Evolution has endowed the big-footed snowshoe hare with a particularly nifty skill. Over a period of about 10 weeks, as autumn days shorten in the high peaks and boreal forests, the nimble nocturnal hare transforms itself. Where it was once a tawny brown to match the pine needles and twigs amid which it forages, the hare turns silvery white, just in time for the falling of winter snow. This transformation is no inconsequential feat. Lepus americanus, as it is formally known, is able to jump 10 feet and run at a speed of 27 miles per hour, propelled by powerful hind legs and a fierce instinct to live. But it nonetheless ends up, 86 per cent of the time by one study, as a meal for a lynx, red fox, coyote, or even a goshawk or great horned owl. The change of coat is a way to remain invisible, to hide in the brush or fly over the snow unseen, long enough at least to keep the species going.
Snowshoe hares are widely spread throughout the colder, higher reaches of North America – in the wilderness of western Montana, on the coniferous slopes of Alaska, and in the forbidding reaches of the Canadian Yukon. The Yukon is part of the Beringia, an ancient swathe of territory that linked Siberia and North America by a land bridge that, with the passing of the last Ice Age 11,000 years ago, gave way to the Bering Strait. All manner of mammals, plants and insects ferried east and west across that bridge, creating, over thousands of years, the rich boreal forest. But in this place, north of the 60-degree latitude, the axiom of life coloured by stinging cold, early snow and concrete ribbons of ice has been upended in the cosmic blink of an eye. The average temperature has increased by 2 degrees Celsius in the past half century, and by 4 degrees Celsius in the winter. Glaciers are rapidly receding, releasing ancient torrents of water into Kluane Lake, a 150-square-mile reflecting pool that has been called a crown jewel of the Yukon. Lightning storms, ice jams, forest fires, rain – these things are suddenly more common. Permafrost is disappearing.
Such rapid-fire changes across a broad swathe of northern latitudes are testing the adaptive abilities of the snowshoe hare, however swift and nimble it might be. Snow arrives later. Snow melts earlier. But the hare changes its coat according to a long-set schedule, which is to say that the snowshoe is sometimes snowy white when its element is still robustly brown. And that makes it an easier target for prey. In 2016, wildlife biologists who tracked the hares in a rugged wilderness in Montana gave this phenomenon a name: ‘climate change-induced camouflage mismatch’. The hares moulted as they always had. It’s just that the snow didn’t come. Survival rates dropped by 7 per cent as predation increased.
In order to outwit its newest enemy – warmer winters – snowshoe hares would need something in the order of a natural miracle, what the biologists, writing in the journal Ecology Letters, called an ‘evolutionary rescue’. Like the Yukon, this pristine corner of Montana was projected to lose yet more snow cover; there would be perhaps an additional month of bare forest floor by the middle of this century, on which snowshoe hares would stand out like bright white balloons.