Snowshoe Hares in a Warming World

The snowshoe hare, which lives in the forests of northern North America, changes its appearance twice a year. In autumn, it gradually replaces its brown summer coat with a white winter coat, and in spring it changes back again. This seasonal change provides excellent camouflage, hiding the hare from predators such as lynx and foxes against snow in winter and against bare ground in summer.

The timing of these changes is controlled mainly by day length, not by temperature or the presence of snow. [A] As the days become shorter in autumn, hormones trigger the growth of white fur. [B] For thousands of years, this system worked well, because the shortening of the days matched the arrival of snow quite closely. [C] Climate change, however, is causing snow to arrive later in autumn and to melt earlier in spring, while day length, of course, remains the same. [D]

The result is a growing mismatch. More and more often, white hares can be seen sitting on brown ground, where predators can spot them easily. Studies have found that hares that are mismatched in this way are much more likely to be killed. Researchers have also discovered, however, that in some regions with little snow, a few hares stay brown all winter. This variation could allow natural selection to help hare populations adapt, at least in some areas.