Part 19 (1/2)
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.
[Ill.u.s.tration: _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 ill.u.s.tration of warning coloration. Perhaps, that is to say, its striking coloration serves as an advertis.e.m.e.nt, 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.]
[Ill.u.s.tration: _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.]
[Ill.u.s.tration: 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 const.i.tutional 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.
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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 (chromatoph.o.r.es) 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 pa.s.sing down the spinal cord, the message travels down the chain of sympathetic ganglia. From these it pa.s.ses 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-gra.s.s, 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.
[Ill.u.s.tration: _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.]
[Ill.u.s.tration: WHEN ONLY A FEW DAYS OLD, YOUNG BITTERN BEGIN TO STRIKE THE SAME ATt.i.tUDE 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 circ.u.mstances, 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 (chromatoph.o.r.es) 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 pa.s.sing cloud makes it change its emerald hue.