I only half joke when I say that I’ve spent most of my career learning how many ways snowshoe hares can die. But that obscure talent has taught us some things, and led to big questions. We found that hares die more in the fall and spring than in winter/summer (and wondered: why?). We watched as hares molted from summer brown to winter white -- timed by ancient rhythms of daylength – and stood out like white light bulbs against increasingly brown snowless ground (and wondered: will they die, or hide, or what?).
A mismatched hare from our field research. Seeing more and more of these over the years got my mind spinning about climate change and potential adaptation. (Mills research photo)
And so, for the last decade we’ve been drilling deep into seasonal coat color change, its consequences, and the potential for animals to adapt to a rapidly changing climate. Our team has grown into a wonderfully collaborative, international group of interdisciplinary researchers and students. Seasonal color-change trait occurs in 21 of the planet’s most charismatic (and photogenic!) mammals and birds across the northern hemisphere, an exquisite camouflage to survive in a predator-filled world. Which got me wondering: If seasonal coat color is directly and with certainty shaped by adaptation to climate, then how might adaptation help wild animals persist in the face of rapid decreases in snow duration?
In our Science paper we first set out to describe, for all 21 species, how seasonal coat color tracks climate. It turns out that not all individuals turn white in winter. Some stay brown. But where in the world do they turn white, where do they turn brown, and what might that mean for adaptation? To answer that we sought the help of some of the best naturalists of the past century. Literally. Our team pored through the old literature where sharp-eyed naturalists had recorded brown vs white winter morphs. And then we fanned out across the world, visiting 26 museums globally to view winter coat color of samples originating from 60 countries (including some collected by Teddy Roosevelt!). Eventually we recorded 2,713 samples, enough to power a major spatial modeling effort describing winter coat color for 8 of the 21 species.
Some of the museum specimens that we used to determine winter coat color for 8 species. Photos by co-author and PhD student Alex Kumar
We lit up areas where all or most animals would be winter white (higher elevations and latitudes with consistent winter snow), and where they’d be winter brown (more maritime, lower elevations and latitudes with ephemeral snow). But we also revealed where winter brown and winter white individuals lived together: ‘polymorphic zones’. Following evolutionary first principles, a conservation focus to maintain polymorphic zones as large and connected populations could have profound consequences. First, it would foster rapid evolutionary shifts towards winter brown coats. Second, it would facilitate natural dispersal of adaptive winter brown genes into adjacent winter white populations.
In short, these could be ‘hotspots’ where evolution could help rescue species from extinction driven by climate change. At a time when scientists are seeking ways that nature might resist environmental changes and recover, these hotspots for evolutionary rescue emerge as areas of high resilience to climate change.
Obviously, we humans must address the root cause of climate change driven by greenhouse gases. But meanwhile, conservation efforts on areas with intrinsic resilience to global warming could help sustain species. Seasonal coat color illuminates how evolutionary rescue could be enlisted to help wild animals persist in the face of climate change. But the concept should apply broadly to any species shaped by adaptation to climate.