The Advantage of Winter Whitening

Why do weasels turn white at all? Obviously, the mechanism must be maintained by natural selection; that is, it would disappear if it did not confer an advantage more often than a disadvantage. For example, if the small stoats of Northern Ireland are descended from a glacial relict population that once whitened regularly (Chapter 1), they must have lost the habit since the climate warmed. Where snow cover is too infrequent and too brief to convey any advantage, then the genes for whitening should soon be silenced or weeded out, especially if a white stoat is especially conspicuous against a dark background. Contrariwise, in northern Canada the penalty for staying brown in winter must be substantial, since no Arctic weasels ever try it—or those that do never live long enough to be observed.

At least three sorts of benefits of winter whitening have been suggested. The first is that white fur is supposed to conserve heat. According to the laws of physics, a black body is a perfect radiator of heat, so some early biologists assumed that a white body would therefore be a less perfect one. But this idea is mistaken, because the radiation of heat from an animal's body is in the far infrared regardless of the color of its coat. Consequently, all animals are "black bodies" with respect to heat loss by radiation (Hammel 1956). It is the length and thickness of the fur, not its color, that determines whether it conserves heat well, and the winter fur of weasels is a poor insulator even in areas where it is always white.

The second suggestion is that white fur is a camouflage that helps a weasel catch its prey. Predators that lie in wait for their prey to pass by unawares, or that sneak up on them quietly from some distance away, rely on camouflage and appropriately stealthy behavior to conceal themselves from their intended victim for as long as possible. This might explain the white coat of the polar bear; however, weasels, brown or white, do not hunt so much by stealth and cunning as by constant, active searching in every possible runway and hiding place (Chapter 6). Even if the match of a white weasel's coat against snow were perfect, which it is not, its movements would give it away—a weasel is hardly ever still except when asleep. The trick of melting into the background works only for animals prepared to move very slowly or to sit immobile for long periods, and that is too much to ask of a weasel.

The third suggestion is that white fur is a camouflage that helps a weasel avoid larger predators. All weasels are small enough to be in danger of attack from hawks, owls, and foxes (see Figure 11.6). Very obvious mismatches between coat color and background would invite the immediate attention of any larger predator, especially raptors. Predation by raptors can be a serious hazard for individual weasels, which explains several things about them, including the old question of why stoats and longtails have black tips on their tails and common and least weasels do not (Chapter 11). It seems most likely that the northern weasels try to match their snowy backgrounds, not to catch a meal but to avoid becoming one.

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