For hares, fashion is a life or death proposition. Whereas Peter Rabbit could always jump down his hole to escape a fox, his cousin, the hare, has to rely on blending in with his environment to avoid detection. But as the climate warms, hares may no longer be able to stay in sync with their environment, according to a new study. The animals will be switching from earthy brown to snowy white or vice versa at the wrong time and becoming targets for hungry predators.

After more than a decade of studying snowshoe hares in the Rocky Mountains, L. Scott Mills, a wildlife biologist at the University of Montana, Missoula, noticed that the animals were beginning to stick out more than usual. In winter their coats turn white; in summer they are a mottled brown. But Mills was beginning to see white hares on brown backgrounds.

To figure out what was going on, he and his colleagues put radio tags on about 50 hares in each of three winters, two of which represented extremes in weather. The year 2010 was quite warm, with snow cover lasting 160 days. The next year was the opposite, one of the coldest on record, and snow persisted on the ground for 190 days.

Each week, the researchers tracked down all the animals they could find and noted how well their coat color matched the local background by determining the percentage of white fur and of snow cover. They considered the hares mismatched if the coat color was 60% or more different from the surrounding 10 meters.

The hares couldn't adjust the date that they started to change color very much, Mills and his colleagues report online today in the Proceedings of the National Academy of Sciences. Each year, the animals started to molt around the same time—10 October in fall and 10 April in spring. "That happened regardless of whether there was a ton of snow on the ground or not," Mills says. In the fall, switching to white took 40 days. In the spring, the changeover to brown took between 30 and 50 days and lasted longer in the colder years, Mills says. "They do have some ability to speed up or put on the brakes [on color change]."

But that ability won't be enough to keep them in sync with future winters, Mills found. He and his colleagues used more than a dozen climate models to determine the temperature and likely snow duration in the study area for 2050 and 2099. By midcentury, the snow season will be a month shorter, and by the end of the century it could be up to 2 months shorter, they report. With the initiation dates for molting fixed, that shift would result in hares being mismatched for as much as 36 days by 2050 and for double that amount of time by the end of the century.

Other studies have gauged how climate changes will affect migration, flowering, and other factors that could put plants and animals out of step with the world around them. But assessing the outcome of those changes can be complicated. In contrast, the hares "provide a really compelling visual effect of climate change," says Daniel Blumstein, a behavioral ecologist at the University of California, Los Angeles, who was not involved with the work. In general, "if animals are caught out with the wrong plumage or fur color, they are particularly vulnerable."

Mills is now trying to quantify the effects of a mismatch to determine just how much easier a wrong-color hare is to catch. He suspects that they will be very easy prey, which is good and bad news. The bad news is that a lot of hares will die. The good news is that there will be a lot of pressure on the hares to evolve a new calendar for molting. Already, some hares in different parts of the country change color at different times of the year, and a few living on the Pacific coast don't change at all. "It makes me optimistic that they can adapt by evolutionary change," Mills says.