Dreaming of a White Winter

If you live in the northern U.S., you may have noticed something missing this past December. That something was snow. My family spent the holidays in upstate New York, which is generally reliable for a white Christmas and some cross-country ski opportunities. We would have been better off packing roller skates. And we weren’t the only ones: snowfall totals for the month of December were remarkably low across the Northeast and the Midwest, with cities like Boston and Chicago recording one and three inches, respectively.

And it wasn’t just in my neck of the woods. The National Oceanic and Atmospheric Association (NOAA) just released its climate summary for 2014, which found that globally last month was the warmest December on record — and 2014 the warmest year! And across the Northern Hemisphere, December snow cover was 130,000 square miles less than the 1981-2010 average. Now, a lack of snow in December may amount to little more than a matter of aesthetics and winter sports choices for many people, but for certain animal species, it’s a very big deal. Take for instance, the snowshoe hare.

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Snowshoe hares are the main rabbit species across most of the northern part of the North America, ranging throughout the boreal forests of Alaska and Canada, and into the northern and mountainous regions of the Lower 48. Hares will eat the twigs and branches of a wide range of plant species, from clover and grass to maple, willow and pine. Following a month-long gestation, young are born with a full coat of brown fur, and capable of hopping and feeding on their own almost immediately. The snowshoe hare’s main habitat requirement is forests with a lot of dense undergrowth, for feeding and to help hide them from potential predators, which include practically every carnivore in the forest: coyotes, bobcats, foxes, raptors, owls, and of course the lynx. The hares’ main defense against being eaten is camouflage: they are a mottled brown color in summer time and turn white in the winter.

The seasonal color change that the hares rely on for camouflage has made them something of a poster child for the phenomenon called “phenologic mismatch.” The shift in coat color is caused by release of hormones in the brain in response to changes in the amount of daylight. Historically, the days began getting shorter in autumn, when snow began to fall, and they started growing longer again in spring as the snow was melting.

However, with the climate warming, the number of days with snow cover is decreasing. In both fall and spring, researchers are seeing more hares whose color no longer matches their environment. By tracking individual animals, they have also learned that when this happens, many more hares fall to predators because they are so easy to spot.

In some locations at the southern part of their range, where snowless winter days are more common, the hares stay brown year-round. But researchers have found that the hares that do change color for winter always do so around the same time, no matter how much snow is around that year. For these animals, a lack of winter snow may literally be a matter of life and death.

For snowshoe hares to be able to navigate climate change, it will be important that forests be managed so as to have lots of cover, so animals out of phase with their environment will have places to hide. Maintaining connections between forests will also allow non-molting hares to move further north as the ratio of snowy winter days decreases.