Winter whites

Seasonal synchrony… ideally

snowshoe-hare-kerry-howard
Photo by Kerry Howard

One day in late November, Parks and Rec hikers well up on the West Glacier trail spotted a fairly small white beast crouched in the brush. It didn’t move when a human approached, dashing off only at the last minute. It seemed so ‘tame’ that some folks thought it was an escaped domestic rabbit. And the ears were ‘too short’ to be those of a hare. However, it was probably a snowshoe hare, now dressed in its winter whites but, alas, very conspicuous against a background of brown twigs and green moss. (Snowshoe hares have shorter ears than jackrabbits, which are also hares and use their long ears to dissipate excess heat—not a problem for snowshoe hares, which often need to conserve body heat).

And not ‘tame’ at all—its heart was undoubtedly thumping hard while the hare behaved as it normally would if it were threatened by a possible predator. Hares’ first line of defense is to hold very still and hope that they haven’t been noticed. This strategy works reasonably well when their coat color matches the background, and their coat color changes seasonally, from brown in summer to white in winter. But it does not work at all if the seasonal change in color is not in synch with the presence or absence of snow on the ground.

The physiological timer for hares’ seasonal change of coat color is thought to be photoperiod (daylength), perhaps somewhat modified by temperature, but it takes several weeks for the color change to happen. Shorter days would typically predict when snow would cover the ground. The timer is set at different daylengths in different regions and altitudes, depending on the historical relationship between daylength and snow cover in the area; in lowland areas of the Pacific Northwest, the timer is turned off and hares stay brown all year.

However, sometimes the actual weather does not match the physiological timer, and the hares may turn brown in a late spring with lots of snow still on the ground and white before winter snows descend. Then their ‘please don’t see me’ strategy fails, and predators have an easy time of it; predators are the major source of mortality for hares, and selection for coat-color matching of the background is strong. By the end of November this year, a heavy snow had fallen and this hare would be suitably camouflaged (until it rains and wrecks the snow….).

Only a few animals in the world exhibit a seasonal change to winter whites. Besides Siberian hamsters in central Asia, there are five genera containing species that turn white in winter: some hares, some weasels, one lemming, a fox, and several ptarmigan, and Alaska has representatives of all of these groups.

When the Arctic fox, a predator of small mammals and birds, turns white in winter, its camouflage presumably give it an advantage in approaching prey. When ermine and least weasels turn white, they may gain a similar advantage in predatory activity but, in addition and perhaps more importantly, their winter camouflage may also help them elude their own predators. Snowshoe hares and Alaska hares, collared lemmings, and three species of ptarmigan are all potential prey and presumably reduce the risk of predation by changing color.

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