The Outline of Science, Volume 1

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 amœboid 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[Pg 141] foam, it is due to the complete reflection of the light from innumerable minute surfaces of crystals or bubbles. 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. 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. 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.