The snowshoe hare (Lepus americanus) has mastered the art of camouflage. In warm times, its coat is a muddled brown, easily blending into a background of trees, rocks, and dirt. As the days shorten and the temperatures begin to drop, it sheds its coat and grows a new one of snowy white, perfect for the snowy days sure to follow.
There’s been worry, however, that this well-honed system could fall apart under climate change: If winters get shorter and snowshoe hares don’t change the timing of their molts, they could be left vulnerable to predators, bright white targets on a brown background.
That worry may be well founded, according to a study published today by PNAS. Researchers from the University of Montana in Missoula and the University of Idaho in Moscow found that the timing of molts, both fall and spring, is tied to the length of day, and the snowshoe hares have no ability to change their fall molt and only a little for the spring.
They draw that conclusion from studying 148 snowshoe hares in the wild over three consecutive Montana winters from 2010 to 2012. Those winters were significant because they were very different and included one that was particularly long and another that was very short. That allowed the scientists to see if the hares adapted to changes in winter length.
The molt from brown to white in the fall was fixed, starting at the same time every year and taking 40 days to complete. In spring, there was a little change in 2011, the year of the really long winter, and the hares completed their transition back to brown 16 days later than in the other two years.
But when the researchers extrapolated from these three years to what may happen later this century, as temperatures rise and winters get shorter, they found that the number of days that snow-white hares can be found in brown landscapes would increase by fourfold by the middle of the century and eightfold by its end.
That’s definitely bad news for snowshoe hares, but it may also bode ill for the Canada lynx (Lynx canadensis, not named particularly well since it’s also found in the United States). The ecology of lynx and snowshoe hares is intertwined, as Smithsonian reported a couple years ago:
The northern [Canada and Alaska] lynx population rises and falls according to the snowshoe hare’s boom-and-bust cycle. The hare population grows dramatically when there is plenty of vegetation, then crashes as the food thins out and predators (goshawks, bears, fox, coyotes and other animals besides lynx) become superabundant. The cycle repeats every ten years or so. The other predators can move on to different prey, but of course the lynx, the naturalist Ernest Thompson Seton wrote in 1911, “lives on Rabbits, follows the Rabbits, thinks Rabbits, tastes like Rabbits, increases with them, and on their failure dies of starvation in the unrabbited woods.” Science has borne him out. One study in a remote area of Canada showed that during the peak of the hare cycle, there were 30 lynx per every 40 square miles; at the low point, just three lynx survived.
The boom-and-bust cycle of hares and lynx isn’t quite as dramatic in the animals’ southern range, where forests are patchier and lynx less common. But all lynx are snowshoe hare specialists, and that specialization may put them at risk. If the hares become more vulnerable to climate change, that doesn’t mean that lynx will automatically get more of them. Other predators (the goshawks, bears, fox, coyotes, and other animals mentioned above) could get to them first, leaving fewer for the lynx. And the lynx, because they’re specialists, won’t be able to fill their tummies with anything else, resulting in, ultimately, fewer lynx.
There are at least nine other mammal species that undergo coat changes in winter similar to that of the snowshoe hare, and unless they’re able to adapt their timing, they will face scenarios also similar to the snowshoe hare. The forces of evolution may help out some of these species, though — as the researchers note in PNAS, natural selection should not be discounted.
Image courtesy of flickr user DenaliNPS