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Winter Wildlife

December 12, 2011
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Mule Deer

A slightly darker coat may give mule deer a modest thermal advantage during winter.

The colder days of winter, and the increased energy demands they require of warm-blooded creatures, are handled in various ways. Some species fly, walk or swim to milder climates, avoiding the hardship altogether. Others retreat to underground burrows or holes excavated in the snow for insulation during a period of hibernation, thereby reducing their energy requirements to survive until spring. However, the winter wildlife that remain above ground and active during the cold season must face winter head-on and rely upon various adaptations to preserve themselves.

For mammals, exchanging a thinner, sleeker coat of summer hair for a dense, long pelage is one adaptation that helps conserve energy and increase the odds of survival. For some species, such as red fox and coyote, the color of the summer and winter hair exhibits little change. Other animals, including mule and whitetail deer, take on a slightly darker, more muted color during the winter, a change that isn’t particularly dramatic, but definitely noticeable to the informed observer.

whitetail jackrabbit

The whitetail jackrabbit wrapped in winter coat.

Why change? The muted tones help the animals blend in with the more drab landscape of winter, providing some measure of camouflage against the keen eyes of predators. Slightly darker coats may also offer a modest thermal advantage. Darker colors absorb heat more readily than lighter tones.

However, the most dramatic winter adaptations in color belong to a diverse array of birds and animals whose color changes to match their surroundings. This amalgam of species includes several varieties of ptarmigan, ground-dwelling birds similar to grouse. It also covers a range of mammals, both predator and prey. Weasel, arctic fox, jackrabbit and snowshoe hare are among the mammals whose coats turn white with the coming of winter. However, not all members of a species necessarily undergo this dramatic color change. A white coat, be it to camouflage a hunter or the hunted, is only useful in regions covered by snow. Thus, long-tailed weasels in northern climates turn from brown to snow white in winter. Their southern counterparts do not.

Winter Wildlife Transformation

Most seasonal changes in animal behavior, be it the bugling of bull elk during the mating season, the migratory instinct of a flock of mallards, or the shedding of a buck deer’s antlers in late winter, are triggered by changes in the photo-period. Photo-period is the amount of daylight at any given point on the calendar. The molting of an animal’s summer coat is triggered by the shortening days of autumn. Thus, at essentially the same time every fall, animals that turn white in the winter shed their darker coats for shades of pale.

Although the concept of acquiring a white coat to blend into an animal’s surroundings for the sake of predation or eluding predation is an easy concept to grasp, the equation is not quite so simple. Why, for instance, is this adaptation not equally distributed between members of highly established predator-prey relationships? Jackrabbits, for example, turn white, but one of their primary predators, coyotes, do not. The same is true for big-footed snowshoe hare that acquire a snowy coat while their two major mammalian predators, lynx and bobcat do not.

short-tailed weasel

Short-tailed weasels turn white as snow during winter to match their surroundings.

The reverse is sometimes true as well. Several species of weasel (short-tailed, long-tailed and least) turn white while the cottontail rabbit, bird, mouse and other animals that they commonly prey upon remain dark. In fact, the only predator-prey relationship of which I am aware involving two species that both turn white in winter include arctic fox and arctic hare. Thus, the adaptive pressures over time that have induced some species to adopt a different color scheme in winter have obviously not triggered similar reactions in both predator and prey.

Another question concerns the transition periods between the seasons and periods of climatic aberrations that make a white winter coat a liability rather than an advantage. A number of times I have observed jackrabbits in the fall, before the snow has fallen, in their white winter coats. Want to make it easy to spot a hopping jackrabbit? Turn it white. In years when winter is delayed or snowpack is sparse, it would seem that doffing a brown coat for a white mantle is a step backward in adaptation, not a move that enhances an animal’s ability to survive.

The peculiar ability of some animals to trade a summer coat of color for white hair in the winter holds both elements of miracle and mystery. For my part, I am glad I can purchase an overcoat in any color I want.

 

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