled into a spiral of two and a half turns known as the
"cochlea" from its likeness to a snail shell. A ridge of bone
runs along the inner side of the spiral, and two thin mem-
branes extend from it to the outer side so that the main tube
is subdivided into three parallel tubes. The upper and lower

172 THE SENSES OF ANIMALS

tubes join at the tip of the spiral so that they are really one,
and the fluid inside can pass from one to the other. The
other ends of this tube come to the surface of the bone on
the inner side of the middle ear cavity, and both are closed
by membranes stretched across the little windows through
which they look. The membrane that closes the upper tube
is attached to the stirrup-bone whose movements cause a
wave to run up the upper tube and down the lower one.
When this happens the membrane at the other window
bulges out or in as the one driven by the stirrup moves in or
out. The pressure in the outer tubes, the upper and lower
ones, is transferred through their membranous walls to the
fluid in the middle tube, which is completely cut off from
that in the outer ones. And it is in the middle tube that the
nerve endings lie.

The floor of the middle tube contains many thousand
fibres stretching from side to side, and it increases in width

UPPER TUBE

OVERHANGING SHELF
ABOVE HAIR CELL

HAIR CELL

MIDDLE TUBE

NERVE

BONY SHELF

STRETCHED
FIBRES

LOWER TUBE

Fig. 1 1 . a cross-section of one turn of the spiral cochlea, showing the upper,
lower and middle tubes and the bony shelf and the "hair" cells.

HEARING 173

from the bottom to the top of the spiral so that the fibres
become longer as the top is approached. The fibres have
been likened to the strings of a harp, and it has been sug-
gested that they are tuned to resonate to different notes so
that waves of different frequencies in the surrounding fluid
make the corresponding fibres vibrate. At the inner side of
the floor there is a ridge of cells running the length of the
tube under a little shelf that projects over it. The free ends
of the cells in the ridge are covered with minute "hairs"
pointing towards the overhanging shelf; the other ends are
surrounded with a network of nerve fibres. The hair cells
are really touch cells and send an impulse along their nerve
fibres if they touch the overhanging shelf. When waves of
vibration pass through the upper and lower tubes the fibres
in the floor of the middle tube that are tuned to those par-
ticular notes are thrown into vibration so that the hair cells
above them are bounced up and down and the hairs touch
the shelf above. At each touch they are stimulated, a physico-
chemical change takes place in the cell and a nerve impulse
is sent along the associated nerve fibre towards the brain. As
Professor Pumphrey has said, the inner ear is a frequency
analyser of great sensitivity, resolving power and range.

This explanation of the way in which the ear works has
been challenged, and other theories which postulate that
the different frequencies received are sorted out in the brain
rather than the inner ear have been suggested. There is,
however, one anatomical feature that strongly supports the
explanation. The spiral tube is the only part of the body
that is the full adult size in the infant — it is nearly as large
in a new-born baby as in a full-grown man, and it scarcely
increases in size at all during the period of growing up. If
it was small in proportion to the rest of the body in a baby
we should expect children to be deaf to low notes and to be
able to hear notes far above those that an adult can hear.
To a slight extent this is true, for children can hear notes
somewhat above the normal range for adults, but in the
main the range of hearing in a small child is the same as
that in an adult. The reason for this is probably that the
resonant fibres in the inner ears of both are practically the
same length.

174 THE SENSES OF ANIMALS

The fishes possess only an inner ear, but all the vertebrates
higher than fishes have at least a middle ear as well. The
middle ear and the outer ear, where present, represent the
first gill opening of the primitive fishes which, in the sharks
and rays, has already become separated firom the other gill
slits and is represented by the spiracle, a small opening firom
the mouth to the outside just behind the eye. Two of the
three little bones in the middle ear of the mammals repre-
sent some of the bones round the jaw-joint in lower verte-
brates; in the mammals they have been diverted to a new
use. In the vertebrates, lower than mammals, that have a
middle ear — the birds, most reptiles and the amphibians —
there is only one middle-ear bone ; the two others of mam-
mals are still part of the jaw mechanism. This single bone
corresponds to the stirrup-bone in the mammals and con-
nects the ear-drum to the membrane covering the window
in the inner ear. It is a more or less straight rod, but the
outer part of it is generally gristly and consequently not
quite rigid so that it can bend and avoid damaging the inner
ear if excessively loud sounds cause too much vibration.

In birds the inner ear is not coiled and is much shorter
than in mammals — at the most it is slightly curved — but
near the junction of the upper and lower tubes there is a
further patch of hair cells embedded in a cap of jelly that
contains minute limey particles. This is called the "lagena"
and it is believed to respond particularly to low notes ; when
the particles vibrate they touch the hairs of the hair-cells
and thereby start a message along the nerves to the brain.
It is probable that the acuity of the lagena is less than that
of the more complicated part of the ear, and consequently
birds probably distinguish a much greater range of high
notes than of low ones. It is possible, too, that there is a gap
in their range of hearing by the two methods, so that there
is a deaf spot between the upper and lower frequencies.

Birds can obviously hear their own songs and cries — the
intricacy of their songs shows that their auditory acuteness
and discrimination must be at least as good as ours and
perhaps better. Furthermore, the extraordinary power of
mimicry possessed by some birds show that they must be
able to hear extremely well since some — the mynah even

HEARING 175

better than any parrot — can imitate human speech in spite
of the differences in voice production and modulation be-
tween man and bird. It is therefore particularly puzzling to
find that the middle and the inner ear in mammals have
apparently a more complicated structure than birds ; on the
face of it the hearing of mammals should be much better
than that of birds, but observation gives no support to this
suggestion, and the functional difference between the two,
if any, is not obvious.

The value of a distant-receptor sense such as hearing is
greatly increased if, in addition to receiving sounds, it is
able to locate their source. An animal with an ear on each
side of its head can readily locate the direction from which
a sound comes by unconsciously comparing the intensity of
sound received in the two ears ; it may then turn its head so
that the intensity is equal in both and it faces the source.
In addition practice and experience may enable it to pin-
point the source by the difference in intensities without turn-
ing the head. Head-turning depends upon the importance
the animal attaches to the sound; a familiar one may be
unconsciously noted but a strange one may rivet the atten-
tion. The movable ear pinna of many mammals, as already
mentioned, is also a great help in the location of sound
sources.

The physics of a hearing system with two symmetrical
ears, one on each side of the head, is such that the location
of a sound source in the horizontal plane is easy, but it is
impossible to locate a source accurately in the vertical plane
without turning the head up or down and searching. This
arrangement seems to be adequate for most animals —
although we live in a three-dimensional world most land
animals live and move mainly in a two-dimensional one —
but it is not accurate enough for most of the owls. The owls
that hunt their prey at night locate it entirely by ear. Mice
in the grass, however, do not make a continuous noise and
the owl therefore has no chance to move the head and search
in order to pinpoint the source of a slight transient sound ;
it must be able to locate the source immediately on receiving
a brief signal. And the ears of owls are unlike those of all
other vertebrates : they are not symmetrical.

176 THE SENSES OF ANIMALS

Birds have no ear pinnae, for they would interfere with
the streamHned shape so necessary to them in rapid flight ;
the tuft of small feathers that covers and protects the entrance
to the ear-tube leading to the drum is faired into the general
surface so that it is practically invisible as a separate part
of the plumage. In the owls the entrances of the ear-tubes
are modified so that the two sides are asymmetrical, the
degree of asymmetry being greater in some species than
others; in the extreme cases the distortion is so great that
the shape of the skull itself is asymmetrical in this region. The
effect of these arrangements is to make the "polar diagram"
for the reception of sound different on the two sides ; in some
owls the direction of greatest sensitivity is greatest above the
horizontal axis on one side, and below it on the other. The
polar diagrams of symmetrical ears produce an ambiguity
in locating sound sources in the vertical plane, although they
have none in the horizontal plane. The asymmetrical ears
of owls resolve the ambiguity by giving polar diagrams some-
what resembling those that would characterize ears set one
above the other, which would have no ambiguity in the
vertical plane but would have one in the horizontal. The
asymmetry gives the owls the best of both systems, for as the
ears are not symmetrical in either horizontal or vertical
planes they have no ambiguity in either of them. Owls are
thus able instantly to locate the direction of the source of a
transient sound, and to pounce with astonishing accuracy
on the prey which they cannot see. There is a further point
associated with the great sensitivity and accuracy of the
owl's hearing; they would be vitiated by any sounds arising
from the bird's flight. The wings of most birds make plenty
of noise when they fly, for example the hum of humming-
birds, the whirr of the wings of small perching birds and
game birds, the swish of falcons and eagles, and the music of
the swan's pinions. The plumage of owls, however, is so
soft that no sound comes from their wing feathers when they
fly, nor is there any sound from the body feathers as the
stream of air flows over them. Owls are not particularly
swift or strong fliers, and no doubt they have sacrificed some
flight efficiency in exchange for the silence essential for their
method of hunting by ear.

HEARING 177

The ears of the reptiles, the amphibians and the fishes are
simpler in structure than those of the mammals and birds.
In the first place not only do none of them possess an ear-
pinna, but none of them possesses an external ear ; the ear-
drum, in those that have one, is on the surface and practically
continuous with the general surface of the skin — at most it
lies in a shallow depression, except in the crocodiles where it
is protected by a movable flap of skin. The inner ear, too,
is less complicated, for there is no cochlea, and the sound-
receiving part is the lagena alone. It is thus probable that
the hearing in all these animals is much less efficient in
discriminating between the quality of different sounds
than in the mammals and birds, and that it is less sensitive
to the higher notes. For all that, the hearing of some of these
creatures must be very much more than rudimentary — most
frogs are noisy creatures, at least during the breeding season,
and all species have their own characteristic songs, some of
them with sweet bell-like notes. The hearing of lizards
appears to be acute ; though they are not vociferous animals
some of them have voices. Although snakes have no ear-
drum they are probably not, as popularly supposed, deaf,
but then fishes have no ear-drum and they are certainly not
deaf.

The fishes have neither external nor middle ear, no ear-
drum nor special bone to conduct sound to the inner ear.
The inner ear is primitive compared with that in the other
vertebrates, for only a small part of it is concerned with
hearing. This part is the lagena, a patch of hair-cells covered
with jelly containing limy particles called the otoliths or ear-
stones. In the sharks and rays the stones are minute and
numerous, but in the ordinary scaly fish they are large and
few, though only one, and that a small one, lies in the lagena
— the rest may serve other functions as we shall see later.
Water is a better conductor of sound waves than air because
it is incompressible. The body of a fish consists of over ninety
per cent water and consequently sound waves are conducted
directly into the inside of the fish without the necessity for
any amplifying mechanism to convey them to the ear. Vibra-
tions carried directly to the inner ear in this way are sufficient
to agitate the otoliths and stimulate the sensory cells into

178 THE SENSES OF ANIMALS

sending nerve impulses to the brain, though some fishes
mentioned below have a modification that greatly increases
the efficiency of hearing.

The range of hearing in fishes is almost certainly not as
great as in other vertebrates; it is likely that although the
hearing of fishes may be acute, it is not very discriminatory
so that all notes sound much the same to them. It is easy to
show that fishes are not hard of hearing by gently tapping
the side of an aquarium, but the sounds to which fish respond
need not originate under water for fish in ponds very soon
learn to come to be fed in response to the ringing of a bell
or other sound signal given from the bank.

Returning for a moment to snakes, we can understand
that they probably hear much as do fishes. It is true that
they live in air and that air-borne sounds are not likely to
be distinctly appreciated, but vibrations, especially those of
longer wave-length, may well be transmitted to them through
the ground, with which they are in closer contact than most
vertebrates. Although snakes have no ear-drum the middle
ear is not wholly degenerate, for the little rod of bone is
present, albeit embedded in muscular and fibrous tissue, and
vibrations of sufficient strength to be conducted through the
tissues will reach the inner ear as they do in fishes.

Throughout the vertebrates the basic pattern of ear struc-
ture is the same, and consequently if we exercise due caution
we may legitimately draw inferences about the hearing of
other vertebrates from our own subjective perception of
sounds. In the invertebrates on the other hand such assump-
tions cannot be so easily made. Most of the invertebrates
respond to air- or water-borne vibrations, but it is impos-
sible for us to guess the nature of the sensations produced
in them by those vibrations. In many there are no organs
whose function we can identify as being concerned with
hearing; in many others which have organs consisting of a
small bag of sensory cells associated with one or more
otoliths the function generally appears to be associated
with balancing or orientation in space rather than with
hearing, although vibrations impinging upon them would be
expected to impart motion to the otoliths. There are some
invertebrates, particularly among the insects, that un-

HEARING 179

doubtedly do hear sounds; they emit sounds, and can be
observed to respond to them.

The noisiest insects are the crickets and grasshoppers, and
the unrelated cicadas ; the well-known songs of these insects
appear generally to be produced by the males, and serve
to attract the females to them. It is in these insects, too, that
auditory organs are most highly developed. In the grass-
hoppers the "ears" are contained in the middle sections of
the front legs. They consist of rows and clusters of sensory
cells connected with nerve fibres lodged in cavities which
communicate with the outside through small slits. The
sensory cells are graduated from small to large along the
length of the row, and present an appearance reminiscent
of the arrangement of fibres in the cochlea of a vertebrate.
It is indeed possible that they are tuned to respond to sounds
of different wave-lengths. However this may be, the insects
do not appear to have very acute discrimination for it is
possible to attract the females of many species with regularly
repeated chirps of wave-lengths widely different from those
produced by the males. In the cicadas the song is so astonish-
ishingly loud that it is difficult to believe that the sense of
hearing is particularly acute, especially as the auditory organ
is closely associated with the vocal organ situated on the
underside of the body. No one who has not heard it can
imagine the deafening noise that can be produced by some
of the tropical cicadas ; it is usually produced by the males
when they are sitting on a tree-trunk, but at least one
Brazilian species can sing also when it is flying about. The
female has an auditory organ like the male's, but she has
no sound-producing structures and consequently is dumb —
"Happy the cicadas' lives, for they all have voiceless wives."
One must suppose that the male's ear is useless to it while it
is singing for it must be impossible for it to hear anything
above its own voice.

Many other insects that are not known to make any sounds
apart from the buzzing of their wings have structures that
appear to be auditory organs in various parts of their bodies ;
many moths, for example, have such structures in their
thorax or abdomen. It is possible that these insects make
sounds of so high a frequency that they are inaudible to us,

l8o THE SENSES OF ANIMALS

and there is evidence that some moths can "hear" such
frequencies, as we shall see in the next chapter. On the other
hand some tropical butterflies [Ageronia] make a loud click-
ing noise, like that made by drawing a finger along the teeth
of a comb, as they fly — not a continuous sound but short
bursts at irregular intervals. No structure that can be identi-
fied as a hearing organ has been identified in any butterfly,
and yet it is impossible to believe that the production of
this sound is without any meaning in the biology of these
creatures.

On the other hand insects possess many structures that
are obviously sense organs, but whose function is unknown.
These "sensilla" take the form of rods of cells, often of
considerable complexity, associated with nerve fibres. They
are found on almost any part of the surface of the body
and may be lodged in pits, or project as hairs, or merely lie
in contact with the cuticle. Some certainly serve the sense
of touch, others probably those of smell and taste, but some
may perhaps serve the sense of hearing. Whatever the truth
may be it seems probable that when insects perceive sound
vibrations the majority do so more by the sense of touch than
of hearing as we know it. Even the most highly elaborated
insect ears cannot, from their structure, be supposed to
analyse sounds of different wave-lengths as can the ear of a
vertebrate nor, from a consideration of the insect's nervous
system, could they be expected to do so. There is not room
in an insect for the vast number of nerve cells that make
up that part of the brain which interprets the messages
received from the ear in a vertebrate.

The fishes possess, in addition to the ear, another means
of perceiving vibrations or changes of pressure in the water
surrounding them. We have no such sense and consequently
it is impossible for us to know the nature of the sensations
produced by it. Nevertheless experiment has revealed much
of the function of this sense in the life of fishes. In most
fishes it is easy to see a line running along the side of the
body from head to tail. A small tube filled with fluid runs
under the skin beneath the line and is connected with a
number of branched tubes that ramify over the head. The
tube — the lateral-line canal — opens through the skin by

HEARING

l8l

Fig. 12. A Lung-Fish {Lepidosiren paradoxa from ihe swamps of South America)
showing tlie lateral line and the tubes connected to it ramifying on the head.

numerous small holes, and contains many sensory cells that
are really touch receptors. These structures respond to
vibrations of very low frequency, vibrations too slow to be
perceptible to us as sound, and may give an effect of "touch
at a distance". By this means fishes probably become aware
of objects in the water as they approach them. J. R. Norman,
quoting Dr. Barton, says, "this sense enables a fish to rush
about in a rocky pool, swerving this way and that to avoid
obstructions, for the water resistance is the greater the nearer
such water is to a solid body." In trying to understand the
nature of the lateral line sense it is relevant to note that in
the course of evolution the ear was derived from one of the
sense organs of the lateral line.

In the sharks and rays there are other sense organs in the
skin covering the head that may have a similar function.
These consist of numerous pits opening on the surface by
pores, but not connected to each other by canals. Their
internal structure is similar to that of the lateral line organ,
but their exact function is not known.

XVI

ECHO-LOCATION

Fifty years ago the only way for a sailor to find the depth
of the water on which his ship was sailing was to let down
a line with a weight on the end until it touched the bottom :
the length of line paid out told him the depth. Although this
method was rather slow and cumbersome it worked well
enough in relatively shallow water, but it was extremely
difficult to use it successfully in really deep water. Nowadays
no one would dream of wasting time with sounding-lines, for
machines have been invented that can give a continuous
record of the depth of water below a ship without letting
down anything over the side. The echo-sounder which began
to come into use in the early nineteen-twenties was the
answer to one of the sailor's major problems.

The principle of this machine is very simple. If the hull
of the ship is tapped sharply the sound travels away in all
directions and some of it reaches the bottom whence it is
reflected back as an echo. The echo can be heard if a suffi-
ciently sensitive microphone is used in the ship. If the time
interval between sending out the sound and receiving the
echo is noted then it is easy to calculate the distance the
sound has travelled out and back because the rate that sound
travels in water is known — it is about 3,150 miles an hour,
or 4,700 feet a second, much faster than its rate of approxi-
mately 700 miles an hour in air. In practice the machine
does the listening and calculating so that the sailor need
only read the depth on a scale, or watch the profile of the
bottom being continuously drawn on a chart. It was the
invention of sensitive microphones and electronic amplifiers
that made this device possible.

Although the principle of the echo-sounder was invented

by man and developed by the aid of his civilized technology,

the same principle had been, all unbeknown to him, in use

183

ECHO-LOCATION 183

for millions of years by several sorts of animals. In these
animals, too, it had been brought to a pitch of perfection
that man has not yet been able to imitate, and in them the
apparatus is packed into a space so small that his electronic
instruments appear absurdly cumbersome in comparison.
Furthermore it is rather surprising that until man made his
inventions he had barely an inkling of the existence of echo-
location in the animals, whereas if he had discovered what
the animals were doing he might have made his inventions
nmch sooner, though it would have been difficult for him to
develop them for practical use until technology had provided
him with the means.

The animals that we know make use of echo-location are
the bats, the whales and dolphins, and some of the birds —
there may of course be many others in which it is awaiting
discovery. The way of life of all these animals is such that
they are active at times or in places where the sense of sight
is diminished or impaired ; the bats fly and hunt by night,
the whales and dolphins live in the sea where even in clear
water the brightness of the light rapidly diminishes with
depth below the surface, and in turbid water is often ex-
tinguished at very moderate depths, and the birds that use
echo-location build their nests in the darkness of extensive
caves. These animals are able, without seeing, to find their
way about, to avoid obstacles, and some of them at least to
catch their prey. They not only locate the objects in their
surroundings and know their distance and direction, but get
information about their size, shape and, doubtless, many
other details.

Soon after man's invention of radio-location was brought
inco practical use it was given the convenient made-up name
of "radar", and when the analogous use of sound echo-
location by animals was discovered it was no more than
natural that it should be called "sonar" — a name which is
now in universal use. The essence of sonar is generally the
giving out of very short, abrupt bursts of sound which can
bounce back as echoes, although it is now known that there
is also another way by which location can be achieved. In
the first method, throwing out short bursts of sound and
catching them on their return can be compared to throwing

184 THE SENSES OF ANIMALS

an india-rubber ball against a wall and catching it as it
bounces back ; the shorter and more cleanly cut off the bursts
the less risk there is of them becoming blurred and distorted
in the process. They must be repeated at intervals such that
the echo of one burst arrives back before the next burst is
given out, for if more than one is in the air at the same time
it would be impossible to know which echo belonged to
which emission. For this reason the nearer an object is the
more quickly the bursts can follow each other because the
distance the sound has to travel is less.

In most sonar the frequency of the sound waves used is
very high, so high that they are inaudible to human ears,
though perfectly audible to the animals that use them. There
are several advantages in using high frequencies, for one
thing they are reflected better than lower ones from very
small objects. To us, whose main sense is that of sight, and
whose whole way of life is centred round sight, the accuracy
and efficiency of sonar is almost unimaginable. Bats can fly
in the dark among a forest of obstacles without colliding
with them, and can pursue and capture small insects in rapid
flight, twisting and dodging to catch them with astonishing
speed and accuracy.

The story of bats' sonar has been elucidated chiefly by
Griffin and his colleagues and others in America. Their first
experiments were simple and confirmed what had long been
known : that blindfolded bats are not handicapped in flight.
They then showed that if a bat's ears are covered it is not
able to avoid collisions when flying, and is in fact very
reluctant to take to the wing at all. If only one ear is covered
the bat can fly with moderate success but makes some col-
lisions, showing that its efficiency in locating its surroundings
is impaired. These experiments proved that bats are made
aware of neighbouring objects that they cannot see by means
of sound waves reflected from them. When the experimenters
covered the mouth and nose of the bats, but left the ears
uncovered, they found the animals were unable to fly without
collisions, proving that the sound waves reflected from objects
are produced by the vocal apparatus of the bats themselves.

On making a closer investigation the experimenters found
that bats make ultrasonic squeaks at frequent intervals nearly

ECHO-LOCATION 185

all the time. The frequency of the vibrations given forth
varies, but is most commonly about 50,000 a second ; at this
pitch each squeak lasts for about two-hundredths of a second
or less. When bats are at rest they make ultrasonic squeaks
about ten times a second, but as soon as they start flying the
squeak rate goes up to about thirty a second ; the faster the
squeak rate the more information about the position of
surrounding objects is obtained in a given time — and a bat
in flight needs the information much more urgently than one
at rest. If a bat flies through a narrow aperture the squeak
rate becomes faster and faster as the bat approaches it, and
then returns to normal when it is safely through. (See
Plate 12.)

Bats are often gregarious creatures and in some places they
roost in enormous numbers huddled close together in the
roofs of caves. When they take to the wing and start giving
out ultrasonic sounds there might appear to be a danger that
this babel of high-frequency sound would cause complete
confusion and put the sonar out of action. Two things prevent
this occurring: first, ultrasonic waves do not travel far in
air, but quickly die away just as ordinary sound gets fainter
and finally fades out as distance increases. The distance from
which the ultrasonic squeaks of bats are able to return a
useful echo is only about five yards, so that unless the bats
are close together they do not interfere with each other's
sonar. This probably also explains the amazingly quick
swerves and aerobatics that bats indulge in, for they have to
be very quick to turn when flying at full speed if they get
warning of an impending collision when they are only five
yards or less from an object. The second thing that prevents
confusion is that the frequency of the vibrations made by
any two bats is not necessarily the same, in fact it is extremely
unlikely that they will be of exactly the same pitch ; a dif-
ference of very few vibrations a second is probably enough
to distinguish them, so that if many bats are flying together
there is no danger of confusion because each will recognize
its own voice.

The matter is not, however, quite as simple as it may
appear because the squeak made by each bat is not generally
of constant pitch or frequency, and so a bat cannot recognize

l86 THE SENSES OF ANIMALS

its own voice as a definite pitch differing if only a little from
that of others. The squeak of most kinds of bats sweeps
through a very wide range of frequencies in a five-hundredth
of a second, falling from a high note to a lower one. This
frequency modulation probably helps the bats' power of
discriminating the echoes returned from objects at different
distances.

As mentioned above, there is another way of locating
objects by means of reflected sound waves, provided the
object and the locator are moving with relation to each
other. If you are standing on the platform of a railway
station and an express train blowing its whistle dashes
through without stopping, the note you hear appears to rise
in pitch to a peak as the train approaches and then to fall
away again as it recedes. This is not just because the noise
is louder when it is nearer, for the note given out by the
whistle is constant in pitch. This "Doppler effect" is caused
by the source of the sound being in motion ; if the note
emitted has, say, a frequency of 5,000 a second, more than
5,000 vibrations a second reach your ear when the whistle
moves towards you and less when it moves away. The sonar
system of certain kinds of bats makes use of the Doppler
effect. These are the Horseshoe bats, so named from the
presence of a bare flap of skin, shaped something like a
horseshoe, on the face surrounding the nostrils. The sonar
squeaks of the Horseshoe bats, unlike those of the others we
have already discussed, are not frequency-modulated but
consist of practically a single frequency. The squeaks or
bursts of sound last much longer than those of the other bats
and may occupy as much as a tenth of a second instead of
one two-hundredth at most. There is, too, another difference :
the Horseshoe bats emit their squeaks through the nose,
whereas the others emit theirs through the mouth; and the
horseshoe of skin, or "nose-leaf", acts as a trumpet to con-
centrate the ultrasound into a narrow beam which can be
likened to that of a searchlight. The Horseshoe bat twists
and turns its head about, directing the beam in different
directions as it scans its surroundings, and by means of the
Doppler effect it is able to fill in the details of the information
it obtains by the direct reflection of echoes. (See Plate 13.)

ECHO-LOCATION 187

We have likened the sonar of bats to the radar of man,
which was first developed for use in war, for locating hostile
aircraft in the dark or when far beyond the range of vision
— radar has since proved to have uses equally valuable
to the arts of peace. In war as soon as any new device
is produced by one side the other seeks means for frust-
rating it, and many different ways were invented for
jamming or otherwise rendering radar useless. Surprising as
it may seem, it appears that nature too has long ago invented
countermeasures to combat the sonar of bats, for it is now
reported that certain moths have ears sensitive to ultrasonic
vibrations, and that they take violent evasive action on being
"looked at" by a bat's sonar system so that the moths can
escape; man has not been alone in inventing air-borne
"anti-bandit" apparatus! Perhaps we may yet find that
some moths actually have a system of jamming the bat's
sonar.

Few people have ever heard a whale's voice, and until
recently zoologists believed that whales were completely
dumb. Yet Arctic whalers had long known that at least one
kind of whale, the Beluga or White Whale, sometimes makes
sounds audible to men on the deck of a sailing ship. When
these creatures were near or underneath the ship a whistling
musical trill was heard so often that the whalers nicknamed
them "sea canaries". Zoologists considered that the sounds
were probably made by the whales releasing a fine stream
of bubbles from the blowhole, and that they were not a true
voice.

The widespread use of hydrophones for undersea listening
during the war showed that the popular conception of the
"silent depths" of the ocean was far from correct. Many sea
creatures, from shrimps and crawfish to fishes, dolphins and
whales, keep up a chorus that sometimes rises to little short
of a general uproar — to ears that can hear it. Animals that
make noises can, presumably, also hear them. Here again
the whalers have long known how easily a nearby whale is
frightened away by a sudden sound, as of a bucket dropped
on deck. Asdic, another war-time device used for echo-
locating underwater objects by ultrasonic vibrations, has
given us more knowledge about whales, for it was soon

l88 THE SENSES OF ANIMALS

noticed that dolphins and whales are frightened away when
this apparatus is in use. It is likely therefore that whales
can hear not only ordinary sounds but those of far higher
frequency than those audible to human ears.

In recent years techniques have been developed for keep-
ing the smaller whales in captivity in aquaria larger than
many swimming baths — "oceanarium" is the name invented
for them in America. These fascinating marine zoos provide
a splendid chance for zoologists to get at close quarters with
animals that have for so long been remote and elusive ; and
as might be expected much new knowledge has been gathered
as a result.

In the first place it was soon evident that whales are highly
intelligent creatures compared with many other mammals;
this is perhaps due to the relatively large size and complex
convolutions of the whale's brain. Then they are unex-
pectedly gentle and docile, and gain confidence in their
keepers within a few days — they are, too, unexpectedly and
sometimes embarrassingly philoprogenitive. They are quick
learners, and, without any coercion, easily learn circus tricks
for the amusement of visitors to the oceanaria; one was even
broken to harness and trained to tow a surf-board with a
bathing belle riding on it. Moreover they invent games for
their own amusement, such as a sort of water polo with
objects dropped into the pool— one dolphin took a mis-
chievous dislike to women dressed in black, and many a nun
visiting his oceanarium left soaked to the skin after he had
greeted her with an unexpected splashing. (See Plate 14.)

Zoological experiment soon followed simple observation,
and it was established that whales produce a wide range of
noises and can hear ultrasonic vibrations up to 80 kilocycles
a second — the upper limit of human hearing is about 20
kilocycles a second in adults. Some years ago the late Arthur
McBride, when curator of the oceanarium in Florida, noted
evidence that sound is used by dolphins in navigation. He
found that the animals cannot be caught in a fine-meshed
net but jump over the line of corks supporting the headrope.
They can, however, easily be caught in a net of ten-inch
square mesh, but as soon as one is trapped and drags the
headline with the corks under water, the others at once make

ECHO-LOCATION 189

for the gap thus produced and escape. They are able to do
this in turbid water and on moonless nights, and McBride
inferred that they were locating the obstructions by echo-
sounding, by listening to their own voices reflected from the
fine-meshed net or the corked line. Experiments to test this
suggestion have amply demonstrated its truth, and it has
been shown, particularly by Schevill and Lawrence in
America, that dolphins can quickly find a fish no larger than
a herring in water so turbid that light can penetrate less than
a yard. The sonar system of whales is analogous to radar,
and consists of a series of impulses whose echoes give infor-
mation about the range and bearing of the objects scanned.
The series of ultrasonic pulses is made at repetition rates of
less than ten to over four hundred a second ; although the
ultrasonic component is inaudible to human ears the series
of impulsive clicks produces a note that can be heard, the
slower rate sounding like knocks, the faster like snarls, whines
or the creaking of rusty hinges. The accuracy of direction-
finding is aided by the whale weaving the head from side to
side as it approaches an object, so that it can orientate itself
with the signals received by each ear exactly balanced ;
furthermore the pulse repetition frequency is increased as
the animal approaches an object and the time for the return
of the echo becomes less.

The sonar of whales is surprisingly subdued, and it is often
difficult for underwater listening apparatus to distinguish it
from the background noise. But whales make other sounds
that are far from subdued — whistles, grunts, groans and the
liquid trills of the sea canaries. These sounds are communica-
tive, conversation if you will ; they are rarely uttered by
solitary captives, but two or three animals kept together are
very loquacious, and the hubbub made by a large school of
dolphins is astonishing.

Nearly all the observations on whale voices have been
made on the toothed whales, because this division of the
Cetacea, although it includes the mighty sperm whale, con-
tains the smaller species that can be kept in captivity; the
whalebone whales that feed by filtering plankton from the
sea are so huge that no oceanarium is large enough to hold
them, even if they could be captured unharmed. Seldom

igO THE SENSES OF ANIMALS

has any sound been heard in the wild from a whalebone
whale, and there was no evidence that this was being used
in echo-location. But when one remembers the necessity of
avoiding collision with the bottom and submerged objects
when travelling at high speed, or in great depths where it
is completely dark, or at night, the probability that the
whalebone whales, too, have a sonar system is high. Much
yet remains to be discovered, not only about whales, but
about other sea creatures; it has just been found that some
seals have a sonar system, and they also can earn their
livings in turbid water where no light can penetrate.

A few years ago a "tame" dolphin appeared off one of
the New Zealand bathing beaches and fraternized with the
swimmers ; it even permitted small children to ride astride
its back. The legends of the ancients about the affection of
the dolphin for mankind may not have been without founda-
tion. Whales have no facial muscles and consequently they
are unable to alter their expression. In view of the intelli-
gence and playfulness revealed by the study of dolphins in
oceanaria one wonders if it is entirely fortuitous that the
dolphin's mouth is so curved that it appears to human eyes
to be set in a fixed and slightly mischievous grin. (See
Plate 15.)

The skull of the whales difTers widely from the usual
pattern in mammals ; the relative sizes and positions of the
bones forming it are much modified, and in many species
they show a high degree of asymmetry. There is also a
complicated system of air spaces situated around the base
of the skull and connected through the Eustachian tube with
the mouth and lungs. The bones of the middle ear are much
modified in shape, as is also the membrane of the ear-drum.
All these structures are so unlike those in the other mammals
that zoologists formerly thought that the middle ear in these
animals could not function for the transmission of sounds,
and that whales and dolphins heard by bone conduction, that
is, that sound waves penetrated the body of the creatures
and set up vibrations in the bones of the skull from which
they were transmitted directly to the inner ear. Fraser and
Purves have recently made a detailed study of the hearing
sense of whales, and have shown that the bone-conduction

ECHO-LOCATION igt

theory is quite erroneous; although the drum and middle
ear are unusual in shape and details they function as in other
mammals, and hearing is acute and by far the most impor-
tant of the special senses to these animals.

Whales, like all living creatures, are composed of a very
high proportion of water, and consequently water-borne
sound waves tend to pass through them without interruption
— they are in fact almost transparent to sound. Sound waves,
however, find the junction of air and water an almost im-
passable barrier; about ninety-nine per cent of sound is
reflected at the meeting of the two media. The air spaces
below the skull of the whales, connected with the middle ear
by the Eustachian tube, probably function, in part at least,
by making use of this effect; they stop the sound waves from
going straight through the animal by translating them into
air waves that can be sent on to the inner ear through the
action of the drum and the middle ear.

It is significant too that in the dominant group of fresh-
water fishes, the Ostariophysi, which includes the Carp,
Roach, Tench, Bream, Cat Fishes and many others, the air-
filled swim-bladder is connected to the inner ear. The swim-
bladder is primarily a hydrostatic organ which adjusts the
buoyancy of fishes so that they tend neither to sink nor to
float and so saves the necessity for muscular effort in main-
taining constant depth. In many fishes part of the swim-
bladder is arranged so that it lies in touch with the fluid-filled
spaces that fill the inner ear, but in the Ostariophysi there
is a chain of little bones, derived from the first four joints of
the backbone, which connects the front end of the swim-
bladder with the inner ear. The similarity of these bones
to those of the middle ear in mammals is striking and it
is almost beyond question that their function is similar.
The water-gas interface of the swim-bladder must certainly
act as an interceptor for the sound waves that would other-
wise pass through the fish, and the vibrations produced are
transmitted by the little bones — the "weberian ossicles" to
the inner ear. Hearing in these fishes must be very much
more acute and discriminatory than in those which have no
such arrangement. Sharks and dogfish, which have no swim-
bladder and keep themselves from scraping on the bottom

192 THE SENSES OF AxNIMALS

by the angle of attack of their large front paired fins acting
like the planes of an aircraft, can perceive only those sounds
that act upon the otoliths of the inner ear direct. They must
therefore have a much less efficient sense of hearing, for the
greater proportion of the sound waves reaching them must
pass through them with little resistance and travel on without
being perceived.

Few kinds of birds make use of sonar, and those that do
appear to confine their sounds entirely to the audible part
of the sound spectrum (to human ears) and not to use ultra-
sonic vibrations. These are birds that nest far inside extensive
caves where no light penetrates ; no nocturnal bird is known
to make use of sonar in finding its way about or hunting its
food — though perhaps there are some as yet to be recognized.
Those that have been most closely studied are the pretty
little swiftlets of Borneo and Malaya, some of which make
the much prized edible birds' nests. These swiftlets nest in
enormous numbers in the huge caves of their native lands
and are able to find their way about, and to home on to
their own nests, in complete darkness by means of a sonar
system using sounds of a very low frequency when compared
with that of bats. A very convincing and simple experiment
was made on these birds by Lord Medway. He released some
of them in a room at night and they flew about in the dark
without colliding with the walls or each other, all the time
keeping up a continual twittering. The moment the electric
light was switched on the twittering stopped although the
birds cont'.nued their flight, reverting to sight instead of
sonar for finding their way. A tape recording made at the
time is a beautiful record — every time the light is switched
off" the twittering starts and every time the light is switched
on silence descends. Another bird that uses sonar is the
peculiar Oil bird, Guacharo, or Diablotin, of Venezuela and
some of the West Indian islands. The diablotin is about as
big as a crow and has a wide mouth beset with bristles some-
what like that of the nightjar; it is completely nocturnal
and sleeps all day in deep dark caves where it congregates
in large numbers. In the evening it awakes and "with croak-
ing and clattering that has been likened to that of castanets,
it approaches the exit of its retreat, whence at nightfall it

ECHO-LOCATION I93

issues in search of its food, which so far as is known, consists
entirely of oily nuts or fruits . . . some of them sought, it
would seem, at a very great distance, for M. Funk . . . states
that in the stomach of one he obtained at Caripe he found
the seed of a tree which he believed did not grow nearer
than 80 leagues." It has been shown that these birds find
their way about in the darkness of the crowded caves by
means of sound echo-location. (See Plates 17 and 18.)

Finally there remains to be mentioned another method of
orientation analogous in some ways to sonar, but which
makes use of pulses of electrical energy to create an electric
field that informs its possessor about its surroundings. This
is found in some of the fishes, and its existence was discovered
only a few years ago. It has long been known that some
fishes, such as the torpedo and the electric eel, possess electric
organs that can give a shock strong enough to stun their
prey; it has equally been known that others, such as the
skates and rays and some of the fresh- water fishes of Africa,
have much smaller and less powerful electric organs, the
function of which was unknown. The electric organs in some
fishes are formed from highly modified muscular tissue, and
in others from similarly modified glandular tissue. When-
ever a muscle fibre in any animal contracts an electrical
change is produced in it; it is therefore conceivable that in
the course of evolution some muscle fibres may have lost
their contractile function and become specialized for the
production of electricity. A similar argument can be applied
to glandular tissue. But the evolution of electric organs has
been a great puzzle to biologists, for it seemed impossible
that the powerful organs of the torpedo and electric eel could
have evolved by a single mutation, and yet no reason could
be seen for their evolution by smaller stages because no
function could be found for the comparatively feeble or
''rudimentary" electric organs of other fishes.

The riddle was solved when Lissmann discovered that the
low-powered electric or^ns of certain fishes which live in
the turbid muddy waters of some of the rivers in Africa have
a very important function. These fishes generate pulses of
electrical energy which produce a symmetrical electric field
around them when they are floating in mid-water, but which

»3

194 THE SENSES OF ANIMALS

is distorted by nearby objects when they are near the bottom
or the bank. The distortions are reflected to the fish, and
appear to be received by the sense organs of the lateral line,
so that the fish is made aware of its surroundings although
it cannot see them. If suitable electrodes are put into the
water where these fish are swimming, and are connected
with an amplifier, each electric pulse can be translated into
a sound so that the pulses are heard as a series of clicks.
Once this discovery had been made the evolution of the more
powerful electric organs of the torpedo and electric eel be-
came comprehensible ; moreover it was found that although
these fishes can give a strong shock occasionally they are in
addition continually giving mild ones wherever they move
about, in order to probe their surroundings. The electric
fishes have indeed anticipated man, for by using electro-
magnetic waves instead of sound for echo-location they have
evolved a form of true radar.

The sense of sight is so dominant with us that it is difficult
for us to imagine how the world must appear to all the
animals that get their main information about it not by sight
but by echo-location. Is there any reason why it should
appear materially different? On the whole there seems to be
none, for it is not the means by which signals are received
from the outside world that matters, but the mental image
that is built up in the brain as the result of decoding the
signals. And it is obvious from the behaviour of these animals
that their mental image built up from the receipt of echoes
is about as complete and detailed as the image we build up
as a result of light falling on the receptor cells at the back of
our eyes.

XVII

SMELL AND TASTE

The sense of smell is the third of the distant-receptors that
we have to consider, but it is so closely associated with the
sense of taste that the two must be dealt with together. Both
smell and taste are chemical senses and function by minute
particles of odorous or tastable substances coming into
contact with the cells in the end organs of the nerves serving
these senses. We have less detailed knowledge of these senses
than we have of sight and hearing for several reasons ; in
the first place our sense of smell, though it may be very
acute in detecting some odours, is not a sense of vital impor-
tance to us and is not nearly so highly developed as it is in
many other animals. Secondly there is no objective method
by which we can classify or measure smells — at the most we
classify them as pleasant, unpleasant or indifferent — and for
the rest we describe them in terms of other senses such as
sweet, sour, heavy or smooth, or liken them to the odours
of familiar things, as when we say a substance has the smell
of violets or of rotten eggs.

In vertebrates the olfactory nerve-endings lie in the nose
and in mammals they lie in the upper part of it. The cavity
of the nose is divided into right and left halves by a partition
in the middle, and much of the cavity is filled by three thin
sheets of bone (the turbinal bones) attached to the outer side
and rolled up like scrolls with the ends facing out and in.
The degree of development of the scrolls varies widely in
different kinds of mammals ; in some they are very com-
plex but in others comparatively simple or even almost
non-existent. Their chief function appears to be that of
warming and moistening the air breathed in, and of filtering
out from it any particles of floating dust. The lower ones at
least are not particularly concerned with the sense of smell,
for the nerve endings of the sense are concentrated in the

196 THE SENSES OF ANIMALS

upper part of the nose cavity. The elephant has a very acute
sense of smell yet the scroll bones in its nose are almost
rudimentary and the long trunk performs the function of
warming, moistening and filtering the air breathed in. The
lower part of the nose-cavity is the passage used in breathing
and odorous particles diffuse from it into the upper part
where they produce the sensation of smell when they touch
the receptor cells. If we consciously try to smell a scent we
sniff, to draw air up into the part where the receptors lie.
So, too, do other animals — we have all seen a dog scenting
the air, holding its head up and turning this way and that
as it sniffs about to pick up smells wafted to it.

The receptor cells for smell are nerve cells lying in the
membrane lining the nose-cavity with their outer ends reach-
ing the surface where they are divided into several minute
hair-like threads ; the lining surface of the membrane is kept
moist by the secretion of numerous small glands. Odorous
substances are those that give off a vapour which can be
drawn into the nose with the air and that will dissolve in
water or oil, so that they can come into contact with the
nerve endings and act upon them chemically to start a
message on its way to the brain. Things that produce no
vapour, or which produce a vapour that is insoluble, give
no sensation of smell. It is not necessary for any great
quantity of vapour to be given off, for some substances con-
tinue to be strongly scented for years with no measurable
loss of weight and others are perceptible even to our noses
in extremely high dilution — it is said that we can detect
musk diluted one part in eight million parts of air, and the
evil-smelling substance mercaptan diluted one in twenty-five
thousand million. One wonders just how many, or rather
how few, molecules are needed to stimulate a single nerve-
ending of smell.

Recent research has shown that in mammals some of the
cells in the lining of the nose contain free vitamin-A and
protein-bound carotenoids. The latter are probably the sub-
stances that receive energy from molecules of odorous matter.
The visual purple of the eye, rhodopsin, is also a protein-
bound carotenoid, and the action of light upon it causes a
geometrical isomeration or rearrangement of the shape of

SMELL AND TASTE I97

its molecules. It is probable that the action of molecules of
odorous substances causes a similar molecular change in the
carotenoids of the nose and starts a message travelling along
the nerve to the brain.

When the receptor cells of any sense are stimulated they
generally respond only to the "correct" stimulus; for
example, the eye receptors respond only to light and those
of the ear only to sound : conversely the eye is deaf and the
ear is blind. The messages that pass along the nerves from
the receptors are, however, always of the same nature and
can be recorded as changes in electric potential. The brain
must therefore sort them out so that any message coming
from the eye means "light", or from the ear means "sound".
This is well illustrated when you get a blow in the eye and
"see stars" — the pressure resulting from the violent blow
has stimulated the receptors in the retina, but because the
message received by the brain comes along the eye nerve it
interprets the message as light and not as pressure. The
restriction of the receptors to responding to only one sort of
stimulus is carried further, as we shall see, in the sense of
taste, where each kind of taste has its own receptor. Whether
there are separate receptors for each of the infinite variety
of smells we do not know, but it does seem necessary that
there should be, because it is difficult to imagine how dif-
ferent substances stimulating the same receptor could be
distinguished by the brain from the message received : on
the other hand, although we cannot classify smells it is pos-
sible that the receptors in our noses can; there may be a
limited number of basic smells and the variety may be pro-
duced by blending two or more with the result that most of
them are "chords". The nose might then perceive them
somewhat as the ear perceives musical chords, the brain
receiving a number of messages from as many receptors and
combining them to give perception of a single specific odour.
It may not be without significance that the language that
perfumers use in describing the scents produced by their art
borrows many of its terms from that of musicians.

The sense of smell is of great importance to most mammals
in seeking and testing their food, in recognizing each other
and the trails left in passing, and the scent deliberately laid

igS THE SENSES OF ANIMALS

to mark out territories, and in receiving warning of the
approach of enemies. It is obvious, as Maxwell Knight has
pointed out in Part I, from the behaviour of the other
mammals that their sense of smell is very much more delicate
and has immensely greater powers of discrimination than
has ours. In man the sense of smell has, in a large measure,
fallen into disuse and lost its accuracy ; in our civilization it
has even become a somewhat disreputable sense. It is per-
fectly acceptable to smell a flower or fruit in order to enjoy
a delicious scent, or to appreciate the bouquet of wine, but
it is not generally regarded as well-mannered to sniff at an
egg or slice of meat to test its freshness — such actions are
regarded as animal-like and almost indecent. Having lost
our sense of smell we affect to despise what we lack.

Aquatic mammals, too, can have little use for a sense of
smell for, being air-breathers, they must all shut their noses
when their heads are beneath the surface of the water.
Whales, dolphins and porpoises use their noses, which have
moved to the top of the head and become blow-holes, only
when they come up to breathe, and although they may have
some sense of smell they can have few opportunities for using
it. As we have seen, they have developed other senses, par-
ticularly that of hearing, to an extraordinary degree of per-
fection, no doubt at least in part as a compensation for being
denied the use of their noses. Little is known of the lives of
the sea-cows, the manatee and dugong of tropical coasts and
rivers, but as they too spend most of their time under water,
and open their noses only when they come to the surface to
breathe, it is probable that they do not possess an acute sense
of smell.

The seals, on the other hand, certainly preserve a good
sense of smell. Although they cannot use their noses when
hunting their prey under water, they do use them when they
come out on to the land. Seals generally congregate in large
numbers on their breeding grounds, and when a seal mother
looks for her pup in a crowded "rookery", she identifies it
by smell, and can pick it out from hundreds of other pups
against the overpowering background aroma. (See Plate
i6.)

Birds are generally held to have little or no sense of smell

SMELL AND TASTE I99

but there are exceptions, notably among the tubinarine
birds — the petrels and albatrosses. In these birds the nostrils
take the form of little tubes projecting on the top of the
beak, unlike the slits of most other birds. When they are
not on their nesting-grounds the albatrosses and petrels
generally live far away at sea, often many hundreds of miles
from the nearest land, for they are truly oceanic. Some
people who collect birds for museums have found a way of
luring these birds within range of the gun by exploiting
their sense of smell. The method is simple : when you are
in mid-ocean you persuade the captain (if you can) to stop
the ship and lower a boat. You get into the boat with your
gun and other tools of your trade, and also take a primus
stove, a frying-pan and a supply of dripping and scraps of

Fig. 13. The head of a Fulmar Petrel showing the tubular nostrils charac-
teristic of all the petrels.

fat. As you row gently along you keep pouring hot fat on to
the water, and when you have gone a mile or so you put
out the stove and row gently back along your track. There
may not have been a bird in sight when you set off, but on
your return you find the smell of the cooking has called up
the petrels from afar, and you can slaughter what you wish
as they eat the food you have provided. Among the birds,
therefore, the petrels at least must have a good sense of
smell, and sometimes use it in finding their food.

The sense of smell is important to reptiles, and is probably
particularly acute in most snakes and lizards, which have a

200 THE SENSES OF ANIMALS

modification of tlie nose unlike that of other animals. In
addition to the usual nose-passage with nerve-endings for
smell in its upper part, these creatures have a further arrange-
ment for smelling. Tv^o little pits lie in the roof of the mouth
towards the front; they are derived from the nose-cavity
during the course of development, and their linings are
richly supplied with nerve-endings similar to those in the
upper part of the nose-cavity. The pits, which are called
"Jacobson's organ", are used in conjunction with the forked
tongue. The tongue is not poisonous, nor is it a sting, as is
sometimes supposed. The well-known flickering action of
the tongue in snakes and lizards has long been regarded as
a means of testing the environment by the sense of touch.
Doubtless the tongue is a delicate tactile organ and often
functions by actually touching objects, but if a reptile is care-
fully watched the observer can easily see that frequently the
tongue merely flickers in the air without toucliing anything
and then is quickly withdrawn. The flickering tongue is
picking up odorous particles floating in the air, and trans-
ferring them to Jacobson's organ when it is withdrawn. In
many snakes there is a small gap in the lips at the front of
the mouth so that the tongue can be flickered in and out
without opening the mouth. Evidently Jacobson's organ
increases the sensitivity of the sense of smell, but exactly how
it does so is not known; it may be that the method of
breathing in these reptiles does not draw enough air into the
upper part of the nose-cavity to allow a quick perception
of air-borne particles.

Although many of the frogs and toads find their food by
sight some of the amphibians use the sense of smell for this
purpose. The species that live wholly or part-time on land
have noses with nerve-endings in them of the type already
described. There appears to be some kind of sense of smell,
too, in the moist skin of some amphibia — the skin through
which much of the breathing takes place in some species, to
such an extent that certain salamanders have no lungs at all.
Those amphibians that live wholly in the water can scarcely
be said to have any sense of smell, if we mean thereby the
ability to detect air-borne vapours. The same remark applies
to the fishes, and we shall therefore defer considering the

SMELL AND TASTE 20I

sense of smell in these aquatic animals until we have reviewed
the sense in those invertebrates that live in air.

The insects are by far the most numerous of those inverte-
brates but members of some of the other groups are also
terrestrial, the most important being the spiders and their
relations, millipedes, and some of the molluscs such as snails
and slugs. Some of the Crustacea and many of the worms
live on land but they, like numerous other isolated examples,
are the exceptions from the general habits of their groups
which are aquatic. Practically all the land-living inverte-
brates give some response to air-borne vapours, and thus
show that they have a sense of smell ; in many of them, how-
ever, there appear to be no special sense organs for smell,
or none that have been identified as such. Earthworms, for
example, are attracted from a distance by the presence of
food but the receptors for smell appear to be scattered
through the skin. In snails and slugs the receptors are be-
lieved to be more concentrated in the skin of the tentacles.
In spiders, too, there are specialized sense organs in different
parts of the body, but little is known of the particular function
of the different kinds, although it is known from their be-
haviour that certain spiders have a sense of smell.

Insects have a well-developed sense of smell, and in some
of them it is so acute that it far surpasses anything that comes
within the range of our own experience. Much work has
been done on this sense of insects yet little is known of how
it functions. As previously mentioned, insects have numerous
sense-organs scattered over their bodies but because we do
not understand the exact use of many of them they are
merely called sensilla ; some of these certainly serve the
sense of smell. It is known that the sense is chiefly concen-
trated in the antennae of insects, and in the feet of some
butterflies, but it is also present to a lesser extent in the
general body surface ; furthermore the antennae bear more
than one kind of sensillum and consequently it has not been
possible to determine which kind is that for smell. Of course
it is possible, though improbable, that more than one kind
is sensitive to smells.

The "assembhng" of the males of some kinds of moth
round a female, which was mentioned in Part I, page 98, is

202 THE SENSES OF ANIMALS

effected by the sense of smell with an acuity that is quite
unimaginable to us. We have already speculated on how
few molecules of an odorous substance are needed to stimu-
late a smell-receptor ; the assembling of moths must be an
extreme case, and one wonders whether a single molecule
is enough. On the other hand, an extremely small quantity
of matter contains a great many millions of molecules, so
that the number given off by the female moth may be very
large — did not someone say that we must all contain at
least one molecule that once formed part of the body of
Julius Caesar?

The extreme sensitivity of assembling male moths to the
scent of the female may be because it is the only one that
they can perceive, so that no other scents can distract or
confuse them. It is also remarkable that in the species that
have this faculty the antennae of the males are very much
more elaborate in structure than those of the females, and
consist of feathery combs which greatly increase the surface
area on which the receptors are arranged.

Many other insects besides moths have an acute sense of
smell ; usually it is important to them in finding their food.
Anyone who has travelled in the wilder parts of tropical
Africa, where there are no amenities of civilized sanitation,
will have noticed the extraordinary promptitude with which
the scarab beetles, which lay their eggs in a ball of dung that
they gather up, assemble the moment their favoured material
is provided.

Among the butterflies there are many kinds that use the
sense of smell for another purpose — the males produce what
to us are sweet-scented perfumes in their courtship of the
female as aphrodisiacs or sexual stimulants to signal their
presence and stimulate her mating reactions. The perfumes
are produced by special glands and scales, and in some
species they are diffused into the air by special brushes that
act as powder puffs, but the exact kind of sensillum that
receives them in the female is not known. A close comparison
of the sense of smell in insects is not, however, possible with
ours because the particles that stimulate the sense of smell
in insects are not, as far as we know, dissolved in a film of
water covering the receptors, but act in the dry. The nature

SMELL AND TASTE 203

of the sensation reaching the insect's consciousness, if indeed
it is conscious in our meaning of the word, remains unknown
to us.

The sense of taste very closely resembles that of smell, for
it is the perception of things through their chemical nature
— not of things at a distance but of things in contact with
the inside of our mouths. The distinction is clear in land
animals because for them smell is airborne whereas taste de-
pends upon contact. In animals that live in water the dif-
ference is less, and the two tend to merge into one — taste
ranging from a distance to actual contact.

In land animals taste is confined to the mouth and is
mainly concentrated upon the tongue, where the cells reac-
tive to dissolved substances are clustered into small organs
known as taste-bulbs. The structure of the taste-bulbs might

SURFACE OF T0N»5U£

TASTE
BULBS
on Sides
of
depression

Fig. 14. A highly magnified section through part of the tongue showing taste
bulbs arranged at the sides of slight depressions of the surface.

Fig. 15. A single taste bulb more highly magnified showing the thread-like
ends of the receptor cells projecting from the surrounding cluster.

204 THE SENSES OF ANIMALS

be more appropriately likened to flower-buds than bulbs.
They consist of a cluster of cells, representing the petals,
surrounding the receptor cells the ends of which project as
fine threads, and might represent stamens and stigma. The
ends of the taste-nerves ramify round the bases of the receptor
cells, and convey signals towards the brain when the receptors
are changed chemically by the contact of tasteable substances
in solution.

There are only four tastes to which our taste-bulbs respond
— sweet, sour, bitter and salt. Other animals may, of course,
taste a greater or lesser number of tastes but of that we know
nothing, although it is unlikely that the other mammals
differ from us, whatever may be the case with other verte-
brates, and still more with the invertebrates. Whenever we
enjoy a meal we realize, if we take the trouble to think about
it, that we are aware of much more than the four tastes, in
fact the enjoyment comes from all the delicious and varied
flavours rather than from the four basic tastes. The flavours,
however, are not tasted but smelt; they diffuse from the
mouth into the air at the back of the nose and act upon the
endings of the smell nerves there. If you have a really bad
cold, so that the smell receptors are completely blanketed,
the most fragrant cup of Brazilian coffee merely tastes bitter-
sweet if you take sugar, or just bitter if you don't. In similar
conditions it is said that you cannot distinguish between an
apple and an onion, apart from the sweet or sour of the apple.
There is, however, more to food than just taste and flavour;
its texture, which is perceived by the touch receptors that
are numerous in the tongue, is of almost equal importance
— was not H. G. Wells's Mr. Polly turned from suicide by
the mention of savoury fried bacon, all steaming hot and
crisp? It was the "crisp" that did it.

Each taste-bulb generally responds only to one kind of
taste, and the bulbs are arranged on the tongue mainly in
groups of a kind — those for sweet are mostly at the tip, and
those for bitter mostly at the back. You can chew up a
quinine tablet with the front teeth, keeping the bits at the
front of the mouth with the tip of the tongue, without tasting
the intense bitterness. If you wash it down quickly with a
draught of water so that it rapidly swishes over the back of

SMELL AND TASTE 205

the tongue and does not come into contact with the taste-
bulbs there you will hardly notice it ; but if you are clumsy
and let it touch the bulbs at the back you will appreciate its
character to the full. Why does strong bitterness make so
many people start sneezing? This is a point that has not,
I believe, been investigated. As far as we know all the other
mammals, and indeed all the other vertebrates, taste as we
do, though many of the flavours that they obviously enjoy
would be disgusting to us. On the other hand, many of our
likes and dislikes may be due to education and custom rather
than any innate preference. If cheese was eaten only by some
remote tribe of savages we should probably think it was
revolting to enjoy the flavour of the coagulated secretion of
the modified skin-glands of a cow after it had undergone
bacterial decomposition. And how many people really enjoy
the taste — or flavour — of their first glass of beer? Not for
nothing have we the expression — "an acquired taste".

It is when we come to aquatic animals that we find it
difficult to draw the line between taste and smell. Both are
chemical senses that respond to substances dissolved in the
water, coming from objects at a distance or in contact. How
close must the contact be before smell gives place to taste?
It is very doubtful whether the sensation felt by an aquatic
animal is different whether water containing dissolved sub-
stances comes from far or near. It is, perhaps, best to lump
the two together under the blanket label "chemo-receptor".
The skin of fishes contains numerous sense receptors scattered
all over it, some of which resemble in structure the taste-
bulbs of higher vertebrates, and in a few fishes they also
occur inside the mouth. The nose in fishes does not, with
very few exceptions, lead into the mouth, but consists of a
pit on the snout lined with chemo-receptor cells. In the bony
fishes it is generally divided into two, so that there are two
separate nostrils on each side of the head. The surface area
of the inside of the pit is increased by the lining being thrown
into numerous folds or ridges. It is sensitive to many sub-
stances in very great dilution, as is shown by the way in
which some kinds of fish are immediately attracted to food
put into the water at a distance. Experiments have shown
that they are indeed attracted by chemo-reception and not

2o6 THE SENSES OF ANIMALS

by sound, sight, or the other senses. It is surprising that
substances diffuse through the water with such speed, for
we know that when a lump of sugar is dissolved in a cup
of tea the concentrated solution remains at the bottom until
it is stirred to mix it evenly. Nevertheless some molecules
must break away and travel throughout the whole, for a
suitable flavour introduced into the water several feet from a
fish is noticed almost at once. Newts and other amphibia
that live for long periods or permanently in water have a
similar taste-smell faculty.

Do fishes that live in the sea taste the salt? It is unlikely
that they do as long as they are in sea-water with the normal
concentration of salt, but it does not follow that they cannot
taste salt. In the first place any nerve-ending that is stimu-
lated above a certain maximum suffers from fatigue and
stops sending impulses to the brain ; even our eyes and ears
refuse to function if over-stimulated, and thus we speak of
a blinding flash or a deafening noise. But there is another
reason for not noticing a constant stimulus, and this probably
fits the case of fish in sea- water better. A sense receptor may
be working perfectly and sending a stream of messages to
the brain, but if the brain is not interested it ignores them
and they remain unnoticed. When you are sitting reading
you do not notice the ticking of the clock on the mantelpiece,
although a message telling you of each tick is being sent to
your brain. If the clock stops you immediately notice it be-
cause the constant stream of messages that you have been
ignoring has ceased, and you become aware of the change.
Similarly fish in the sea probably do not notice the salt, but
should they move into water of greater salinity, or of less as
where a river runs into the sea, they probably taste the
difference at once.

Most of the invertebrate animals that live in the water,
apart from crustaceans, insects and their relatives, have soft
skins — even sea-urchins have a thin covering of skin on the
outside — and their skins, like those of fishes, are covered
with chemo-receptor and other sense organs. The simpler
kinds of animals respond to the presence of certain chemi-
cals by moving towards or away from them, but their
structure is so primitive as to preclude the possibility of their

SMELL AND TASTE QOJ

being aware of the stimuli in the way that we should be.
Their reactions appear to be automatic, and to correspond
more with the movements of the cations to the cathode in
electrolysis rather than conscious reactions. In the more
complex invertebrates definite chemo-receptor organs are
recognizable and they are particularly conspicuous in some
of the gastropod molluscs such as the whelk, periwinkle and
many others. These creatures breathe by means of gills that
are contained in a cavity near the front part of the body.
Sea-water is drawn over the gills through an opening the
edges of which are usually extended as a tube known as the
siphon. When the water enters the cavity it impinges on a
complicated chemo-receptor before it passes over the gills.
This water tester is thrown into numerous frilly tufts so that
its surface area is large and can hold many receptor cells. It
not only tests the water for suspended matter so that the gills
do not get clogged with muddy particles, but also acts as a
chemo-receptor. Whelks, which are carnivorous, find their
food by "smell" and can easily be caught in numbers by
letting down a small net baited with meat or fish on to the
bottom where they live. A striking example of the use of the
water tester for finding food is shown by a sea snail, Bullia,
that lives on the shores of South Africa. In tliis creature
there is no doubt that the water tester is for smelling, or
tasting at a distance. The snails live buried in the sand near
low tide mark, but feed upon beach carrion stranded near
the high water mark. Substances from the food dissolve in
the water when the waves reach it and are carried down
to the snails as the waves retreat. As soon as the scent
reaches the snails they come up out of the sand and let the
waves carry them up the beach to strand them near the tide
mark where their sense of smell leads them straight to the
food.

XVIII

TOUCH

The sense of touch is conveniently classified with the other
senses that lie in the skin, those of pain, heat and cold. All
these, like taste and smell, are contact senses, but they are
physico-receptors rather than chemo-receptors. Touch, like
smell and taste, can nevertheless to a certain extent act at
a distance, as we shall see.

The nerve-endings in the skin are of several kinds, and
they are on the whole less complicated in structure than
those of the other senses already described. In the simplest
the nerve branches into a number of slender twigs close
beneath the outer skin, but in others it ends in a small speck
of jelly enclosed within concentric layers of supporting cells
like the skins of an onion. These more complicated end
organs are known as "corpuscles", and they are classified
into different sorts by the details of their structure. Touch
corpuscles lie close beneath the surface of the skin and are
particularly numerous under the bare skin of the palms of
our hands and soles of our feet. They respond to contact of
the skin with any object, that is to say, they respond to
light pressure ; other corpuscles situated deeper below the
surface respond to heavier or deep pressure. They are not
evenly spread over the surface of the body but are placed
most close to each other on the tip of the tongue, next
closely on the under side of the finger-tip, less closely on
the palm of the hand, the back of the hand, and much more
sparsely elsewhere. The spacing can easily be mapped by
moving a stiff bristle over the skin and marking each spot
where it can be felt.

The sense of touch is served not only by the touch cor-
puscles which respond to things in contact with the skin,
but also by the nerve-endings associated with the roots of

the hairs. These by responding to minute movements of

208

TOUCH 209

the hairs give a sensation of touch with things that are not
in actual contact with the skin. Each hair grows from a
small pit in the skin and the tactile nerves end both at the
base of the hair within it and in the walls of the surrounding
pit. The nerve-endings are particularly numerous round the
roots of the long hairs that grow on the snouts and elsewhere
in mammals. These long hairs are popularly called whiskers,
but they are not the equivalents of the facial hairs that adorn
the countenance of man. In addition to those on the snout,
mainly on the upper lip, these special hairs commonly occur
above each eye, under the chin, on the wrists and heels, and
less commonly elsewhere. Mankind is without these special
hairs, except for the characters in certain popular strip
cartoons — had we appendages such as the cat's eyebrow
whiskers we could more easily avoid bumping our heads in
going through low doorways.

The "whiskers" are usually assumed to be simple organs
of touch, but in some animals they may be something more.
In many of the aquatic mammals they are very long and
thick, so thick that in some large seals they resemble quills.
In the seals, the otters and the African River Shrew [Pota-
mogale) the prominent whiskers on the upper lips are set in
conspicuous pads of tissue that thicken the lips and give a
characteristic shape to the snout. The bottoms of the pits
from which these whiskers sprout are enlarged and form
bulbous swellings, each supplied with a large nerve branch.
These mammals are not very closely related to each other
in the zoological classification, but they have one point in
common, they are all aquatic. Many of them are able to
find their food in water that is so turbid with suspended
mud that little or no light penetrates more than a few inches
or feet below the surface. They are thus prevented from
hunting by sight, and we know that with their noses shut
they cannot hunt by smell ; how then do they find their
food? The only possibility seems to be that they do so by
touch at a distance, using their highly specialized whiskers
to detect turbulences in the water caused by other objects,
including their prey. There appear to be no experiments
recorded on this matter, and it will be interesting to know
the results of any tests that may be made. The whales and

2IO THE SENSES OF ANIMALS

dolphins have no functional whiskers of this sort, but then
we know they have a very efficient sonar system for use in
such surroundings.

The largest whiskers of all are found in the walrus, but
in this animal they have been put to a different use, although
they may of course retain their tactile function to some
extent. In the walrus they are used as spoons for feeding.
The walrus lives largely on clams which it digs out of the
mud at the bottom of the sea with its tusks, and it uses its
bushy whiskers for shovelling them into its mouth when it
has dug them up.

Among the other aquatic vertebrates whisker-like struc-
tures are commonly found in the fishes. The barbels of fishes,
however, although similar in function as organs of touch,
are very different in structure. Hairs are non-living products
of the skin, but barbels are extensions of the living part of
the body, and consist not only of skin but of some of the
underlying supporting tissue as well. Their mode of action,

Fig. 1 6. a Catfish [Saccobranchus) showing the barbels on the snout and lower
jaw.

too, is different; they are richly covered with touch end
organs so that nerve impulses can start in their very tips
whereas impulses can start only from the cells surrounding
the bases of hairs. The most familiar barbels are those on

TOUCH 211

the chin of such fishes as cod, haddock and, among fresh-
water fishes, the barbel. In some fishes there is more than
one barbel on the chin, and in others there may be a pair,
or several pairs, on the upper lip and snout as well. Among
fresh-water fishes the British loach has six barbels on the
snout, and the catfish of continental waters has two very
long barbels on the snout and four shorter ones on the
lower jaw. In some of the deep-sea fishes the barbels reach
a fantastic length and may be longer than the body. Barbels
are obviously of great importance as touch-organs to fishes
that live in turbid water, or at great depths of the sea, or
that feed mostly at night, but they are also one of the
most used organs in some fishes that live in clear water.
It is a fascinating sight to see red mullet in an aquarium
using their barbels. Under the chin of these fish there are
two rather long barbels which can be folded away under
the lower jaw so that they cannot be seen when the fish is
swimming in mid water but can be brought out to project
slightly forwards below the chin when the fish is seeking
food at the bottom. They are very mobile and assuredly
very sensitive ; the fish moves them about rapidly, gently
touching everything below it as it explores the shingle and
sand. They are moved about by muscles, but the exact way
in which they function does not appear to have been re-
corded. In use the part between the fish and the bottom
appears to be rigid, although it has no bone in it, but the
part in contact with the bottom is soft and moulded to the
shape of the objects it touches. If the fish moves nearer the
bottom more of the barbel becomes soft ; if it moves away
more of it becomes rigid. The mechanism by which this
change is attained must be of some complexity.

In the invertebrates with naked bodies the sense of touch
is generally distributed all over the skin, but there is also
often a concentration of touch-receptors on feelers and
tentacles of different sorts which nearly always occur at the
head end. Most of the aquatic worms, the snail-like molluscs,
and a host of other creatures are thus provided. In the
crustaceans, the insects, and the other animals with jointed
body-armour, there are one or more pairs of feelers, or
antennae, often of great length, as in the crayfish. These

212 THE SENSES OF ANIMALS

TENTACULAR
CIRRI

Fig. 17. The head end of a marine worm {Nereis virens) to show the eight
"feelers" (tentacular cirri) beside the four eyes.

are richly provided with touch-receptors, but they also bear
receptors for other senses such as that of smell or taste. In
addition most of these animals bear receptors on the cuticle
that covers the armour, such as the sensilla already men-
tioned, which occur on the bodies of insects. Some of these
are certainly touch-receptors, but others are probably chemo-
receptors, or sound- and vibration-receptors.

The sense of touch has considerable powers of discrimina-
tion when many of the receptors are stimulated at the same
time. If only a few are stimulated we have the sensation of
something the nature of which we cannot determine touch-
ing us; if the touch is light it merely "makes the place
tickle". If a larger area of skin comes in contact with the
object, so that many receptors respond, we know whether
the object is rough or smooth, sticky, soft or hard. It is
peculiar that some of the sensations produced are pleasant
to us and others are not ; we dislike the feel of sticky glue
or rough sandpaper, but enjoy the feel of sleek fur, cool
marble or polished jade. Now technology has provided us
with a new sensation ; the frictionless plastics that are now
being introduced for the bearings of machinery are outside
our normal experience — if you hold a piece in your hand
you are aware of its presence through its weight, yet you

TOUCH 213

cannot feel it because its extremely low coefficient of friction
does not allow it to stimulate the touch receptors.

The receptors for heat and cold are very much fewer
than those for touch ; they can be mapped in a similar way
by drawing a hot or cold metal point over the surface of
the skin. If spots that respond are marked it can be seen
that the receptors for heat lie in different places from those
for cold, and that both those for heat and cold are separate
from those for touch. The temperature-receptors, however,
with the exception mentioned below, do not respond to
particular temperatures but to relative ones. A heat-receptor
is stimulated only by a temperature greater than that of the
skin, and a cold-receptor only by one below it. This is easily
shown by placing one hand in a bowl of hot water and the
other in a bowl of cold, long enough for the temperature
of the one hand to be raised and for that of the other to be
lowered. If now both hands are put into a bowl of luke-
warm water it will feel warm to the cold hand and cool to
the warm one. Water of one temperature is thus stimulating
the cold-receptors of one hand and the heat-receptors of the
other because it is hotter or colder than the skin of the
respective hands.

The receptors for pain are more numerous even than
those for touch, probably thereby indicating that they are
the most important of the skin receptors. Pain is a very
efficient guard for the body in that it prevents animals
damaging themselves by contact with objects that might
injure them. It is impossible to answer the question so often
asked, whether, or to what extent, animals feel pain. Only
the animals know that, and we can merely judge by their
reactions to stimuli that cause pain to us. By this criterion
the answer is that warm-blooded animals, the birds and
mammals, undoubtedly do feel pain, though their reactions
are often such that we must infer that some of them appear
to feel it less acutely than we do. There is no reason for
supposing that their receptors are less sensitive than ours,
but it may well be that the sensations produced in their
brains differ from the equivalent ones in ours. Just as people
who coddle themselves become ridiculously sensitive to heat
and cold, so it is probable that the softness of civilization

214 THE SENSES OF ANIMALS

has made us over-sensitive to pain. A toughening course
can teach almost anyone to be comfortable in surroundings
so hot or so cold that they would be highly distressing to a
person in soft condition — how many adults would care to
go out in winter weather with bare legs and ankle socks, as
so many children do without discomfort? It is the same with
pain, and one has only to watch a boxing match to realize
that the brain is able to ignore a great many messages from
the pain-receptors that would be interpreted as very painful
by an untrained person. In like manner no doubt the rough
and tumble of the life of a wild animal teaches it to ignore
the lesser painful stimuli and to attend only to those that
mean danger. In the cold-blooded animals, both vertebrate
and invertebrate, the sense of pain appears to be less acute,
for many of them are able to sustain injuries from which we
should die of shock, without reacting in a way that indicates
they are suffering severely. Their reactions show that they
do have a sense of pain, but the sensation is likely to be much
more transient than in higher animals and probably ceases
with the withdrawal of the stimulus. The matter of injury-
shock is, however, rather beyond our subject ; it is concerned
with other things, such as biochemistry and psychology, in
addition to the senses.

The receptors for pain and for temperature both seem to
be of two kinds, those that are normally used for detailed
discrimination and those that respond to stronger stimuli.
The latter are known as the receptors of protopathic sensa-
tion; they need a very much stronger stimulus in order to
react, but once the sensation is aroused it is intense, long
lasting, and very distressing. The protopathic sensations are
those of very high or very low temperature and of great
pain. Everyone knows how acute, intense and long-lasting
the pain of a burnt finger can be long after the hand has
been snatched away from the stimulating flame; or how
excruciating it is when the fingers are dried and warmed
after they have been chilled by injudiciously joining in a
snowball game without gloves. The protopathic pain sensa-
tion is so crippling in its effects that some of the tricks of
unarmed combat are designed to produce it, whereas the
Queensberry Rules are designed to avoid it.

TOUCH 215

In addition to the senses of pain, touch, cold and heat,
the skin of some animals contains receptors for other senses.
There is a generalized chemical sense that in us is mainly
localized in the lining of the mouth and nose, which responds
to things that give sensations of astringency, pungency and
so on, that are neither tasted nor smelt. This sense is probably
of greater importance in the animals with naked and moist
skins, such as the amphibians and fishes, and many of the
invertebrates.

In the sharks and rays among the fishes there is, too, an
elaborate arrangement of sensory pits in the skin over most
of the head. The pits are large enough to be plainly visible
to the naked eye after the removal of the skin, through which
they open by small and rather inconspicuous pores. These
"ampullae of Lorenzini" are filled with jelly and are well
supplied with nerve-endings. Experiments have shown that
they respond to changes in water-pressure, and it is probable
that they function as organs of "touch at a distance", for
changes in pressure will be produced when the fish move
near other objects, or when other moving objects are near,
or when wave movements of the water impinge on fixed
objects. The fish are thus able to feel things without touch-
ing them, through alterations in the water-pressure caused
in these diverse ways.

A special development of the temperature sense is known
in a small group of animals — the ability to detect infra-red
radiation, the radiation of longer wave-length than that
producing the sensation of red light in our eyes, but of
shorter wave-length than that producing the sensation of
heat in our temperature receptors. This adaptation is found
in some of the snakes that live on small mammals such as
mice. They are the pit-vipers, which take their name from
the presence of a small pit in the skin on the side of the
head below and in front of the eye. The pit-vipers are
poisonous snakes found throughout the Americas, and all
the warmer parts of Asia ; they include the rattlesnakes, the
Copper-head or Moccasin, and the dreaded "Fer de lance".
The pit is plentifully supplied with nerve endings which
respond to stimulation by the infra-red radiation given off
by the warm bodies of small mammals and birds. The snakes

2l6 THE SENSES OF ANIMALS

can thus locate tiieir prey in the dark, for most of them hunt
by night, though they are often seen out in dayhght basking
in the warmth of the sun. (See Plate 19.)

It has been suggested that owls similarly find their prey
in the dark by seeing the infra-red radiation given off from
the bodies of small animals. There are however theoretical
grounds for rejecting this idea, and experiment has con-
firmed that the eye of owls does not respond to infra-red
radiation. Infra-red radiation, "black light" as it has been
called, has been used for illuminating small mammals in the
dark so that they can be photographed by films sensitive to
those wave-lengths. Most small mammals can be equally
easily observed by red light visible to our eyes, which pro-
duces little or no response in theirs.

There is another sense, or response to a stimulus, that
remains to be mentioned. It is found in animals that live
in water, and is now well known in the fishes. It is a reaction
to electric currents passing through the water, and is
probably quite different from the sense that responds to the
electric field set up by the electric fishes in their echo-
location. This reaction is exploited in the recently developed
techniques of fishing by electric currents. If an electric
current is passed through water in which fish are living they
are attracted to the cathode, provided the current is of suit-
able strength. Thus the anode can be made a metal plate
fixed under a boat and the cathode the ring of a hand net ;
when the net is dipped in the water the fish swim to it and
can be lifted out. This works well in fresh-water but the
method is not so easy to use in sea-water which is about
500 times a better conductor than fresh-water, and so short-
circuits the electrodes. In sea-water the current must be in
the form of pulses of short duration but high current density.
Methods have also been invented for attracting fish elec-
trically to the mouth of a pipe lowered over the ship's side
through which they are pumped on board. By adjusting the
strength of the current fish can be attracted, repelled, stunned
or killed. Electrified fences can be used for keeping fish away
from such places as the intakes of turbines, or for leading
them in a desired direction. There is a point about the
middle of a fish's body that is most sensitive to electric

TOUCH 217

Stimulation ; ii" the fish receives the stimulation ahead of this
point it backs away, but if behind it swims forward. It is
pecuUar, too, that large fish are more sensitive to a given
current than the smaller ones.

XIX

MONITORS

The senses which we have been considering in previous
chapters tells animals what is happening around them, but
it is equally important for animals to know what is going
on inside their own bodies. Indeed, often the information
about internal conditions helps an animal to know more
about the external ones.

How is it that you can take your handkerchief out of your
pocket and blow your nose in the dark? It is not primarily
by touch, although that sense is involved in manipulating
the handkerchief, for your hand goes straight to your pocket
and does not grope over the surface of your coat searching
for it by touch. And when you have taken it out you raise
it directly and accurately to the nose. Similarly in complete
darkness you can put a sweet into your mouth — the one
mouth you can never see except as a reflected image in a
mirror.

The senses that enable these feats to be done and monitor
the bodily processes are the proprioceptive (or self-perceiv-
ing) senses ; most people take them so much for granted that
they do not know they possess them. These senses are served
by nerve-endings in most parts of the body, especially in the
muscles, joints and tendons, and in part of the ear. Those
in the muscles, joints and tendons form the muscle sense, and
together they tell the position of the limbs and the other
parts of the body both in movement and at rest. In the
muscles the nerve-endings lie among the cells and are stimu-
lated by movements; in the tendons they lie among the
fibres and respond when they are stretched ; in the joints
they lie in the membranes covering the ends of the bones
and are stimulated by contact between the different parts

of the joint surfaces. The messages sent to the brain by these

218

MONITORS 219

nerve-endings are essential in making co-ordinated move-
ments of the different muscles as, for example, in grasping
an object between the fingers and thumb, or in moving the
legs when walking. Thus a horse can be taught to walk in
a gymkhana along a row of bottles set out on the ground,
stepping in the spaces between them and not knocking one
over, although he cannot see his hind feet — he has seen the
bottles and his muscle sense tells him where his feet are in
relation to them. Even on a cold winter's day when the feet
are numb through walking through the snow we know where
they are and one can go on putting one before the other —
the sense of touch has been put out of action by the cold, but
the muscle sense in which the nerve-endings are not so near
the surface is unaffected.

The proprioceptive sense that is served by part of the
internal ear tells us which way up we are and how we are
changing direction when we move — part of it is a gravity-
receptor and part of it an angular-acceleration-receptor. In
all the vertebrates, except a few of the most primitive such
as the lamprey, the structure of these receptors is essentially
similar. It consists of two small membranous bags connected
with three semicircular canals arranged in planes at right
angles to each other. They are all embedded in the bone of

AMPULUE

SEMICIRCULAR
aNALS

r SUPERIOR
LATERAL
I POSTERIOR

COCHLEA

Fig. 18. A diagram of the three semicircular canals arranged at right angles to
each other. The ampullae of the canals open into one of the membranous
bags (the utriculus) which is connected via the second bag (the secculus) to
the cochlea. The sausage-shaped bag below is the endolymphatic sac.

220 THE SENSES OF ANIMALS

the skull close to the cochlea, the part of the ear concerned
with hearing, which is connected to one of the bags. All are
filled with fluid which is continuous throughout the canals,
the bags and the cochlea.

Inside each of the semicircular canals and near one end
there is a pimple covered with elongated cells bearing
filaments like hairs which project into the fluid. In each of
the little bags there is a similar mound, but suspended among
the hairs there are many grains of limy matter. The last are
the gravity-receptors, and the pressure of the limy grains
on the hairs stimulates the cells into sending messages to the
brain along the nerve-fibres that ramify among them. Any
change of the body position moves the grains so that they
stimulate cells in different parts of the mound and enable the
animal to keep its balance. In the bony fishes the grains are
not separate, but form a comparatively large chip of bone,
the otolith, which differs in shape in different kinds of fish,
so much that it is not difficult to learn to what kind of fish
an otolith may have belonged. Fish otoliths are rather hard
and are not quickly digested by animals that feed on fish —
birds, dolphins and other fishes; they often remain in the
stomach long after all the rest has been digested and so it
is possible to find out what kind of fishes an animal may
have been eating from an examination of the otoliths in its
stomach. There is another use that the naturalist can make
of otoliths. The otoliths increase in size as the fish grows,
but the fish grows fastest during the summer abundance of
food, and consequently the otolith is marked with rings
somewhat like those in the trunk of a tree. By counting the
number of rings in the otoliths it is possible to know the age
of the fish from which it was taken.

The patches of sensory hair-cells in the semicircular canals
are stimulated when the fluid flows over them. The fluid
flows along the canals whenever the head moves ; if the head
is moved to one side the fluid flows away from the cells on
that side of the head and towards them on the other. The
changes in pressure stimulate the hair cells, which start the
appropriate messages on their way to the brain.

One of the semicircular canals lies in the horizontal plane,
the other two in the vertical plane but at right angles to

MONITORS 221

each other; the three thus He in the three planes that are
seen, for example, where three sides of a box meet at any of
the corners. As a result of this arrangement movement of the
head in any direction sets the fluid moving in one or more
of the canals, and the sensory cells are stimulated. At first
sight one might imagine that two canals, one vertical and
one horizontal, would be enough to give an indication of
head movements in any plane. If, however, there were only
two, a movement about the axis of the junction of the two
planes would not produce any movement in the fluid,
whereas with three there is no possible movement that would
not cause the fluid to flow. There are, in fact, some animals,
the lampreys, that have only two canals, not, as one might
expect, vertical and horizontal, but both vertical — it is the
horizontal one that is missing. Yet lampreys apparently
swim as efficiently as the fishes, move up or down in the
water, turn from side to side and keep on an even keel, as
well as animals with three canals.

It may be that the lampreys use their eyes to help them
in keeping their balance. If the canals are put out of action
in a pigeon, for example, the bird cannot stand, walk or fly,
but is in constant unco-ordinated movement. After a time,
however, it regains partial control over its movements and
appears more or less normal. If then its eyes are covered it
is as bad as ever; the eyes have evidently taken over some
of the functions of the canals, and it relies on visual images
to orientate itself. Similarly the lamprey may use its eyes to
provide the information that it lacks through the absence of
the horizontal canal. To put against this suggestion there is,
however, the case of the lamprey's near relation, the Hag
fish. This creature has only one semicircular canal and no
eyes, and yet it appears to suflfer no handicap. Nobody knows
the answer to this riddle.

In the invertebrate animals we have seen that the ear
differs widely from that in the vertebrates. The proprio-
ceptive balancing sense although basically the same also
differs greatly in detail. In the first place there is usually no
separation of the part used as a gravity-receptor from that
for appreciating angular acceleration. The conspicuous
exception to this generalization is found in the octopus,

222

THE SENSES OF ANIMALS

where there is a gravity-receptor and three rows of sensory
cells set in planes at right angles to each other which in
function strongly resemble the three semicircular canals of
the vertebrates. In many invertebrates the sensory cells and
the calcareous grains are contained in a small bag, as in the
jelly-fish described on page 133. They appear to combine the
functions of gravity-receptors with angular-acceleration-
receptors and perhaps also respond to vibrations that we
perceive as sounds.

In many of the Crustacea these otocysts, or more correctly
statocysts, are open to the exterior and filled with water. In
the crayfish they lie at the bases of the second pair of feelers ;
every time the animal moults their linings are shed with the
rest of the shell, and with the lining the contained granules
are also lost. As the new shell hardens the animal picks up

AntenmHe

STATOCSST

Fig. 19. (a) A freshwater crayfish showing the second pair of feelers
(antennules) .

(b) Enlarged view of an antennule showing the entrance to the
statocyst.

{c) The arrangement of hair-like sensory cells and sand granules
inside the statocyst (highly magnified) .

minute sand grains and places them in the statocyst where
they serve the same function as the calcareous granules in
other animals where they are secreted by the tissues of the
body. If a moulting crayfish is supplied with very fine iron

MONITORS 223

filings instead of sand it puts some of them into the statocyst
where, under the influence of gravity, they press upon the
sensory cells at the bottom. If a strong magnet is held above
the animal the filings are lifted from the bottom of the
statocysts, and the animal turns upon its back so that they
again press upon the cells at the bottom ; the animal, adapted
to respond to gravity, is completely misled by the trick.

Most insects have two pairs of functional wings, the front
and hind, but the flies, comprising the order Diptera, have
only one, the front. The hind pair of wings in the flies is
represented by two little rods shaped like drumsticks that
project from the side of the body just behind the wings. When
a fly is flying, the little rods, known as halteres, vibrate at a
speed so high that they produce a gyroscopic effect which

HALTERE

Fig. 20. A mosquito {Anopheles) at rest with the hind feet raised from the
perch. The haltere, or modified hind wing, is visible below the base of the
front wing.

enables the fly to orientate itself and keep on an even keel.
They are proprioceptors that monitor the animal's flight.
The common house-fly, that can alight upside down on the
ceiling and is so agile at dodging when you try to swat it,
must have a very complex nervous mechanism for co-ordinat-
ing its movements in which the halteres play an important
part. The apparently erratic movements led some wag to
wise-crack, "Time flies. We cannot — their flight is too
irregular."

Other proprioceptive senses are served by a complex of
sensations from different kinds of sense-organs and chemical

224 THE SENSES OF ANIMALS

changes in the blood that act directly upon specihc centres
in the brain. Although there are no hunger or thirst end-
organs we know when we are hungry or thirsty and react
accordingly — our proprioceptive nerve-endings tell us when
the stomach is empty, but the amount of sugar in the blood
also informs us when we need more nourishment. Hence a
few lumps of sugar or a good sweet "cuppa" pick up your
flagging spirits when you feel exhausted by physical exertion.
On the other hand the pangs of hunger are a very different
thing: only those who have suffered from real starvation
know the excruciating pain which is probably caused by the
over-strong contractions of the muscles in the wall of the
intestine when it is empty of food. Some American Indian
tribes are said to have been in the habit of eating clay to
"stay the pangs of hunger" by filling the intestine and giving
the muscular walls something to grip upon although the
material contained little to afford nourishment. Many sorts
of seal fast for a month or more when they come ashore for
breeding each year, and their stomachs, although empty of
food, then commonly contain a considerable quantity of
shingle or pebbles. This was so well known to the old fur-seal
hunters on the coasts of South Africa that they called a seal's
stomach its "ballast bag". The seals probably swallow the
shingle while they are on the beaches, though no one has
ever seen them doing it, and no one knows whether they do
it deliberately or by accident in the turmoil of life in the
crowded rookeries. It has been suggested that seals fill their
stomachs with indigestible matter merely to stay the pangs
of hunger while they are fasting, but no evidence to prove
that this theory represents the truth has been forthcoming.
Just as we are made aware of hunger and thirst, we are
aware of the necessity for excretion through our proprio-
ceptive senses, which also give us information even more
subtle. Why, after a period of rest and relaxation do we
yawn and stretch? During repose the blood tends to dilate
the comparatively soft-walled veins, and to stagnate in them
instead of hurrying back to the heart to be impelled again
through the arteries. In stretching and yawning we squeeze
the veins by the action of the muscles and by deeply filling
the lungs and so send the sluggish blood racing back into

MONITORS 225

circulation. Then our proprioceptors tell us that we are
feeling refreshed and vigorous again.

Some very recent experiments have shown that certain
animals have senses of which we have no conception by
analogy with our own. A kind of prawn is able to respond
to differences in the pressure of water produced by differences
in depth of only a few centimetres. The surface membranes
of all cells are electrically charged and the charges on the
cells in different parts of the body are often different. When
such differences in electric potential occur on different parts
of the bodies of animals that live in water an electric current
flows from one to the other. Although the current is extremely
minute in the prawn it is sufficient to produce some electro-
lysis so that an excessively thin layer of gas covers parts of
the body, too thin to be seen through any ordinary micro-
scope. Water is incompressible and a prawn's body, per-
meated everywhere with fluid, consists of over 90 per cent
of water so that any change in pressure is distributed evenly
throughout it. Thus no differences of pressure are produced
that could act upon a sense-organ. But the minute film of
gas is compressible, and the appropriate sense-organs of the
animal can respond to the alterations in the pressure of the
gas. The animal is thus able to respond to differences in
absolute pressure, in contrast to the transient differences in
pressure to which fishes respond through their lateral-line
organs. It is difficult to decide whether this sense is really
proprioceptive or exteroceptive — a response to internal or
external stimuli.

And what of the migrations and homing abilities of birds
and other animals? Much thought has been devoted to the
subject of bird navigation but, without in any way decrying
the careful and brilliant research that has been devoted to
the subject, I cannot help thinking that we are as far from
a solution of the problem as ever. The facts of bird migration
are well known, including the migration of young cuckoos
which fly off unerringly to their winter quarters long after
the adults have left our country, and which have, as far as
we know, nothing but their innate instincts to guide them.
Many other animals are expert at homing. Even the limpet
on the surf-beaten rock prowls about at high tide grazing on

226 THE SENSES OF ANIMALS

the algae, and returns to the one spot that exactly fits its
shell when the waves retreat. Pigeons can be trained to home
from great distances by being sent further and further away
at each trial. The limpet explores its surroundings and gets
to know its way about its home; and so, too, do homing
pigeons, at least to some extent. Similarly wasps and bees
find their way home by using landmarks with which they
have become familiar. If the landmarks are moved their
homing is badly upset and they have to learn the new
pattern. Even birds can be completely put out by moving
their nest quite a small distance ; they will bring food to the
place where the nest was, but leave their young only a few
yards away to starve. They react to the old landmarks and
cannot quickly learn the new pattern or, indeed, appreciate
the altered conditions.

But some animals show an ability for homing which does
not depend upon learning the landmarks and knowing the
locality. The Shearwaters that were taken from their nests
in Pembrokeshire and flown in closed boxes to America
could have had no landmarks to recognize in the thousands
of miles across the Atlantic, yet some of them were safely at
home a week later. Dogs and cats that have moved house
with their owners to distances of a hundred miles and more
have been known to disappear and turn up at their old home
days or weeks later. Frogs and toads make long journeys
each spring to their breeding ponds, overcoming all obstacles
and passing by other ponds that seem to us to be as desirable
as those to which they go, (See Plate 20.)

Nobody knows how they do it, although much experiment
has been done on the homing and migration of birds. It
has been suggested that birds may respond to the earth's
magnetic field, or to the Coriolis force produced in a moving
body by the rotation of the earth, but experiment has not
been able to support such ideas. Much work has been done
upon the theory that birds navigate by unconsciously observ-
ing the altitude of the sun, and working out their bearing by
this and the use of a postulated time-sense or "internal
clock". The theoretical internal clock is necessary for obtain-
ing longitude by comparing local time with that of the point of
departure. The suggestion that there may be some way of

MONITORS 227

measuring the passage of time seems not unreasonable at first
sight, and we can find examples fi-om our own experience.
In the first place our own physiological processes give some
indication — we do not need a clock to tell us that dinner-
time is approaching. On the other hand this is because we
are in the habit of eating our meals at approximately constant
times day after day; were we, like many wild animals, to
feed whenever the chance of getting food occurred our
physiology would tell us merely that we were hungry and
would give no indication of the time of day.

Many people are, however, able to wake with great
accuracy at an unusual predetermined hour, especially when
they have a strong reason for doing so, such as the necessity
of catching an early train. This facility is so striking that it
may well lead to a belief that we have some internal way of
registering the lapse of time ; but what of the person who is
so anxious lest he oversleep that he wakes every half-hour or
so throughout the night? If he has an internal clock his sub-
conscious mind appears to set little store by its accuracy. It
is possible that waking at the desired hour is achieved by
using external stimuli. If the hour is an unusual one the
anxiety not to miss it may cause sleep to be lighter than
usual so that external stimuli from the sense organs are not
rejected or ignored by the brain. It is not completely impos-
sible that we may subconsciously count the ticks of the bed-
side clock or watch ; although we may not consciously know
the number of quarter-seconds that must go, perhaps we do
subconsciously. Other external stimuli may be used, such as
the striking of the distant church clock, which normally we
do not hear at all. If the use of such external signals be
denied there is the alternative that we may similarly count
our own heart-beats or respirations, though as their rates are
not so constant as the escapement of a clock it is hard to see
how they could produce any great accuracy.

Strong support of the theory that external stimuli are used
is given by the way in which all sense of time is quickly lost
in their absence. Cave explorers, for example, if by some
accident they are deprived of their illumination and have
to wait until they are rescued, after a moderate interval
have no idea of the hour, or even of the day of the week,

228 THE SENSES OF ANIMALS

when they have neither sight nor sound to guide them.
Ahhough it is not proved that men or birds have a time-
sense which is conveniently labelled an "internal clock",
some animals do appear to have an internal rhythm of some
sort that can regulate their behaviour. Some American
experimenters kept a Ground Squirrel for two years under
conditions with no seasonal fluctuations. The temperature
was constant at a little above freezing, an artificial day and
night of twelve hours each was maintained, and ample food,
water and bedding were provided. In spite of the removal of all
external seasonal stimuli the squirrel hibernated only at the
time that its relations in the wild were hibernating and was
active, like them, only from June to September. As nothing
external should conceivably have given a clue to the time
of year the squirrel must have had some sort of internal
seasonal clock to maintain the rhythm.

Even if the birds are proved to have a similar time
mechanism the mystery of their navigation is not solved.
Latitude and longitude can be determined with the aid of
a sextant and a chronometer (and a nautical almanac) but
knowing your position in these terms is no help to naviga-
tion unless you have a chart — if not a physical one, at least
a mental one. It is essential to know the geographical posi-
tion of the place of departure ; further, you must know what
place it is you wish to reach and its position, and you must
also know that you wish to reach it. Is it conceivable that
birds are provided with all these aids to navigation, even
subconsciously and figuratively? And do migratory fishes
observe the altitude of the sun and possess similar internal
clocks and mental charts?

It seems more probable to me that navigation, and some
kinds of homing performed by animals, depend upon some-
thing we have not yet discovered. It was not until man
invented echo-location — sonar and radar — that he found
that animals had been using them for finding their way
about for millions of years, utilizing the senses with which
he was already familiar. Is it possible that there is some way,
that we have not yet discovered, of using the known senses
for navigation? Perhaps when some elaborate instrument
is invented for the purpose we shall suddenly realize that the

MONITORS 229

same principles are in use all around us by other animals.
Whatever the truth may be it is certain that no animals
navigate consciously on their migrations. In the first place
it is highly improbable that an animal can have any sort of
mental chart or picture of the geography of the globe or,
secondly, that it knows where it is in relation to distant
places. It is probable that it finds itself travelling in a certain
direction with no more realization of what it is doing than
a man on a bicycle has when he turns a corner — he just goes
round without conscious effort. When, however, the animal
finds itself in surroundings that it has inhabited before, its
memory of local landmarks and former habit will lead it to
its exact destination ; but that, of course, is not navigation.
Throughout these chapters we have seen that animals,
from the simplest to the most complex, are endowed with
sense-organs that enable them to adapt their behaviour to
the nature of their surroundings. Animals are as it were shut
up in their own bodies and are desperately trying to find
out what is going on outside. The simpler senses tell them
something about the objects with which they come in contact,
the more complicated ones about things which are at a
distance. Equally important are the senses that tell what is
happening inside the body. The sum of the information
supplied by the senses is used for the two primary purposes
of life, if we may speak teleologically, or assume that it has
purpose — staying alive by finding food and avoiding dangers,
and reproducing by finding a mate and leaving progeny to
fill the gap that will be left at the final error in interpreting
the messages from the sense organs, or at a misfortune against
which they give no protection. All the more complex
sense-organs are, in effect, transformers for turning stimuli
of all kinds into physico-chemical changes at the nerve-
endings. The changes start impulses travelling along the
nerves and they are relayed either by the central nervous
system or by shorter paths to the muscles and other structures
that can make the appropriate response. Apart from the
shorter reflex paths, the senses are of no avail without the
brain to interpret and co-ordinate the messages received
from the end-organs. In spite of all the beautiful mechanisms,
the brain can easily be mistaken in its interpretation,

290 THE SENSES OF ANIMALS

especially in new or unfamiliar situations, and although the
acuity of the senses is often astonishingly high, the senses are
often deceived so that "fings ain't by no means always wot
they seems t'be".

INDEX

Adder, 55 ; its elliptical pupil, 28 ;

sense of smell, loo-i
African River Shrew {Potamogale) ,

209
Albatross, their sense of smell, 103,

199

Albinism, and ocular defects, 28

Alligators, 46 ; good vision on land
and under water, 48-9; good
nocturnal vision, 49 ; hearing, 72

Amoeba, its reaction to stimuli,
12576

Amphibians; variations of sense
perception, 17; sight: ade-
quacy, 24 ; the nictitating mem-
brane, 25 ; observations and
experiments, 35-46; eye socket
incomplete, 140; hearing:
importance of, in certain groups,
71-2; observations, 77-8; struc-
ture of the ear, 174, 177;
SMELL : scenting powers, 9 1 ,
102-3, 200; experiments, 103;
TOUCH : experiment to test re-
sponse, 1 1 9

Ampullae of Lorenzini, 215

Anableps ("Four-eyed Fish"), its
optical mechanism, 152-3

Angular - acceleration - receptor,
219-20, 222

Antennae : organs of touch, smell
and taste, 17, 109, 116; of
ichneumon flies, 1 18

Ants, sensitive to odours, 98

Apes : colour vision of anthropoid
apes, 68 ; sense of taste, 1 1 1 ;
sense of touch, 114, 120-1

Aquatic worms, touch-receptors
of, 211

Archer fish. See Toxotes

Arthropods, 160 (an^/ j^g Insects)

Ascidians (Sea-squirts), sight of,
157

Asdic, 187

Amelia (jelly-fish), nerve network

of, 133-4
Axons, 128

Badgers, 1 3 ; and nocturnal vision,
29; scenting power, 106; use of
scent for marking out territory,
107; sense of touch, 120

Balancing sense, 219-23

Barbel (fish), 21 1

Barbels, as tactile organs, 2 10- 11

Barn Owl, sight of, 60, 63

Barrington, F.J. F., 112

Barton, Dr., 181

Bats : eyes of, 67 ; hearing, 70, 75,
84, 86; use of sonar, 183, 184-6;
Horseshoe Bats' use of Doppler
effect, 186

Bears, short-sightedness of, 65

Bees, 71, 226

Beetles, 16; as food of owls, 16-21 ;
no significance in their hum-
ming noise, 71
See also Burying beetles

Bellairs, Dr. Angus d'A., Reptiles,

Beluga of White Whale. See under
Whales

Birds : sight : importance of, 19;
colour vision, 26-7, 55-6, 150-1,
154; observations and experi-
ments, 55-65; acuity, 56, 58-
instinctive recognition of shape,
56-8; song birds and high-
frequency sounds, 70 ; their eye-
lids, 139; eyelashes, 140; keep
pupils fully open, 142; optical
mechanism, 147-8, 150-1 ; in-
ternal structure of the eye, 148;
hearing: acuity and varied
uses, 73, 80, 81; response to
alarm calls, 73-4; purpose of

231

232 INDEX

Birds {cont.)

their songs, 81 ; mimicry, 82-3,
1 74-5 ; experiments, 83 ; struc-
ture of the ear, 1 74-6 ; location
of direction of sound, 175, 176;
use of sonar, 183, 192-3 ; smell :
most birds deficient in, 19, 92,
103; good in petrels and alba-
trosses, 1 98-9 ; TASTE : well-
developed, 19, no; touch:
delicate tactile perception, 19,
1 14-15 ; field observations, 119;
monitors : homing and migra-
tion, 225, 226-9

Blackbirds : hearing, 73 ; alarm
call, 73-4

Blind Worm. See Slow Worm

Bloodworms, as food for newts, 44

Brain: in man and animals, 135-
136; and sight, 150; of molluscs,
158; of whales, 188

Bream, 191

Bristow, W. S., World of Spiders, 23

Brown Rat, hearing of, 75

Budgerigars, mimicry of, 82

Burying beetles, scenting powers
of, 16, 98-9

Bush-babies (galagos), hearing of,
84, 86-8

Bushby, L. C, 118

Butterflies : sight : colour vision,
167; hearing: lack of auditory
organ, 180; smell: field obser-
vations and experiments on, 97;
male production of perfumes,
202

Camouflage, 135

Carp, 191

Cat Fishes, 191

Caterpillars: reaction to light, 22;

reaction to vibrations, 76 ; sense

of taste, no; their ocelli, 162-3
Cats: homing ability, 15, 226;

good nocturnal vision, 29, 84-5 ;

hearing, 84, 85 ; sense of taste,

in; sense of touch, 1 20
Catde, 85
Cells, specialization of functions

and co-ordination of, 127

See also Nerve cells ; Receptor

cells ; Sensory cells
Central nervous sytem, 131, 134

Cephalopods, eyes of, 158

Cephalothorax, 161

Chameleons : unique structure of
their eyes, 49-50; observations
and experiments on their sight,
50-1

Chemical sense, generalized, 215

Chemo-receptor organs, 205-7

Chemo-tactic organs, 90

Chimpanzees, their sense of touch,
1 20-1

Chlorophyll, its function in Proto-
zoa, 127

Chub, 26

Cicadas, auditory organs of, 1 79

Cilia, of Protozoa, 126

Cobra, 53

Cockchafers, as food of owls, 62

Cochlea of the ear, in mammals,
171, 220

Cockles, eyes of, 157, 158

Cod, 21 1

Cold-receptor, 213

Colour vision, 146; of birds, 26-7,
55-6, 156; limited vision of most
reptiles, 47-8 ; of lizards, 52 ;
most animals colour-blind, 150;
theories of, 1 54-5

Common or Smooth Newt, 45

Common or Viviparous Lizard:
colour vision, 47, 52 ; acuity of
sight, 49 ; observations on sight,
5 1 ; hearing, 72-3 ; observations
on hearing, 79-80

Copper-head or Moccasin, 215

Corpuscles, 208

Crabs, their sense of taste, no-i i

Crayfish : its touch-receptors, 211-
212; its statocyst, 222-3

Crickets : stridulation, 7 1 ; experi-
ment on their hearing, 76;
auditory organs, 179

Crocodiles, 46 ; good vision on land
and in water, 25, 48-9; keen
sense of smell, 25, 100; their
nictating membrane, 25 ; good
nocturnal vision, 49 ; hearing, 72

Crows, mimicry of, 82, 83

Crustacea: sense of sight, 157, 160,
164, 167; touch-receptors, 211

Cuckoo, 57-8; auditory acuity,
81-2; migration, 225

Curlew, its sense of touch, 92, 1 14

Dahlia Wartlet (sea-anemone) ,
128; reaction to stimuli, 129

Death's Head Moth, 71

Deer : acuity of senses, 66 ; acuity
of hearing, 75, 85 ; excretion of
scent from glands, 107

Diablotin, its use of sonar, 192-3

Dogfish, 191

Dogs: homing ability, 15, 226;
eyesight, 28 ; acuity of hearing,
84 ; location of sounds, 85 ;
scenting powers of sporting
dogs, 104-6; sense of taste, iii

Dolphins, 210; use of sonar, 183,
187-8; reaction to Asdic, 187,
188; docility and playfulness,
188; sense of sinell, 198

Doppler effect, 186

Dormouse, experiment of scenting
power of, 107

Dragonflies: visual acuity, 16, 34,
164; number of lenses in their
eyes, 23; observations on sight

o^, 33-4
Ducks: scenting power, 19, 92;

sense of touch, 1 14
Dugong, its sense of smell, 198

Eagles, 148

Ear(s) : mechanism and structure
in mammals, 169-73; of fishes,
1 74 ; of birds, 1 74-6 ; of reptiles,
amphibians, fishes, 177-8; com-
parison of vertebrates and in-
vertebrates, 1 78-9 ; of insects,
179-80; of whales, 190-1 ; pro-
prioceptive sense served by inner
ear, 219-23

Ear-flaps. See External ears

Earthworms : negative reaction to
light, 21, 22, 137; method of
catching, 32-3 ; as food for
newts, 44; responsive to touch,
119; their receptors for smell,
201

Echo-location (sonar) : the prin-
ciple of, 1 82-3 ; animals that use
it, 183; its essence, 183-4; high
frequency of sound waves used,
184; by bats, 184-7; the Dop-
pler effect used by certain bats,
186; by whales and dolphins,
187-91; by the Ostariophysi,

INDEX 233

191 ; by birds, 192-3; use of an
electric field to probe surround-
ings, 193-4

Echo-sounder, 182

Edible Frog : observations on sight
of, 39-40; territorial defence
behaviour, 77

Electric currents, reaction of fishes
to, 216-17

Electric eel, 193, 194

Electric organs, of fishes, 193

Elephant : hearing of, 74, 84 ;
touch, 1 2 1-2; smell, 196

Elephant Hawk Moths, observa-
tions and experiments on, 97

Elliptical pupil, and nocturnal
vision, 28

Eltringham, Professor, 167

Emperor Moth : visual acuity, 1 6-
1 7 ; scenting power, 98

External ears or "ear-flaps"
(pinnae) : possessed only by
mammals, 83, 170; of elephants,
84; of bush-babies, 86

Eyed Lizards, 49

Eye(s) : number of, in spiders, 23,
161 ; simple eyes (ocelli) and
compound eyes, 23, 160-7;
unique structure of chameleon's,
49-50; structure of, in higher
animals, 138; optical system of,
140-5; internal structure of a
bird's eye, 148; of molluscs,
157-60

Feeding behaviour. See Movement
of prey

Fer de lance, 215

Fishes : sight : good sight of most
fishes, 18, 25-6; optical mech-
anism, 147, 152-3; in deep and
shallow waters, 15 1-2; colour
vision, 154; hearing: sensitivity
to vibrations, 73 ; possess only
an inner ear, 174; structure of
the ear, 177-8; use of sonar,
1 9 1-2; use of an electric field to
probe surroundings, 1 93-4 ;
smell: scenting power, 91-2,
99-100, 200; barely distinct
from taste, 205-6 ; taste : part
played by smell in, 1 10 ; touch :
response to vibrations, and the

234 INDEX

Fishes (cont.)

lateral line, i8, 180-1, 194;
barbels as organs of touch, 210-
1 1 ; sensory pits in sharks and
rays, 215; reaction to electric
currents, 216-17

Flagellum, of Protozoa, 1 26

Flies, their special hairs serving as
tongues, 109

"Four-eyed Fish". See Anableps

Foxes : short-sighted 65 ; hearing
75; scenting power, 106; sense
of taste 1 1 1

Frogs, brain of, 135; sight: and
moving prey, 24, 135; observa-
tions and experiments 35-40 ;
climbing ability, 37-8; a pecu-
liar use of their eyes, 140;
HEARING : auditory sense 14-15 ;
importance of sound 71-2 ; func-
tion served by croaking 72,
77-8; field observations, 77;
smell: scenting powers, 91,102,
200 ; TOUCH : sensitivity over
whole body, 119; monitors :
homing ability

Fruits eaten by lizards, 52

Galagos. See Bush-babies

Geckos, observations and experi-
ments on their sight, 49

Geese, scenting power of, 19, 92

Goldcrest, 81

Gorillas, their sense of touch, 1 20

Gosse, P. H., 128-30

Grass Snake, experiment on its
sense of smell, 10 1-2

Grasshoppers : stridulation, 7 1 ;
experiment on its hearing, 76-7 ;
auditory organs, 179

Gravity-receptor, 219-20, 222

Grayling, 26

Great Warty Newt, 45

Green Lizard, 47, 49, 5 1 , 52

Greyhounds : sight, 28 ; scenting
power, 106

Haddock, 21 1

Halteres, of insects, 223

Hares : eyesight and defence 28 ;

hearing 75^ 85
Hawker Dragonflies, 16, 33-4
Hawks, 148

Hearing, sense of: definition, 14,
169; and communication be-
tween animals, 69; response to
sounds, 69-70; more vital than
sight to some animals, 70 ; field
observations and experiments : larger
invertebrates, 76^7 ; amphibians,
77-8; reptiles, 78-80; birds, 80-
83; mammals, 84-8; — mechan-
ism and structure of the ear,
169-73; inner and middle ears,
174; structure of birds' ears,
174-6; location of direction of
sound, 175, 176; structure of
ears of reptiles, amphibians,
fishes, 177; comparison of verte-
brates and invertebrates, 1 78-9 ;
auditory organs of insects, 1 79-
80; and the sensilla of insects,
180; the lateral line in fishes,
1 80- 1
See also Echo-location

Heat-receptor, 213

Hedgehog: sight, 28; scenting
power, 107; sense of touch, 120

Hedge-sparrows, 73

Homing ability, 15, 225-6, 228

Hornbills, eyelashes of, 140

Horses, 85, 219

Horseshoe bats, their use of sonar,
186

Hounds, scenting power of, 104

Hunger, proprioceptive reaction
to, 224

Hydra : their limited sensory equip-
ment, 15-16, 128; experiment
to test sense of touch, 1 1 8

Ichneumon flies, their sense of
touch, 1x8

Infra-red radiation, detection of,
by pit- vipers, 215-16

Insects : variations in dominant
senses, 16-17; sight: two kinds
of eyes, 23 ; field observations,
33-4; optical mechanism, 160-
167; colour vision, 167; hear-
ing : importance of, 71; stridu-
lation, 71 ; experiments on, 76-
77; auditory organs, 179-80;
smell: organs for detecting
scent, 90-1, 97, 201-3; field
observations and experiments,

INDEX

97-g; scents for defensive pur-
poses, 99 ; TASTE : taste-by-
touch, 16; special hairs serving
as tongues, 109; discrimination,
no; TOUCH : hairs as organs of
touch, 1 15-16; the antennae
and ovipostors of ichneumon
flies, 118; their sensilla, 180;
MONITORS : the proprioceptors,
223

Jacobson's organ, 17, 91, 200
Jelly-fish : few special sense-organs,
127-8; nerve network, 133-4

Kestrel, its sight, 58-9

Ladybirds, 99

Lagena : of birds, 1 74 ; of fishes,

177
Lampreys, 219, 221
Lapwings, 135
Larvae, their negative reaction to

light, 22-3
Lateral line, of fishes, 18, 180-1,

194

Lemurs, their use of scent for
marking out territory, 107

Limpet: its sight, 157, 167-8;
homing instinct, 225-6

Little Owl : its sight, and field
observations on, 60-2

Lizards, 46; sight: and moving
prey, 25, 5 ^ 52-3; eyelids, 26,
139; limited colour vision of
some species, 47, 52 ; observa-
tions and experiments, 49-53 ;
best sighted of all reptiles, 49 ;
hearing: sensitivity, 72-3, 177;
observations and experiments,
78—80 ; SMELL, dependence upon,
18, 100; the tongue a detector
of smells, 91, 200; experiment
on, 102; olfactory mechanism,
1 99-200 ; TOUCH : sensitivity,

119

Lob-worm, 32

Long-eared Owl : sight, 60 ; its
ear-tufts used for display, 61

Lurchers (greyhound cross), scent-
ing powers of, 106

Macaws, their sense of touch, 1 1 5

235

McBride, Arthur, 188-9

Mammal Society of the British
Isles, 27

Mammals : sight : characteristics,
27-8; nocturnal vision, 28-9;
observations and experiments,
65-8 ; mostly lack colour vision,
68, 150, 154; hearing: sensiti-
vity, 19-20, 74-5, 83; the only
creatures with external ears, 83-
84; experiments on, 84; struc-
ture of the ear, 169-73; smell:
importance of, for varied pur-
poses, 19, 197-8 ; scenting power,
89, 93-4 ; field observations and
experiments, 103-8; scenting
powers of sporting dogs, 105-6;
TASTE : taste-smell relationship,
III; behaviour of carnivores
with pregnant female prey, 1 1 1-
112; discrimination, 1 12-13;
touch : whiskers as sensitive
tactile features, 115; variations
of manifestation, 119-20; of
badgers, hedgehogs, cats, ra-
coons, 1 20 ; monkeys and apes,
1 20-1; whiskers as tactile or-
gans on aquatic mammals, 209-
210

Manatee, its sense of smell, 198

Marsh Frog: observations on its
sight, 39-40; territorial defence
behaviour, 77-8

Mealworm, 41

Medway, Lord, 192

Memory, in birds, 82-3

Metazoa, 127-8

Mice, 41; sight, 28; hearing, 75;
scenting power, 1 06

Micro-climates, 94

Microscope, for observing lower
organisms, 30-1

Migration, 225-9

Millipedes, 201

Mimicry, of birds, 82, 1 74-5

Moles : experiments on sight of,
67-8 ; sense of touch, 120

Molluscs: eyes of, 157-60; touch-
receptors, 2 1 1

Mongoose : attacks on snakes, 54 ;
uses scent for marking out terri-
tory, 107; behaviour before eat-
ing prey, 1 1 1-12

236 INDEX

Monkeys : colour vision, 68 ; sense
of taste, III, 113; sense of touch,
114, 120

Mosaic vision, 23, 165

Moths : HEARING : reception of
high frequency sounds, 180;
their counter to the sonar of
bats, 187; smell: organs for
detecting scent, 97 ; males de-
tect scent of females with
antennae, 97, 98, 201-2

Movement of prey : importance of,
in feeding behaviour, 16, 17, 24,
27, 41 ; responses of lizards, 25,
51-3; responses of dragonflies,
34; responses of tree-frogs, 38;
in water, and responses of
turtles, terrapins, pond tor-
toises, 48

Muscle sense, 218-19

Mussels, eyes of, 158

Mynahs, mimicry of, 82, 1 74-5

Ommatidia (tubular cameras),
164-7

Orang-outang, its sense of touch,
120

Ostariophysi, and sonar, 191-2

Ostrich, its eyelashes, 140

Otocysts. See Statocysts

Otters, their whiskers as tactile
organs, 209

Ovipostors, of ichneumon flies, 1 18

Owls : SIGHT : nocturnal and di-
urnal vision, 60-1 ; observations
and experiments on, 61-5;
optical mechanism, 148-9, 151;
alleged ability to detect infra-
red radiation, 216; hearing:
importance in hunting behav-
iour, 73; location of direction of
sound, 175, 176; structure of
ear, 175, 176

Oyster-catchers, 135

Oysters, eyes of, 157, 159

Natterjack toad, 40-1
Nautilus, eye of, 158
Neck, short. See Short neck theory
Nerve cells : in sea-anemone, 1 28,

131, 133; nature and function,
1 3 1-4; in Aurelia (jelly-fish),
133; receptor cells for smell,
196-7; receptor cells in taste-
bulbs, 204

Nervous impulse, basically a
physico-chemical change, 131-

132, 134

Newts : scenting power, and ex-
periment on, 17, 102-3; voice-
lessness, 17, 72; field observa-
tions and experiments on its
sight, 43-6 ; sense of taste, 1 1 o ;
responsive to touch, 1 19
Nictitating membrane, 25, 139
Night-jars, their sense of touch,

Nocturnal vision : of amphibians,
24 ; of mammals, 28-9, 84-5 ; of
owls, 60-1

Norman, J. R., 181

Oak Eggar Moth, 98
Ocelli (simple eyes), 23, 160-4
Octopuses: eyes of, 157, 158; their
gravity-receptorSj 221-2

Pain, receptors for, 213-14

Palmate Newt, 45

Parrots : mimicry of, 82, 83 ;

sense of smell, 92-3; sense of

touch, 1 14-14, 119
Penguins, 142

Perch, its scenting power, 91
Petrels, their sense of smell, 103,

199
Pigeons, homing ability of, 226
Pike, scenting power of, 91
Pinnae. See External ears
Pit vipers : sense of touch, 115;
ability to detect infra-red radia-
tion, 215-16
Planarians, their limited sensory

equipment, 15-16
Plumose anemone, 130
Poacher, as field naturalist, 106
Pointers, scenting powers of, 105
Pond tortoise, its response to move-
ment of prey in water, 48
Porpoise, its sense of smell, 1 98
Prawn, its response to differences

in water pressure, 225
Proprioceptive senses, 218 ff".
Proprioceptors, of insects, 223
Protopathic sensation, receptors of,

214
Protozoa, 125; limited sensory

equipment, 1 5 ; perception of
light, 21 ; susceptibility to vibra-
tions, 71 ; reaction to stimuli,
126-7; eye-spots of, 156
See also Amoeba
Pumphrey, Professor, 171, 173

Rabbits, 85

Racoons, their sense of touch, 1 20

Radar, 183, 187

Rats : eyesight, 28 ; hearing, 75 ;

experiments in intelligence of,

96 ; sense of taste, 1 1 2
Rattlesnakes : sense of touch, 115;

ability to detect infra-red

radiation
Rays : hearing, 1 74, 177; sensory

pits in, 181, 215; electric organs

of, 193

Receptor cells: for smell, 196-7;
in taste-bulbs, 204; for touch,
211-13; for temperature and
pain, 213-14; gravity receptor,
2 1 9-20 ; angular-acceleration-
receptor, 219-20

Redshanks, alarm note of, 74

Reptiles : variations of sense per-
ception, 17-18; sight: varia-
tions in role of the eye, 24-5 ;
eye-coverings, 25, 139; optical
mechanism, 46-7, 147; field
observations and experiments,
47-54; colour vision, 47-8, 52,
154; hearing: variations in,
72 ; observations and experi-
ments on, 78-80 ; structure of the
ear, 177; smell: the tongue a
detector of smells, 91 ; olfactory
mechanism, 199-200; touch:
experiment to test response, 119

Retrievers, their scenting powers,
105

Rhinoceros, its short-sightedness,
65, 67

Ringed plover, 135

Roach 1 9 1

Robins their hearing, 73

Rodents, their hearing, 74-5

Roe Deer, their excretion of scent
from glands, 107

Rotifers : their limited sensory
equipment, 15-16; suscepti-
bility to vibrations, 71

INDEX 237

Salamanders : field observations
and experiments on their sight,
43, 46 ; sense of smell, 200
Sand Lizards : colour vision, 47,
52 ; acuity of sight, 49 ; field
observations on their sight, 51 ;
hearing, 73 ; field observations
and experiments on their hear-
ing, 78-80
Savage, Dr. Maxwell, 91
Scallop, eye-spots of, 1 59-60
Scarab beetles, their sense of

smell, 202
Sea anemones : responsive to touch,
119; few special sense-organs,
127-8; co-ordination of their
cells, 128; nerve-cells, 128, 131,
133; reaction to chemical sti-
muli, and to light, 129-30
Sea-cows, their sense of smell, 198
Sea snail {Bullia), its sense of

smell, 207
Sea-squirts. See Ascidians
Seals : good sight on land and in
water, 29 ; lack naso-lachrymal
ducts, 139; optical mechanism,
153; use of sonar, 190; sense of
smell, 198; their whiskers as
tactile organs, 209; their fast
before breeding, 224
Sensations : additional to sense of
touch, 114; in cold-blooded
animals, 1 15
Sensilla, of insects, 180, 201
Sensory cells, function of, 13 1-3
Sensory pits, in sharks and rays,

2'5 . . . .

Shape, birds' instinctive recogni-
tion of, 56-8
Sharks: scenting power, 91, 99-

100; hearing, 174, 177, 191-2;

sense organ similar to lateral

line, 181
Shearwaters, homing ability of,

226
Sheep, 85
Shield bugs, 99
Short-eared Owl : its sight, and

field observations on, 60-1
Short neck theory, 56-8
Shrews : experiments on their

sight, 67-8 ; sense of touch, 1 20

238 INDEX

Sight, sense of: subordinate to
smell and hearing in most
animals, 20 ; perception of light
among lower forms of life, 2 1 ;
positive and negative reactions
to light, 22-3 ; in spiders, 23,
161 ; in insects, and mosaic
vision, 23; in amphibians, 24;
in reptiles, 24-5 ; in fishes, 25-6 ;
in birds, 26-7 ; in mammals, 27-
28 ; position of eyes and size of
eyeballs indicate habit and be-
haviour, 28; marine mammals,
29; field observations and experi-
ments: use of microscope, 30-1 ;
studying animals in captivity,
31 ; worms, 32-3; insects, 33-4;
spiders, 34-5 ; amphibians, 35-
46; reptiles, 46-55; birds, 55-
65; mammals, 65-8; — the
fundamental principle dictated
by physical properties of light,
137; structure of the eye, i 38;
eyelids and lack of eyelids, 139;
most land vertebrates possess
third eyelid, 139; eyelashes,
139-40; the incomplete eye
socket of amphibians, 140;
optical system of the eye, 1 40-5 ;
a blind spot in all vertebrates,
145; wide field of view in most
animals, 1 46 ; stereoscopic sight,
146; optical mechanism and
internal structure of eye in birds,
147-9, 1 50- 1 ; use of eyes learnt
by experience, 149; fallibility,
149-50; and the image in the
brain, 150; of fishes in deep and
shallow waters, 151-3 ; seals and
whales, 153-4; theories of colour
vision, 1 54-5 ; evolution from
eye-spots to eyes, 1 56-7 ; eyes of
molluscs, 157-60; simple eyes
(ocelli) and compound eyes,
1 60-7 ; diffraction and refrac-
tion, 167-8

Skates, 193

Sleep, 70

Slow Worm : acuity of sight, 49 ;
response to movement of prey,
52-3 ; experiments on its sight,
53 ; hearing, 73 ; experiments
on its sense of smell, 102

Slugs : sense of taste, 1 1 1 ; respon-
sive to touch, 119; their recep-
tors for smell, 201

Smell, sense of: closely linked with
sense of taste, 16, 90, 109, 113,
195, 203; a chemical sense, 89;
factors affecting scents given off
and perceived, 90; and micro-
climates, 94; limitations of
laboratory experiments, 95-6 ;
field observations and experiments:
insects, 97-9; fishes, 99-100;
reptiles, 1 00-2 ; amphibians,
102-3; birds, 103; mammals,
103-8; — mammalian use of
scent for marking out territory,
107; scents given off by female
mammals in season, 107-8;
smells cannot be classified ob-
jectively, 195 ; olfactory mechan-
ism of vertebrates, 195-7; ™"
portant to most mammals for
varied purposes, 197-8; birds,
198-9; reptiles, 199-200; am-
phibians, 200; insects, 201-3

Smith, Malcolm, 40

Smooth Newt, 45

Smooth Snake, 102

Snails : sense of taste, in; respon-
sive to touch, 119; sight, 157;
their receptors for smell, 201

Snakes : sight : problemis, 24-5 ;
devoid of moveable eyelids, 25,
139; their elliptical pupils, 28;
faulty observation of, 53 ; snake-
charmers and the cobra, 53 ;
short-sighted, 53, 54; trail prey
by scent, 53-4; part played by
eyes in capturing prey, 54-5 ;
HEARING : lack ears, but sensi-
tive to vibrations, 18, 72, 78,
177, 178; smell: dependence
upon, 1 8, 1 00-2 ; trail prey by
scent, 53-4; the tongue a de-
tector of smells, 91, 200; field
observations and experiments,
1 00-3 ; olfactory mechanism,
199-200; touch: among pit
vipers, 115; keen response to,
119; detection of infra-red radia-
tion by pit vipers, 215-16

Snipe, their sense of touch, 92, 1 14

Sonar. See Echo-location

South American Giant Toad, 41,

43.
Spaniels: their sight, 28; scenting

powers, 105

Spiders : sight : and courtship
dances, 16; number of eyes, 23,
161 ; observations and experi-
ments, 34-5 ; HEARING : experi-
ments on, 76 ; SMELL : special-
ized sense organs for, 201 ;
touch: keen response to, 118;
web-spiders' reliance on, 16

Spiracle, 174

Spotted Salamander, 46

Squids, eyes of, 157, 158

Squirrels: scenting power, 106,
107; hibernation, 228

Starfish, responsive to touch, 119

Starlings, mimicry of, 82, 83

Statocysts, 222

Stereoscopic sight, 146

Stoats : sight, 66 ; sense of taste, 1 1 2

Stridulation, 71, 76

Swiftlets, their use of sonar, 192

Synapse, 128

Taeliafelina (sea-anemone), 128

Taste, sense of: closely linked with
sense of smell, 16, 90, 109, 113,
195, 203 ; a chemical sense, 109 ;
taste-bulbs, 109, 203-4; of
aquatic animals, no; fish and
amphibians, iio; birds, iio;
taste smell relationship, iio,
III; mammals, in; behaviour
of carnivores with pregnant
female prey, 1 1 i-i 2 ; carnivores'
avoidance of gall-bladder, 112;
discrimination by rats, stoats,
monkeys, 1 12-13; response to,
human and animal, 204-5 ;
lack of distinction between
taste and smell in aquatic
animals (chemo-reception),

205-7

Taste-bulbs, 109; structure of,
203-4

Tawny or Brown Owl : its sight,
60 ; field observations, 62-3

Temperature-receptors, 2 1 3

Tench, 191

Terrapins, 46; response to move-
ment of prey in water, 48;

INDEX 239

hearing, 72; sense of taste, no;
experiment to test response to
touch, 119

Terriers, scenting powers of, 105

Third eyelid. See Nictitating mem-
brane

Thirst, proprioceptive reaction to,
224

Thorpe, Dr. VV. H., 83

Thrushes, hearing of, 73

Tits, 73

Toads : sight : field observations
and experiments, 43-6; a pecu-
liar use of, 140; hearing:
importance of sound, 71-2;
function served by croaking, 72,
77; field observations on, 77;
smell: scenting power, 91, 102-
103, 200; taste: in water, no;
touch: sensitivity to, 119;
monitors: homing ability, 226

Tongue : of snakes and lizards as
detector of smells, 17, 91, 200;
of parrots, tactile use of, 114-15 ;
of snakes and lizards, as means
of testing environment by touch,
200
See also Taste-bulbs

Torpedo, 193, 194

Tortoises, 46; good sight of, 24;
limited colour vision, 47-8;
hearing, 72; smell, 100; taste,
1 10; touch, 1 19

Touch, sense of: and feeling, 114;
in different orders of animals,
114-16; affinity with hearing,
117; field observations and experi-
ments : vorticella, 117; hydra,
118; ichneumon flies, 118;
amphibians and reptiles, 119;
birds, 119; mammals, 119-22;
— response of sea-anemones, 1 29,
simplest of the senses, 1 34 ;
served by corpuscles and nerve-
endings at roots of hairs, 208-9 ;
whiskers as tactile organs, 209-
210; barbels as tactile organs,
2 lo-i I ; in certain invertebrates,
crustaceans, insects, 211-12;
powers of discrimination, 212-
213; receptors for heat, cold,
pain, 213-14; the generalized
chemical sense, 215; sensory

240 INDEX

Touch sense of {cont.)

pits in sharks and rays, 215;
detection of infra-red radiation
by pit vipers, 215-16; reaction
of fishes to electric currents,
216-17

Toxotes (Archer fish), 153

Tree-frogs, sight of, 24, 38-9

Trout, 26

Tubular cameras. See Ommatidia

Turtles, 46; good sight of, 24;
their nictitating membrane, 25 ;
response to moving prey in
water, 48; sense of smell, 100

Unicellular animals, 15

Viviparous lizard. See Common
lizard

Voles: eyesight, 28; as prey of
Short-eared Owl, 61 ; as prey
of adders, loo-i

Vorticella, their sensitivity to
vibrations, 71, 117

Vultures: sight of, 59-60; de-
ficient in sense of smell, 92

Wall lizards, 49, 51, 53

Walrus, 210

Wasps, 71, 226

Water-fleas, their perception of
light, 21, 22

Water Vole, hearing of, 74

Web-spiders. See Spiders

Wenham tube, 30

Whales, 209-10; good sight above
and below water, 29 ; optical
mechanism, 153-4; use of sonar,
183, 189-90; Beluga Whale's
use of sonar, 187; reaction to
Asdic, 187-8; structure of ear,
1 90- 1 ; sense of smell, 198

Whelks, 207

Whiskers, tactile nerves at ends of,
20, 1 15, 209-10

Woles, their sense of taste, 1 1 1

Wood Mouse, hearing of, 75

Woodcock, its sense of touch, 92,
1 14

Woodpeckers, their sense of touch,

ii5> 119
Worms. See Earthworms
Wrens, 73