Winter Whites: How Snowshoe Hares and Ptarmigan Are Influenced by Climate Change & Evolution

By Jan Thornhill

Josée Bisaillon's illustration of Lily wearing her 
"winter whites" in Winter's Coming.

My most recent book, Winter’s Coming: A Story of Seasonal Change, follows Lily, a young snowshoe hare, as she learns about the ways in which other animals prepare for winter’s arrival. While the forest's leaves turn from green to yellow to brown and eventually fall to the ground, Lily is unaware that she, herself, is gradually changing colour from brown to white.

A young snowshoe hare has no idea that it will turn
completely white in the fall. (NPS/Tim Rains)

Snowshoe Hares

Snowshoe hares are one of seventeen northern animals that have adapted to their environments by undergoing a colour change twice a year. In the autumn these mammals and birds grow white fur or feathers so they’ll be hidden against the snow, and in the spring they trade their glorious whites for a variety of muted browns that provide summer camouflage.

Snowshoe hares turn white for the winter (NPS/Jacob W. Frank)

Right now it’s January and winter’s well under way in the northern hemisphere, which means that throughout their range snowshoe hares are safely camouflaged in their “winter whites.” Unfortunately there’s a new glitch in this fabulous winter adaptation: climate change is causing snow cover to arrive later and disappear earlier than usual in many of the areas where snowshoe hares live. 

Snowshoe hare researchers have been keeping track of this shortening of winter for a few years now and, not surprisingly, it’s not a great situation for the hares: for each extra day their coats are mismatched with their surroundings, there is an increase in mortality from predation.

A snowshoe hare in transition. (D. Sikes/Wikipedia)

Unlike people, snowshoe hares can’t just slip on appropriate clothing at will. Their colour changes are triggered by something neither they nor we can control: the changing length of daylight hours. Because of this, as warming trends continue, the snowshoe hare population is going to take a hard hit. The species, however, will likely bounce back as they gradually adapt to climate change. It’s all about evolution: any hares that turn white later than the majority in the fall or that moult earlier than others into their summer browns will have a greater chance of surviving long enough to breed and pass on this advantageous trait to their offspring.

This might be the only chance the species has since, like my character Lily in Winter’s Coming, snowshoe hares don’t have a clue that they spend half the year white and the other half brown. Researchers have found that in the spring, pure white hares do not seek out and crouch in areas where snow remains, but instead choose open areas where they are easily seen. 

Ptarmigan

Soyeon Kim's illustration of a moulting ptarmigan from Is This Panama?

Ptarmigans are another species that turn white in the winter to match their snowy northern surroundings. Although, like the snowshoe hare, the change in a ptarmigan's plumage is linked to changes in daylight hours, researchers have shown that these birds, unlike snowshoe hares, appear to have an awareness of their colour.  

Ptarmigans are camouflaged by white plumage in the winter, and will
also burrow into snow for warmth. (Xander/Wikipedia)

While both male and female Ptarmigans turn almost pure white in the winter, the females' springtime return to cryptic, camouflaging browns happens considerably earlier than the males'. So, while a female in her mottled summer colours is almost impossible to see once the snow melts, the white feathers the male still sports glow like beacons against the greens and browns of their habitat. Which makes them a target for predators such as gyrfalcons. But, apparently, standing out is the whole point: their flashy whites impress the girls, and impressing the girls is more important than hiding from predators. The really interesting thing, though, is that once the females begin egg-laying and are no longer receptive to the males' attentions, the males go out of their way to muddy their white feathers, masking the glaring white with smears of brown dirt for a couple of weeks until their spring moult is complete.

A female willow ptarmigan is well camouflaged in the spring after
she grows her cryptic breeding plumage. (Jan Thornhill)

Male ptarmigans keep their conspicuous white feathers longer than
the females in the spring to attract the girls. (Jan Thornhill)

To prove that this feather-soiling activity wasn't just a coincidence, during a 17-year study, Bob Montgomerie of Queen's University and his team purposely dirtied the white feathers of males early in the mating season with markers when the females were still receptive. This sullying of the birds showy "winter whites" so disturbed the amorous males, that they went to work, primping and preening, making their handsome white feathers once again immaculate within 48 hours.  

Soyeon Kim's illustration of a moulting ptarmigan from Is This Panama?

References:

Zimova M, LS Mills, PM Lukacs and MS Mitchell (2014). Snowshoe hares display limited phenotypic plasticity to mismatch in seasonal camouflage. Proceedings of the Royal Society B: 281(1782).
Mills LS, et al. (2013) Camouflage mismatch in seasonal coat color due to decreased snow duration Proceedings of the National Academy of Sciences of the United States of America 110(18):7360-7365.
Montgomerie, Robert; Lyon, Bruce; Holder, Karen, 2001: Dirty ptarmigan: Behavioral modification of conspicuous male plumage. Behavioral Ecology 12(4): 429-438


Kids' Resources:

Shameless plug for my two most recent books, Winter's Coming: A Story of Seasonal Change (illustrated by Soyeon Kim), and Is This Panama? A Migration Story (illustrated by Josée Bisaillon)