UM-led researchers: Can brown hares ‘rescue’ white hares from climate-change disaster?

There’s no doubt that humans must reduce their global carbon dioxide footprint, says University of Montana wildlife biologist L. Scott Mills. But are there other steps that people can take to minimize extinctions of wild animals as the climate changes?

In recent years, Mills and his students explored that question with researchers from five other countries and arrived at answers that left him with “a profound sense of optimism for the future.”

Their conclusions, published in Science online Feb. 15 and in the March 2 print edition, don’t circumvent the need for aggressive carbon reductions, Mills said. But they do provide hope for 21 species of animals and birds that change color with the seasons and so are at considerable risk as those time markers change.

It all depends, Mills said, “on the actions that humans take now.”

For a dozen years, Mills and his students at UM have studied snowshoe hares and their seasonal color-phase changes from brown to white, an adaptation that helps camouflage the rabbits from would-be predators.

In that time, they’ve seen winter’s snows begin later and melt earlier, leaving white-phase hares unexpectedly exposed against brown landscapes, “like white light bulbs hopping around the forest,” in Mills’ description.

The color change is triggered by the amount of daylight, not by snowfall, so the hares just keep molting from brown to white even when the snow doesn’t fall.

And the greater the hares’ exposure, the researchers quickly learned, the greater the predation – even though the hares seem not to realize they’re mismatched with the landscape and make no attempt to alter their behavior.

Dubbed “the cheeseburgers of the forest” by Mills’ students, snowshoe hares are on nearly every predator’s menu. So even a few weeks of exposure can have disastrous consequences.

Then the Montana researchers expanded their study to include all 21 northern hemisphere species that drop their brown coats for white to blend into the snow. They pulled in an international team of researchers.

Brown and white snowshoe hares at a University of Montana research facility. (L.S. Mills research photos by Jaco and Lindsey Barnard)

That’s when the fun began.

“Much of the work was old-school natural history sleuthing at its best, seeking winter specimens with known coat color and a precise collection location,” Mills said. “Our team pored through the old literature, and visited 26 museums globally to collect 2,700 samples originating from 60 countries.”

For eight of the color-changing species, they were able to collect enough data and specimens to draw a solid conclusion: the Arctic fox, three types of weasel, and four types of hare.

For each species, they found geographic areas (usually southern or coastal areas) where individual animals do not turn white in winter, but retain their brown coat.

Winter-brown snowshoe hares may hold the key to an “evolutionary rescue” of winter-white hares as the climate changes. (L.S. Mills research photos by Jaco and Lindsey Barnard)

“Weasels in the southern U.S. and mountain hares in Ireland, for example, have evolved to remain brown year-round,” Mills said. “This is a genetic adaptation to retain camouflage in areas where snow is intermittent or sparse.”

Then the scientists discovered regions where both brown-phase and white-phase animals coexist through the winter.

In the new report, Mills’ team described how they mapped these “polymorphic zones” for all eight species. These are the zones where both brown-phase and white-phase animals coexist in winter.

The Pacific Northwest, for example, has zones of coexisting brown- and white-phase snowshoe hares.

“These areas hold the special sauce for rapid evolutionary rescue,” Mills said. “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.”

And while these are “hotspots for evolutionary rescue from climate change,” he said, they are not “magic fortresses” that will prevent climate change from affecting the species. Some populations cannot be rescued if global warming continues unchecked.

“Ultimately, the world must reduce carbon dioxide emissions or else the climate effects will overwhelm the ability of many species to adapt,” co-author Eugenia Bragina said. “But by mapping these adaptive hotspots, we identify places where people could help foster evolutionary rescue in the short term by working to maintain large and connected wildlife populations.”

Mills explained further: “A fundamental principle of evolutionary biology holds that the secret sauce for rapid evolutionary change is the presence of rich variation for selection to act on.  In the case of seasonal color change, the ingredients for rapid change are found in the polymorphic populations, with winter-brown and winter-white individuals living together.

“Here in the polymorphic zones, the populations are most likely to rapidly evolve toward winter-brown, and to disperse the winter-brown genes out into the adjacent winter-white populations,” mounting an “evolutionary rescue,” if you will.

Winter-white hares are like “light bulbs in the forest” if the snow melts before they molt, researcher L. Scott Mills says. (L.S. Mills research photos by Jaco and Lindsey Barnard)

That’s where humans again enter the equation.

These zones of winter-white and winter-brown animals must be protected from development, and must remain connected to nearby winter-white populations. If that connection is lost, the winter-brown animals will not be able to aid their winter-white cousins in adapting as the snow cover becomes shorter and shorter in duration.

So “our first and biggest effort” must be to promote large populations and natural connectivity between these polymorphic zones, according to Mills.

“Evolution is not magic or omnipotent,” he added, “and ultimately we must pursue a reduction in the greenhouse gases that cause climate change. But we also needn’t panic. Rather, we must push for large and connected populations that … empower nature’s adaptation engine to sustain wild animals through substantial challenges.

“This is something that any citizen, community leader, land manager or politician could do in the short term to help wild animals persist in the face of climate change.”

Research continues into the polymorphic zones, and into additional species of color-changing animals.

UM’s research partners include North Carolina State University, the University of Natural Resources and Life Sciences in Vienna, the Universidade do Porto in Portugal, the German Remote Sensing Data Center, the Russian Academy of Sciences in St. Petersburg and Russia’s Institute of Systematics and Ecology of Animals.

Five UM graduate students also were co-authors of the paper for Science and were part of the international research team.