Color-Changing Rabbits Can’t Cope With a Warmer World
Snowshoe hares are among the few mammals—around a dozen out of more than 5,500 species—that camouflage themselves to hide from predators. Every autumn, the decrease in sunlight cues the hares to molt from brown to a shade of white that’s perfect for blending in with winter snow. In spring, as days lengthen, they molt back from white to brown.
Now human activities are changing the climate so fast that this nifty evolutionary adaptation is turning from a benefit into a deadly drawback.
Research published Thursday in the journal Ecology Letters shows that with the snowfall season growing shorter overall, snowshoe hares in Montana have become “mismatched” to their surroundings for about nine extra days of the year.
This has made the hares easier for predators to find and caused a 4 to 7 percent decline in their survival rate—depending on just how much they stand out from their surroundings.
Based on current trends in greenhouse gas emissions, the researchers calculated that there will be about four more snow-free weeks a year by midcentury and up to eight by 2100, extending the “camouflage mismatch” period between snowshoe hares and their environment and cutting their survival rate by up to 23 percent.
“Mismatch kills,” said wildlife biologist Marketa Zimova, the study’s lead author and a graduate student at North Carolina State University. “Because of climate change, the amount of snow has decreased, but the hares haven’t adjusted.”
The snowshoe hare population in Montana is healthy right now, she noted. “But there will be so much more mismatch in the future, they could decline by the end of the century” and even face extinction in the next unless they can evolve as fast as the climate is changing—something the species does not yet appear able to do.
Other mammals that molt to blend in with seasonal conditions face the same problem, Zimova said. “The effect could be even more severe on animals that need to change their coat color to remain undetected by both predators or prey,” such as Arctic foxes, she said.
“Of course, there are things we can do to avert that,” she said. “The main one would be to mitigate climate change. But then, we can also foster this evolutionary change by maintaining large and connected populations, which supports high genetic variation, so that the genetic shifts can occur.”
To figure out how climate change was affecting snowshoe hare survival, Zimova and her colleagues trapped and put radio trackers on 186 hares over three years at two study sites in Montana. “Once we collared the hare, we would go back to the site once a week, and using radio telemetry—walking around the woods with giant antennas—we’d locate the animals, take notes and photographs on what color they were, and also the snow cover around them,” she explained.
If one of the collared hares died, the researchers would note whether a predator had killed it.
Zimova was surprised at the range of coat colors she observed among the hares. “There would be some days in spring and in fall when you’d see some hares that were completely brown and some completely white,” she said. “They don’t all change color at once—and it can result in a lot of mismatch.”
Because brown hares out-survived white hares overall, she said, that suggests that evolution could “rescue” the species from climate change.
But the fossil record has shown that in mammals, this natural selection process takes millions of generations.
“This might not be the end of hares,” Zimova said, but “we don’t know yet if they can adapt quickly enough.”