On December 4, 1920, a 14-year-old boy saw something extraordinary while walking in the central Wisconsin woods.

Snowshoe hares, all of them with vibrant white fur, “were hopping about on fallen leaves that had no snow covering,” he wrote. “The month was unusually mild, with practically no snow until the middle of the period.” It was like a vision: The animals almost glowed against the sullen, early-winter soil.

The sight so stuck with him that he described it in a scientific paper 13 years later. By that time, Wallace Byron Grange had demonstrated an intelligence, a precociousness, and a flair for prose style that matched his middle name. At 22, he had been appointed Wisconsin’s first-ever game commissioner; now, at 27, he was a publishing zoologist as well. He was particularly fascinated by snowshoe hares—and their mysterious annual change of costume.

Most snowshoe hares start the year with white coats. They “[hop] stealthily over the crust and loose snow, almost like some phantom creature, rendered relatively inconspicuous,” Grange wrote. “By late winter the surface color ... is somewhat mottled, and by the time of the spring thaws it may even appear dirty.” At this point, their dark summer coat starts to come in, and “as the spring progresses, the surface tone changes from a russet to [a] less showy brown.”

The snowshoe hare, in other words, constantly changes its coat to match its seasonal habitat. Grange loved it. “What is important is the fact that there does seem to be a very definite correlation between the [coat] color and the average climatic conditions ... The success with which this average situation is met is often almost miraculous.”

Snowshoe hares are not the only animal to pull off such a divine feat. Twenty-one species—including Arctic foxes, long-tailed weasels, and mountain jackrabbits—shift their coat color through the year to match the changing seasons.

But where it was once extraordinary to catch snowy-hued animals during a snowless season, it is now far more common. As the world has warmed, snow has become a rarer sight across much of the Northern Hemisphere, and hares and foxes increasingly find themselves in “mismatch” with their environment.

For the last decade, the ecologist L. Scott Mills and his colleagues have been studying how coat-changing animals might be able to survive climate change. On Thursday, their team published a new study—the culmination of years of work combing through museum collections—that pinpoints the parts of the world where conservation work would be most likely to save coat-changing creatures.

An Arctic fox, with a white winter coat, rests at the Zoo de Servion near Lausanne, Switzerland. (Dominic Favre / AP)

Since the 1980s, ecologists have worried about how global warming will wreck the delicate seasonal timing of animals and plants. They fret about the decline of honeybees, about early flowerings and late frosts, about migratory birds arriving too early in the spring to help trees spread their seeds. But it can be hard to tell how much climate change is responsible for these ills, since so many of them can also be tied to pollution, deforestation, or overhunting.

A decade ago, Mills—a bearded, genial professor at the University of Montana who has worked with Arctic foxes in Sweden and snow leopards in the Himalayas—realized that the threat facing snowshoe hares and other color-changing animals wasn’t so complex. “The [success of the] trait is 100 percent determined by climate,” he told me.

But across the 21 species who can change their winter coat color, there are both “brown morphs” and “white morphs”—something determined totally by genetics. White morphs grow white fur in the winter, and brown morphs retain their summertime color year-round. For both morphs, hares are more likely to survive the winter if the color of their coat matches their local climate. As Ben Zuckerberg, a professor of ecology at the University of Wisconsin, puts it, winter coat color has huge “fitness consequences.”

In fact, hares often don’t even realize that they’re mismatched to their environment. In the 1990s, Mills realized this while doing field work near the Canadian border. “As I wandered around in the fall and spring, I started to see more and more of these white light bulbs hopping around the forest,” he told me.

“They don’t run away. It’s sitting there, glowing white on brown, totally still, and you’re five feet away,” he said. “It’s almost embarrassing, like you walked in on someone in the shower. You want to turn your eyes away and say, Dude, you are so not camouflaged right now.”

Since evolutionary success and coat color are so closely correlated, Mills began to wonder whether conservationists could harness evolution to save the animals. If climate and evolution conspired to promote the trait, maybe they can also be used to demote it.

“It’s essentially a law of biology that the most rapid evolution will occur in places that have the most variation,” he said. The key is finding areas where there is variation—which means, in this case, places where there are individuals with both brown-winter and white-winter coats. Mills calls these areas the “polymorphic zones.” If these polymorphic zones are identified and preserved, Mills thinks evolution will take its course and the brown morphs will proliferate, helping the species to survive.

But for most of these species, there had never been any precise mapping of where different coat colors cohabitated. Mills and his colleagues would have to make it themselves. So they fanned out to 26 natural-history museums around the world. They searched through the collections—physical archives of preserved pelts and taxidermy—to figure out where the two coat colors coexisted with each other.

At first, they tried to find the polymorphic zones of all 21 camouflage-changing species. But despite examining more than 3,000 specimens, the team found that there weren’t enough samples to find polymorphic zones for most of the creatures. They could only refer to samples of animals collected during winter, whose precise location was known. Since the bulk of most natural-history archives were collected near the turn of the 20th century, such specificity can be scarce. (But there are perks to that history: One of the samples used in the study was collected by former President Teddy Roosevelt.)

Ultimately, they located the polymorphic zones for eight species: the Arctic fox, three types of weasel, and four types of hare. It’s a mammal-heavy list—it doesn’t include the one bird whose feathers also change with the weather—but Mills insists that’s okay.“Mammals in general are not very highly tuned for self-awareness of—fashion. I won’t put humans in this category,” he said.

“If you compare mammals to birds, birds have all this gaudy wild coloration—the females have coloration, and the males have coloration often even more spectacularly—and that’s because the mating system of birds is often about female choice.”

The ptarmigan, a type of small grouse, changes its camouflage in the fall like the snowshoe hare does. (It is the only bird to do so.) But the similarities stop there.

“The spring comes, and the females molt to brown. But the males stay white after the snow melts, and they remain white until the day that they mate. Then, as soon as they mate, they find a mud hole—or a pile of feces, even—and they roll around in it on the ground,” he said.

This suggests that ptarmigans know that their camouflage doesn’t match the season. The birds show behavioral plasticity: Individuals can change their habits or actions in order to eat, mate, or survive. “But we don’t think that will play a big role for mammals,” said Mills.


Where Can Camouflaged Animals Be Conserved?

These maps represent hours of work spent poring over museum collections. Maps A and B show the most efficient areas of conservation: Red areas are where the polymorphic zones of two species probably overlap, and dark-red areas show where three species probably overlap. The bottom maps, C and D, represent a more detailed picture, showing areas in North America and the United Kingdom where about half of animals are white morphs—and where evolutionary rescue would occur the fastest. E shows a least weasel in camouflage mismatch. (Mills et al. / Science)

Will the idea behind genetic rescue—mashing different populations together in order to save the species as a whole—work? The best evidence comes from the wild. In the 1960s, the Scottish naturalist Adam Watson—“the daddy of coat color,” said Mills—found that hares were even more preposterously well adapted to their local habitat than it seemed at first. Mountain-hare populations across Scotland stayed white often for almost exactly as long as an area’s average snow duration. Mills is now part of a team rerunning that study to see if it still holds in a climate-changed era.

“It’s a remarkable study, given its breadth and sophistication, to link climate projections to animal behavior to conservation,” said Toni Lyn Morelli, an ecologist with the U.S. Geological Survey who wasn’t connected to Mills’s study. “I wouldn’t say this is a surprising story, I would say it’s a hopeful one—for most species.”

Some animals, like mountain hares, don’t exhibit much polymorphism. But the hares are very common, so they don’t need an extraordinary plan for genetic rescue.*

“This study really shows ... the value of museum collections, without which this work would be so difficult as to essentially be impossible,” she added. Indeed, ecologists around the world have increasingly turned to museum collections as a kind of time capsule from an earlier era. Scientists have tracked 100-year-old air pollution in Chicago by photographing birds in the city’s Field Museum archive. Ecologists are also still discovering new species in museum collections.

Zuckerberg, the Wisconsin ecologist who was also not connected to the study, said the work was “pioneering.” But he also found an ominous note in it: If you compare currently conserved areas to the recommended regions on the map, “we’re not doing a very good job right now.”

That’s been the finding of Zuckerberg’s own research. He and his colleagues recently discovered that—due primarily to coat-color mismatch—snowshoe hares have entirely vanished from the central forest of Wisconsin. “Thirty or 40 years ago, snowshoe hares were really common there,” he said. Now, they’re gone.

“The areas where snowshoe hares are no longer appearing now are also the areas where we’ve seen the most rapid changes in snow-cover duration,” he told me. “It’s something you can see in the field. It’s something that we feel has contributed to their range retraction through the state.”

The central forest of Wisconsin is not just any tract of Earth. On December 4, 1920, a 14-year-old boy turned a corner in a wood there and saw something amazing—a dull clearing speckled with snowshoe hares, each the color of a shining cloud. A century later, there are no longer any snowshoe hares to see.


* This article previously misstated the size of the mountain-hare population. We regret the error.