Can Snowshoe Hares Evolve to Cope With Climate Change?

There's something odd about a bright white snowshoe hare motionless and alert—without any hint of snow nearby.

Gleaming white on a brown background of dirt and leaves, the hares, which are native to the mountain ranges of North America, might as well be wearing an "eat me" sign for lynx and other predators.

Scott Mills and Marketa Zimova of North Carolina State University call this "mismatch"—when the hare, which turns from brown to white as the fall becomes winter and back again in spring, doesn't match its background.

Usually, hares seem to time their color change pretty well. Now the average hare is mismatched only about a week out of the year.

But climate change is likely to make such awkward—and potentially fatal—mismatches much more common, the team said this week at the North America Congress for Conservation Biology in Missoula, Montana. (See "7 Species Hit Hard by Climate Change—Including One That's Already Extinct.")

The hares seem to use the length of the day, rather than temperature or presence of snow, to time their "molt."

Mills and Zimova have determined that individual animals don't have much ability to change the timing of their molt to conform with conditions. And as the snow arrives later and melts earlier in a warming world, that may spell up to eight weeks of mismatch by the end of the century.

Adaptable Bunnies

But that doesn't mean the hardy animals are doomed. Hares breed fast, and if they can evolve earlier molt times, they may be able to avoid significant population declines, the team says.

There are three prerequisites for such evolution to occur, Zimova explains: Molt timing must be genetic, there must be existing variation in that timing, and there must be fitness costs to mismatch.

If those three things are true, then mismatched individuals will be eaten at a higher rate and hares with later molts will live to reproduce and pass on their later-molting genes. Over time, the average molt time will shift later for the whole population. (Related: "Ten U.S. Species Feeling Global Warming's Heat.")

On Monday, Zimova presented evidence for two out of three of those prerequisites for evolution.

By radio collaring almost 200 hares and periodically checking on them for three years at two sites in Montana, Zimova was able to show variation in when the coats change color.

This variation is likely to occur in other snowshoe hare populations as well. Biologist Charles Krebs at the University of British Columbia in Vancouver said that the hares he studies in the Yukon Territory show the same pattern. (See a map of global warming's effects.)

"You can go out at the right time of year and find hares that are 90 percent white and those that are 5 percent white," he said.

Zimova also showed there is a real cost to mismatch.

Sometimes, when Zimova and her colleagues tracked the signal from a collar, they would find little more than a gnawed collar and a pile of fur.

When they tracked hare deaths, they were able to show that during weeks when hares are completely mismatched, their chance of death is 7 percent higher. At this rate, given increased mismatch, the population would likely go into serious decline by the end of the century without any evolution.

So the costs of mismatch are high, and there's a good deal of natural variability.

Cool and Docile

Next, Zimova will try to determine whether molt timing is inherited. She's been heading out early in the mornings to trap hares, which she'll later breed in the lab to see if the young hares' molt timing is related to that of their parents.

The morning after her talk, she said she caught two hares, including a tiny, light-brown baby that fit into her palm.

A Ph.D. student, Zimova has been working with the hares since she began volunteering with Mills as an undergraduate.

"The hares are supercool," she said. "They never bite. They are really docile. You never have to sedate them. We can just slip on radio collars in one minute. And they are really soft."

She's hopeful that as the climate warms, the hares will be able to evolve an earlier molt and keep their populations from declining.

The University of British Columbia's Krebs added that her work is important and well done, and that studies like Zimova's demonstrate the continuing value of doing research on the ground.

"You can't sit in the lab and build a computer model. You have to be out in the field."

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