Red Squirrels Show Signs of Adapting to Climate Change

March 13, 2003 / Ned Rozell

One of the noisiest creatures in Alaska is making headlines for its apparent ability to adapt to climate change.

Canadian scientists have found that red squirrels in the Yukon give birth 18 days earlier on average than their great-grandmothers, perhaps in response to warmer springtime temperatures. The researchers say it's the first time a mammal has shown a genetic response to climate change.

The red squirrel is familiar to anyone who has stepped into the boreal forest, which stretches from Alaska through Canada to the northern tier of the Lower 48, but few people know red squirrels as well as Stan Boutin. Boutin is a biology professor at the University of Alberta in Edmonton and a coauthor of the recent paper in which scientists claim that natural selection has favored squirrels that are born earlier, at least in a 1-square kilometer patch of spruce forest in the Yukon Territory.

Boutin and his colleagues, including graduate student Andrew McAdam, know every red squirrel in their study site, which borders the Alaska Highway near Kluane Lake. Since 1989, the scientists have kept track of the 200-to-400 red squirrels within their plot. In springtime, the researchers climb spruce trees to remove newborn squirrels from nests, and then install eartags before replacing the pups. By remaining diligent to this practice and by live-trapping and tagging adults, they have known the life history of every red squirrel on the plot for more than ten years.

Their study is unique in that they have been able to track several generations of red squirrels, a task that is difficult or impossible with other mammals that roam farther or live more secretive lives.

" We're like a census taker in a city," Boutin said. "We know who's who, what real estate they own, and how many kids they have."

Red squirrels establish territories centered on a large cache of spruce cones on the forest floor called a midden. Boutin and McAdam have found that squirrels may live to the age of nine, but few live past three or four, and most don't survive their first year.

" There's a lot of competition for territories," McAdam said. "If they don't get a territory for their first winter, they're toast."

Young squirrels gain their own territories following the deaths or movement of squirrels that held the plots before, or occasionally when mothers give up their territory to one of their offspring. The competition for territories is crazy, and squirrels born earlier have the jump on their rivals because they are first to find vacancies in the woods and are often larger and more competitive, Boutin and McAdam said.

Boutin started studying squirrels in 1984 to look at the energy costs that come with defending a territory, but when McAdam and colleagues Denis Reale and Dominique Berteaux of McGill University in Montreal studied his long-term data set, they saw that a warming environment seemed to be selecting for squirrels born earlier.

The researchers applied statistical techniques developed for livestock breeding to the red squirrel population. They tracked four generations of red squirrels that lived from 1989 to 1998, a period when spring (April, May and June) temperatures in the area warmed by an average of 2 degrees Celsius. During their lifetimes, female squirrels born in 1998 gave birth on April 20th, on average, while females born in 1989 gave birth in early May.

The Canadian red squirrels may show for the first time that a mammal can adapt to a warmer world in a few generations. Biologists think some animals might not be able to adapt quickly enough to survive rapid changes, but the red squirrel seems to have proven that it will chatter on, at least in a slightly warmer world.