Thanks to climate change, the hallmarks of winter’s end—snow melting and plants budding and flowering—are occurring earlier and earlier. While humans might cheer a little less time dealing with snow and slush each year, the animals that have evolved to sync their coat color with their seasonal surroundings are in mortal danger. New research has now pinpointed locations where these animals have the best shot of evolving to blend in with their increasingly snowless surroundings.
Normally, animals living at higher latitudes that alternate between drab, brown “summer” colors and brilliant, white “winter” color have fine-tuned the timing of their coat transition to the onset or thaw of seasonal snow cover. But as climate change alters this timing, these animals are mismatched with their surroundings for long periods, making them easy to spot as prey—or as predators, easy to avoid.
“Snow cover loss is a form of habitat loss, it’s almost comparable to the loss of tropical rainforest,” Benjamin Zuckerberg, a climate change ecologist and associate professor at the University of Wisconsin-Madison not involved with the new study, told Earther.
Previous research led by University of Montana professor L. Scott Mills found that winter white hares mismatched against a snowless background had higher mortality rates than those inhabiting snow fields. As snow cover wanes, this survival disparity could mean plummeting population numbers. The conservation consequences already seem to be mounting: Ptarmigan and snowshoe hares have experienced range decreases.
Now, Mills and an international team of researchers have identified locations where evolution may “rescue” these animals from the worst impacts of the receding whiteness. The study, published today in the journal Science, targets “polymorphic zones” of hares, weasels, and Arctic fox. These zones have a mix of individuals that turn white in the winter, and those that stay brown year-round (as is often the case in the southern reaches of these species’ ranges). It’s this variation in seasonal coat color that makes all the difference.
For the animals to adapt, there needs to be both a strong selective force, and a trait evolution can act on. Coat color certainly could be that trait, as it influences survival and future reproductive output. But without enough coat color variation, even the strongest selective pressure won’t result in an evolutionary shift.
“A population consisting only of white hares cannot adapt to reduced snowpack because there is no variation in coat coloration in that population,” Jill Anderson, an evolutionary ecologist and assistant professor at the University of Georgia not involved with this study, told Earther.
Mills and his team found these important “polymorphic zones” by mapping geographic patterns in winter coat color of eight different mammal species—four hare species and four carnivores—against local climate variables like snow cover duration. The dataset combined georeferenced descriptions of winter coat color from more than 2,700 specimens spanning 60 countries across the range of the species. Using the climate variables and the winter coat information, the researchers developed a predictive models of winter coat color across the geographic ranges of each of the species. As expected, the probability of white winter coat color went up with increasing snow cover duration.
The researchers identified polymorphic hotspots by finding areas predicted to have a roughly even mix of white and brown winter coats. It’s these places that are the most promising for adaptation to climate change-driven snow loss.
“These areas hold the special sauce for rapid evolutionary rescue,” Mills said in a statement. “Because they contain winter-brown individuals better adapted to shorter winters, these polymorphic populations are primed to promote rapid evolution toward being winter brown instead of white as climate changes.”
The percentage of the geographic ranges that were in polymorphic zones varied greatly, depending on the species and on how broadly the polymorphic zones were defined. Arctic fox had between 10 percent and 57 percent of its range consist of polymorphic zones, but mountain hares were far more restricted, with a maximum of 2 percent of their range consisting of these coveted “rescue” areas.
Strikingly, the team found that only 10 percent of these polymorphic zones exist in regions prioritized for conservation.
“It’s surprising how few of these areas are well-conserved using our current network of protected areas,” Zuckerberg noted. “It’s certainly something that’s concerning for managing species that are vulnerable to the effects of climate change.”
While the study provides key information on locations that should be granted conservation consideration as a kind of insurance policy against climate change, Zuckerberg notes that the results may only be applicable to species keenly adapted to high-latitude seasonal swings.
“For other groups of species that are not clearly cold-adapted, it may be less relevant,” he said.
But for now, this research provides a powerful perspective on how—depending on local circumstances—evolution can respond to the relentless march of climate change in some species. By prioritizing these “evolutionary rescue” hotspots in conservation plans, we may give these species a fighting chance.