Snowshoe hares are mistiming their colour change as the climate warms

Snowshoe hares in the Yukon are turning white later in the fall as the duration of snow cover declines, according to a new study.

Hares that wait too late in autumn to shift from brown to white are less likely to survive throughout the winter, the same study shows.

The findings suggest that hares may experience longer periods of mismatch between their coat colour and their environment as the climate warms, which could have consequences for their populations and boreal ecosystems.

Snowshoe hares are found throughout boreal forests in North America, including in the Northwest Territories. Through a process known as moulting, their coats turn brown in the spring to match the bare summer ground. In the fall, they change to white to blend in with the winter snow.

This coat-colour camouflage is critical, given snowshoe hares’ many predators. Because everything eats snowshoe hares, they are sometimes called “cheeseburgers of the forest.”

Lynx, coyotes, wolves, wolverines – and species you might not expect, such as red squirrels, grey jays and ravens – all eat snowshoe hares, said Charles Krebs, professor emeritus at the University of British Columbia and one of the study’s authors.

In some boreal forests, upward of 90 percent of hare deaths can be attributed to predation, the researchers wrote.

“You could find maybe a few two-year-old hares, but they’re very rare,” Krebs said. “The predators get them sooner or later.”

Now, climate change is undercutting hares’ camouflage.

In the Yukon, snow is generally arriving later in the fall and melting earlier in the spring, Krebs said, which can leave hares with the wrong coat colour. Snow conditions are also highly variable from year to year, he added. Given that moulting takes about a month, all this can make it harder for hares to match their timing with the arrival of snow.

For hares, being the wrong colour can be deadly. A study in Montana found that snowshoe hares were three times more likely to die from predation during periods of camouflage mismatch.

The impact of climate change on hares depends on how well they can adapt the timing of their moult to changing snow conditions. Although some research has looked into this question, few studies have examined it over the long term.

In the new work, researchers drew on 43 years of field data to assess how coat colour change is responding to warming and how it affects winter and summer survival.

The data came from a long-term study of snowshoe hares near Kluane Lake in southwestern Yukon, where mean winter temperatures have been increasing by 0.75C per decade, the researchers reported.

Since 1977, Krebs and his colleagues have been live-trapping hares in the area, tagging them and gauging the percentage of their coat that is white. If hares are later recaptured, the tags are used to identify them – information that can be used to estimate survival rates.

Madan Oli, a professor in the University of Florida’s wildlife ecology and conservation department, is another of the study’s authors. He said Krebs has yet to miss a field season.

“Nobody else in the whole world has the kind of data Charlie does,” he said.

Potential ‘disaster’ for the ecosystem

Oli, Krebs and their colleagues focused on data from hares captured in the spring and fall between 1977 and 2020. Using statistical tools, they assessed how the whiteness of hares’ coats has changed and how coat colour affects winter and summer survival.

On average, the percentage of white fur in hares’ coats has declined over the past four decades, the researchers found, with the decline occurring at a faster rate in autumn than in spring.

Because the timing of moulting is known to be influenced by daylight, temperature and snow conditions, it’s likely that this trend is a response to reduced snow cover and snow depth in the study area, the researchers wrote.

The team also found that hares with whiter coats in the fall were more likely to survive over winter. In contrast, coat colour in the spring had no noticeable effect on summer survival.

Hares appear to be trying to adapt, Oli said. But adaptation takes time.

“Climate change is happening more quickly than hares can potentially adapt,” he said.

For now, hares are still doing OK, Oli said. The concern is that, as climate change progresses, the period of potential camouflage mismatch is likely to increase. With rising temperatures, hare survival may decline, he said.

Snowshoe hares are known for their eight-to-11-year population cycles, tightly linked to those of lynx.

They’re also considered a keystone species in boreal forests, Krebs said, meaning they affect a “whole lot of things.”

According to Oli, high levels of mortality in winter seem to cause hare populations to crash. If hares do not adapt to climate change fast enough, their population cycles may be dampened or even collapse.

“That would be a major disaster for the entire ecosystem,” he said.

Hares may also not be the only species struggling to camouflage in a warming world. At least 21 other species of birds and mammals that live in arctic and subarctic regions are known to undergo a colour change from summer to winter, including weasels, arctic foxes and several types of ptarmigan.

“They all, I would guess, are having this problem,” Krebs said, although there’s little research on the topic and how it may affect them.

Oli points out that many of these species aren’t under the same kind of predation pressure as snowshoe hares, however.

“If [hare] numbers are drastically reduced, it can have a lot of negative impacts on a whole host of species,” he said.

Although the study’s strength is long-term data, Oli said the team does not have details on when moulting starts and ends. The researchers also didn’t compare hares’ coat colour to their environment – an important variable, given that snow cover is often patchy in the spring and fall.

The team is now working to fill these gaps. For the past seven years, researchers have been using wildlife cameras to track hares’ colour change as well as their background environment in more detail and have begun analyzing the data.

Beyond providing a more fine-grained way of studying moulting, Krebs said there is an added perk to using the cameras: “You get the most incredible wildlife photos you can imagine.”

This article is produced under a Creative Commons CC BY-ND 4.0 licence through the Wilfrid Laurier University Climate Change Journalism Fellowship.