The Outline of Science, Vol. 1 (of 4)

J >> J. Arthur Thomson >> The Outline of Science, Vol. 1 (of 4)

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26

We have said enough to show how well adapted many animals are to meet
the particular difficulties of the haunt which they tenant. But
difficulties and limitations are ever arising afresh, and so one fitness
follows on another. It is natural, therefore, to pass to the frequent
occurrence of protective resemblance, camouflage, and mimicry--the
subject of the next article.

BIBLIOGRAPHY

ELMHIRST, R., _Animals of the Shore_.
FLATTELY AND WALTON, _The Biology of the Shore_ (1921).
FURNEAUX, _Life of Ponds and Streams_.
HICKSON, S. J., _Story of Life in the Seas_ and _Fauna of the Deep Sea_.
JOHNSTONE, J., _Life in the Sea_ (Cambridge Manual of Science).
MIALL, L. C., _Aquatic Insects_.
MURRAY, SIR JOHN, _The Ocean_ (Home University Library).
MURRAY, SIR JOHN AND HJORT, DR. J., _The Depths of the Ocean_.
NEWBIGIN, M. I., _Life by the Sea Shore_.
PYCRAFT, W. P., _History of Birds_.
SCHARFF, R. F., _History of the European Fauna_ (Contemp. Sci. Series).
THOMSON, J. ARTHUR, _The Wonder of Life_ (1914) and
_The Haunts of Life_ (1921).

IV

THE STRUGGLE FOR EXISTENCE

ANIMAL AND BIRD MIMICRY AND DISGUISE

Sec. 1

For every animal one discovers when observing carefully, there must be
ten unseen. This is partly because many animals burrow in the ground or
get in underneath things and into dark corners, being what is called
cryptozoic or elusive. But it is partly because many animals put on
disguise or have in some way acquired a garment of invisibility. This is
very common among animals, and it occurs in many forms and degrees. The
reason why it is so common is because the struggle for existence is
often very keen, and the reasons why the struggle for existence is keen
are four. First, there is the tendency to over-population in many
animals, especially those of low degree. Second, there is the fact that
the scheme of nature involves nutritive chains or successive
incarnations, one animal depending upon another for food, and all in the
long run on plants; thirdly, every vigorous animal is a bit of a
hustler, given to insurgence and sticking out his elbows. There is a
fourth great reason for the struggle for existence, namely, the frequent
changefulness of the physical environment, which forces animals to
answer back or die; but the first three reasons have most to do with the
very common assumption of some sort of disguise. Even when an animal is
in no sense a weakling, it may be very advantageous for it to be
inconspicuous when it is resting or when it is taking care of its young.
Our problem is the evolution of elusiveness, so far at least as that
depends on likeness to surroundings, on protective resemblance to other
objects, and in its highest reaches on true mimicry.

Colour Permanently Like That of Surroundings

Many animals living on sandy places have a light-brown colour, as is
seen in some lizards and snakes. The green lizard is like the grass and
the green tree-snake is inconspicuous among the branches. The spotted
leopard is suited to the interrupted light of the forest, and it is
sometimes hard to tell where the jungle ends and the striped tiger
begins. There is no better case than the hare or the partridge sitting a
few yards off on the ploughed field. Even a donkey grazing in the dusk
is much more readily heard than seen.

The experiment has been made of tethering the green variety of Praying
Mantis on green herbage, fastening them with silk threads. They escape
the notice of birds. The same is true when the brown variety is tethered
on withered herbage. But if the green ones are put on brown plants, or
the brown ones on green plants, the birds pick them off. Similarly, out
of 300 chickens in a field, 240 white or black and therefore
conspicuous, 60 spotted and inconspicuous, 24 were soon picked off by
crows, but only one of these was spotted. This was not the proportion
that there should have been if the mortality had been fortuitous. There
is no doubt that it often pays an animal to be like its habitual
surroundings, like a little piece of scenery if the animal is not
moving. It is safe to say that in process of time wide departures from
the safest coloration will be wiped out in the course of Nature's
ceaseless sifting.

But we must not be credulous, and there are three cautions to be borne
in mind. (1) An animal may be very like its surroundings without there
being any protection implied. The arrow-worms in the sea are as clear as
glass, and so are many open-sea animals. But this is because their
tissues are so watery, with a specific gravity near that of the salt
water. And the invisibility does not save them, always or often, from
being swallowed by larger animals that gather the harvest of the sea.
(2) Among the cleverer animals it looks as if the creature sometimes
sought out a spot where it was most inconspicuous. A spider may place
itself in the middle of a little patch of lichen, where its
self-effacement is complete. Perhaps it is more comfortable as well as
safer to rest in surroundings the general colour of which is like that
of the animal's body. (3) The fishes that live among the coral-reefs are
startling in their brilliant coloration, and there are many different
patterns. To explain this it has been suggested that these fishes are so
safe among the mazy passages and endless nooks of the reefs, that they
can well afford to wear any colour that suits their constitution. In
some cases this may be true, but naturalists who have put on a diving
suit and walked about among the coral have told us that each kind of
fish is particularly suited to some particular place, and that some are
suited for midday work and others for evening work. Sometimes there is a
sort of Box and Cox arrangement by which two different fishes utilise
the same corner at different times.

[Illustration: THE PRAYING MANTIS (_Mantis Religiosa_)

A very voracious insect with a quiet, unobtrusive appearance. It holds
its formidable forelegs as if in the attitude of prayer; its movements
are very slow and stealthy; and there is a suggestion of a leaf in the
forewing. But there is no reason to credit the creature with conscious
guile!]

[Illustration: PROTECTIVE COLORATION: A WINTER SCENE IN NORTH
SCANDINAVIA

Showing Variable Hare, Willow Grouse, and Arctic Fox, all white in
winter and inconspicuous against the snow. But the white dress is also
the dress that is physiologically best, for it loses least of the animal
heat.]

[Illustration: THE VARIABLE MONITOR (_Varanus_)

The monitors are the largest of existing lizards, the Australian species
represented in the photograph attaining a length of four feet. It has a
brown colour with yellow spots, and in spite of its size it is not
conspicuous against certain backgrounds, such as the bark of a tree.]

Sec. 2

Gradual Change of Colour

The common shore-crab shows many different colours and mottlings,
especially when it is young. It may be green or grey, red or brown, and
so forth, and it is often in admirable adjustment to the colour of the
rock-pool where it is living. Experiments, which require extension, have
shown that when the crab has moulted, which it has to do very often when
it is young, the colour of the new shell tends to harmonise with the
general colour of the rocks and seaweed. How this is brought about, we
do not know. The colour does not seem to change till the next moult, and
not then unless there is some reason for it. A full-grown shore-crab is
well able to look after itself, and it is of interest to notice,
therefore, that the variety of coloration is mainly among the small
individuals, who have, of course, a much less secure position. It is
possible, moreover, that the resemblance to the surroundings admits of
more successful hunting, enabling the small crab to take its victim
unawares.

Professor Poulton's experiments with the caterpillars of the small
tortoise-shell butterfly showed that in black surroundings the pupae tend
to be darker, in white surroundings lighter, in gilded boxes golden; and
the same is true in other cases. It appears that the surrounding colour
affects the caterpillars through the skin during a sensitive period--the
twenty hours immediately preceding the last twelve hours of the larval
state. The result will tend to make the quiescent pupae less conspicuous
during the critical time of metamorphosis. The physiology of this
sympathetic colouring remains obscure.

Seasonal Change of Colouring

The ptarmigan moults three times in the year. Its summer plumage is
rather grouselike above, with a good deal of rufous brown; the back
becomes much more grey in autumn; almost all the feathers of the winter
plumage are white. That is to say, they develop without any pigment and
with numerous gas-bubbles in their cells. Now there can be no doubt that
this white winter plumage makes the ptarmigan very inconspicuous amidst
the snow. Sometimes one comes within a few feet of the crouching bird
without seeing it, and this garment of invisibility may save it from the
hungry eyes of golden eagles.

Similarly the brown stoat becomes the white ermine, mainly by the
growth, of a new suit of white fur, and the same is true of the mountain
hare. The ermine is all white except the black tip of its tail; the
mountain hare in its winter dress is all white save the black tips of
its ears. In some cases, especially in the mountain hare, it seems that
individual hairs may turn white, by a loss of pigment, as may occur in
man. According to Metchnikoff, the wandering amoeboid cells of the
body, called phagocytes, may creep up into the hairs and come back again
with microscopic burdens of pigment. The place of the pigment is taken
by gas-bubbles, and that is what causes the whiteness. In no animals is
there any white _pigment_; the white _colour_ is like that of snow or
foam, it is due to the complete reflection of the light from innumerable
minute surfaces of crystals or bubbles.

[Illustration: _Photo: W. S. Berridge, F.Z.S._

BANDED KRAIT: A VERY POISONOUS SNAKE WITH ALTERNATING YELLOW AND DARK
BANDS

It is very conspicuous and may serve as an illustration of warning
coloration. Perhaps, that is to say, its striking coloration serves as
an advertisement, impressing other creatures with the fact that the
Banded Krait should be left alone. It is very unprofitable for a snake
to waste its venom on creatures it does not want.]

[Illustration: _Photos: W. S. Berridge, F.Z.S._

THE WARTY CHAMELEON

The upper photograph shows the Warty Chameleon inflated and conspicuous.
At another time, however, with compressed body and adjusted coloration,
the animal is very inconspicuous. The lower photograph shows the sudden
protrusion of the very long tongue on a fly.]

[Illustration: SEASONAL COLOUR-CHANGE: A SUMMER SCENE IN NORTH
SCANDINAVIA

Showing a brown Variable Hare, Willow Grouse, and Arctic Fox, all
inconspicuous in their coloration when seen in their natural
surroundings.]

The mountain hare may escape the fox the more readily because its
whiteness makes it so inconspicuous against a background of snow; and
yet, at other times, we have seen the creature standing out like a
target on the dark moorland. So it cuts both ways. The ermine has almost
no enemies except the gamekeeper, but its winter whiteness may help it
to sneak upon its victims, such as grouse or rabbit, when there is snow
upon the ground. In both cases, however, the probability is that the
constitutional rhythm which leads to white hair in winter has been
fostered and fixed for a reason quite apart from protection. The fact is
that for a warm-blooded creature, whether bird or mammal, the
physiologically best dress is a white one, for there is less radiation
of the precious animal heat from white plumage or white pelage than from
any other colour. The quality of warm-bloodedness is a prerogative of
birds and mammals, and it means that the body keeps an almost constant
temperature, day and night, year in and year out. This is effected by
automatic internal adjustments which regulate the supply of heat,
chiefly from the muscles, to the loss of heat, chiefly through the skin
and from the lungs. The chief importance of this internal heat is that
it facilitates the smooth continuance of the chemical processes on which
life depends. If the temperature falls, as in hibernating mammals (whose
warm-bloodedness is imperfect), the rate of the vital process is slowed
down--sometimes dangerously. Thus we see how the white coat helps the
life of the creature.

Sec. 3

Rapid Colour-change

Bony flat-fishes, like plaice and sole, have a remarkable power of
adjusting their hue and pattern to the surrounding gravel and sand, so
that it is difficult to find them even when we know that they are there.
It must be admitted that they are also very quick to get a sprinkling
of sand over their upturned side, so that only the eyes are left
showing. But there is no doubt as to the exactness with which they often
adjust themselves to be like a little piece of the substratum on which
they lie; they will do this within limits in experimental conditions
when they are placed on a quite artificial floor. As these fishes are
very palatable and are much sought after by such enemies as cormorants
and otters, it is highly probably that their power of self-effacement
often saves their life. And it may be effected within a few minutes, in
some cases within a minute.

In these self-effacing flat-fishes we know with some precision what
happens. The adjustment of colour and pattern is due to changes in the
size, shape, and position of mobile pigment-cells (chromatophores) and
the skin. But what makes the pigment-cells change? The fact that a blind
flat-fish does not change its colour gives us the first part of the
answer. The colour and the pattern of the surroundings must affect the
eye. The message travels by the optic nerve to the brain; from the
brain, instead of passing down the spinal cord, the message travels down
the chain of sympathetic ganglia. From these it passes along the nerves
which comes out of the spinal cord and control the skin. Thus the
message reaches the colour-cells in the skin, and before you have
carefully read these lines the flat-fish has slipped on its Gyges ring
and become invisible.

The same power of rapid colour-change is seen in cuttlefishes, where it
is often an expression of nervous excitement, though it sometimes helps
to conceal. It occurs with much subtlety in the AEsop prawn, Hippolyte,
which may be brown on a brown seaweed, green on sea-lettuce or
sea-grass, red on red seaweed, and so on through an extensive repertory.

According to the nature of the background, [Professor Gamble writes]
so is the mixture of the pigments compounded so as to form a close
reproduction both of its colour and its pattern. A sweep of the
shrimp net detaches a battalion of these sleeping prawns, and if
we turn the motley into a dish and give a choice of seaweed, each
variety after its kind will select the one with which it agrees in
colour, and vanish. Both when young and when full-grown, the AEsop
prawn takes on the colour of its immediate surroundings. At
nightfall Hippolyte, of whatever colour, changes to a transparent
azure blue: its stolidity gives place to a nervous restlessness; at
the least tremor it leaps violently, and often swims actively from
one food-plant to another. This blue fit lasts till daybreak, and is
then succeeded by the prawn's diurnal tint.

Thus, Professor Gamble continues, the colour of an animal may express a
nervous rhythm.

[Illustration: _Photo: J. J. Ward, F.E.S._

PROTECTIVE RESEMBLANCE

Hawk Moth, settled down on a branch, and very difficult to detect as
long as it remains stationary. Note its remarkable sucking tongue, which
is about twice the length of its body. The tongue can be quickly coiled
up and put safely away beneath the lower part of the head.]

[Illustration: WHEN ONLY A FEW DAYS OLD, YOUNG BITTERN BEGIN TO STRIKE
THE SAME ATTITUDE AS THEIR PARENTS THRUSTING THEIR BILLS UPWARDS AND
DRAWING THEIR BODIES UP SO THAT THEY RESEMBLE A BUNCH OF REEDS

The soft browns and blue-greens harmonise with the dull sheaths of the
young reeds; the nestling bittern is thus completely camouflaged.]

The Case of Chameleons

The highest level at which rapid colour-change occurs is among lizards,
and the finest exhibition of it is among the chameleons. These quaint
creatures are characteristic of Africa; but they occur also in
Andalusia, Arabia, Ceylon, and Southern India. They are adapted for life
on trees, where they hunt insects with great deliberateness and success.
The protrusible tongue, ending in a sticky club, can be shot out for
about seven inches in the common chameleon. Their hands and feet are
split so that they grip the branches firmly, and the prehensile tail
rivals a monkey's. When they wish they can make themselves very slim,
contracting the body from side to side, so that they are not very
readily seen. In other circumstances, however, they do not practise
self-effacement, but the very reverse. They inflate their bodies, having
not only large lungs, but air-sacs in connection with them. The throat
bulges; the body sways from side to side; and the creature expresses its
sentiments in a hiss. The power of colour-change is very remarkable, and
depends partly on the contraction and expansion of the colour-cells
(chromatophores) in the under-skin (or dermis) and partly on
close-packed refractive granules and crystals of a waste-product called
guanin. The repertory of possible colours in the common chameleon is
greater than in any other animal except the AEsop prawn. There is a
legend of a chameleon which was brown in a brown box, green in a green
box, and blue in a blue box, and died when put into one lined with
tartan; and there is no doubt that one and the same animal has a wide
range of colours. The so-called "chameleon" (_Anolis_) of North America
is so sensitive that a passing cloud makes it change its emerald hue.

There is no doubt that a chameleon may make itself more inconspicuous by
changing its colour, being affected by the play of light on its eyes. A
bright-green hue is often seen on those that are sitting among strongly
illumined green leaves. But the colour also changes with the time of day
and with the animal's moods. A sudden irritation may bring about a rapid
change; in other cases the transformation comes about very gradually.
When the colour-change expresses the chameleon's feelings it might be
compared to blushing, but that is due to an expansion of the arteries of
the face, allowing more blood to get into the capillaries of the
under-skin. The case of the chameleon is peculiarly interesting because
the animal has two kinds of tactics--self-effacement on the one hand and
bluffing on the other. There can be little doubt that the power of
colour-change sometimes justifies itself by driving off intruders. Dr.
Cyril Crossland observed that a chameleon attacked by a fox-terrier
"turned round and opened its great pink mouth in the face of the
advancing dog, at the same time rapidly changing colour, becoming almost
black. This ruse succeeded every time, the dog turning off at once." In
natural leafy surroundings the startling effect would be much greater--a
sudden throwing off of the mantle of invisibility and the exposure of a
conspicuous black body with a large red mouth.

Sec. 4

Likeness to Other Things

Dr. H. O. Forbes tells of a flat spider which presents a striking
resemblance to a bird's dropping on a leaf. Years after he first
found it he was watching in a forest in the Far East when his eye fell
on a leaf before him which had been blotched by a bird. He wondered idly
why he had not seen for so long another specimen of the bird-dropping
spider (_Ornithoscatoides decipiens_), and drew the leaf towards him.
Instantaneously he got a characteristic sharp nip; it was the spider
after all! Here the colour-resemblance was enhanced by a
form-resemblance.

[Illustration: A. PROTECTIVE COLORATION OR CAMOUFLAGING, GIVING ANIMALS
A GARMENT OF INVISIBILITY

At the foot of the plate is a Nightjar, with plumage like bark and
withering leaves; to the right, resting on a branch, is shown a
Chameleon in a green phase amid green surroundings; the insects on the
reeds are Locusts; while a green Frog, merged into its surroundings,
rests on a leaf near the centre at the top of the picture.

B. ANOTHER EXAMPLE OF PROTECTIVE COLORATION OR CAMOUFLAGE

A shore scene showing Trout in the pool almost invisible against their
background. The Stone Curlews, both adult and young, are very
inconspicuous among the stones on the beach.]

But why should it profit a spider to be like a bird-dropping? Perhaps
because it thereby escapes attention; but there is another possibility.
It seems that some butterflies, allied to our Blues, are often attracted
to excrementitious material, and the spider Dr. Forbes observed had
actually caught its victim. This is borne out by a recent observation by
Dr. D. G. H. Carpenter, who found a Uganda bug closely resembling a
bird-dropping on sand. The bug actually settled down on a bird-dropping
on sand, and caught a blue butterfly which came to feed there!

Some of the walking-stick insects, belonging to the order of crickets
and grasshoppers (Orthoptera), have their body elongated and narrow,
like a thin dry branch, and they have a way of sticking out their limbs
at abrupt and diverse angles, which makes the resemblance to twigs very
close indeed. Some of these quaint insects rest through the day and have
the remarkable habit of putting themselves into a sort of kataleptic
state. Many creatures turn stiff when they get a shock, or pass suddenly
into new surroundings, like some of the sand-hoppers when we lay them on
the palm of our hand; but these twig-insects put themselves into this
strange state. The body is rocked from side to side for a short time,
and then it stiffens. An advantage may be that even if they were
surprised by a bird or a lizard, they will not be able to betray
themselves by even a tremor. Disguise is perfected by a remarkable
habit, a habit which leads us to think of a whole series of different
ways of lying low and saying nothing which are often of life-preserving
value. The top end of the series is seen when a fox plays 'possum.

The leaf-butterfly _Kallima_, conspicuously coloured on its upper
surface, is like a withered leaf when it settles down and shows the
under side of its wings. Here, again, there is precise form-resemblance,
for the nervures on the wings are like the mid-rib and side veins on a
leaf, and the touch of perfection is given in the presence of whitish
spots which look exactly like the discolorations produced by lichens on
leaves. An old entomologist, Mr. Jenner Weir, confessed that he
repeatedly pruned off a caterpillar on a bush in mistake for a
superfluous twig, for many brownish caterpillars fasten themselves by
their posterior claspers and by an invisible thread of silk from their
mouth, and project from the branch at a twig-like angle. An insect may
be the very image of a sharp prickle or a piece of soft moss; a spider
may look precisely like a tiny knob on a branch or a fragment of lichen;
one of the sea-horses (_Phyllopteryx_) has frond-like tassels on various
parts of its body, so that it looks extraordinarily like the seaweeds
among which it lives. In a few cases, e.g. among spiders, it has been
shown that animals with a special protective resemblance to something
else seek out a position where this resemblance tells, and there is
urgent need for observations bearing on this selection of environment.

Sec. 5

Mimicry in the True Sense

It sometimes happens that in one and the same place there are two groups
of animals not very nearly related which are "doubles" of one another.
Investigation shows that the members of the one group, _always in the
majority_, are in some way specially protected, e.g. by being
unpalatable. They are the "mimicked." The members of the other group,
_always in the minority_, have not got the special protection possessed
by the others. They are the "mimickers," though the resemblance is not,
of course, associated with any conscious imitation. The theory is that
the mimickers live on the reputation of the mimicked. If the mimicked
are left alone by birds because they have a reputation for
unpalatability, or because they are able to sting, the mimickers
survive--although they are palatable and stingless. They succeed, not
through any virtue of their own, but because of their resemblance to the
mimicked, for whom they are mistaken. There are many cases of mimetic
resemblance so striking and so subtle that it seems impossible to doubt
that the thing works; there are other cases which are rather
far-fetched, and may be somewhat of the nature of coincidences. Thus
although Mr. Bates tells us that he repeatedly shot humming-bird moths
in mistake for humming-birds, we cannot think that this is a good
illustration of mimicry. What is needed for many cases is what is
forthcoming for some, namely, experimental evidence, e.g. that the
unpalatable mimicked butterflies are left in relative peace while
similar palatable butterflies are persecuted. It is also necessary to
show that the mimickers do actually consort with the mimicked. Some
beetles and moths are curiously wasplike, which may be a great
advantage; the common drone-fly is superficially like a small bee; some
harmless snakes are very like poisonous species; and Mr. Wallace
maintained that the powerful "friar-birds" of the Far East are mimicked
by the weak and timid orioles. When the model is unpalatable or
repulsive or dangerous, and the mimic the reverse, the mimicry is called
"Batesian" (after Mr. Bates), but there is another kind of mimicry
called Muellerian (after Fritz Mueller) where the mimic is also
unpalatable. The theory in this case is that the mimicry serves as
mutual assurance, the members of the ring getting on better by
consistently presenting the same appearance, which has come to mean to
possible enemies a signal, _Noli me tangere_ ("Leave me alone"). There
is nothing out of the question in this theory, but it requires to be
taken in a critical spirit. It leads us to think of "warning colours,"
which are the very opposite of the disguises which we are now studying.
Some creatures like skunks, magpies, coral-snakes, cobras, brightly
coloured tree-frogs are obtrusive rather than elusive, and the theory
of Alfred Russel Wallace was that the flaunting conspicuousness serves
as a useful advertisement, impressing itself on the memories of
inexperienced enemies, who soon learn to leave creatures with "warning
colours" alone. In any case it is plain that an animal which is as safe
as a wasp or a coral-snake can afford to wear any suit of clothes it
likes.

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26