Every year, scientists fan out across Colorado's Upper East River Valley to count the yellow-bellied marmots that make their home in rocky meadows bordered by aspen, fir and spruce trees.
Over the last decade, the work has gotten more tiring. Now they know why — the population of squirrel-like critters has vastly expanded as a result of environmental changes brought on by global warming, according to a study published Thursday in the journal Nature.
It's a rare example of animals benefiting from the higher temperatures, which are making life increasingly difficult for polar bears, harlequin frogs and dozens of other species around the world, the researchers said. But in this case the effect is only temporary, since the forces that are causing marmots to thrive are almost certain to spell their doom.
"It certainly looks like a good-news story for now, but it would surprise me if it continues for the long term," said Andrew McAdam, an evolutionary ecologist at the University of Guelph in Ontario, Canada, who was not involved in the marmot study.
For now, the warmer weather means that spring-like temperatures arrive about one day earlier each year. The cumulative effect since 1976 — when scientists began collecting detailed data on the marmots as part of an unusual long-term research effort — is that the animals now emerge from hibernation nearly a month sooner than they used to, said study leader Arpat Ozgul, a postdoctoral research associate at Imperial College London.
"These guys are active for only four to five months of the year," so that extra month increases their waking time by at least 20%, he said.
They use it to eat. When the marmots first poke their heads out of their burrows, they are surrounded by moist and nutritious green plants. The extra month of calories helps sustain them after vegetation dries out in the summer. It also means they are fatter when they settle in for their next hibernation, which begins around September.
Another consequence of the earlier start to spring is that baby marmots arrive sooner. Like their parents, the pups benefit from the additional time to put on weight.
To quantify the change in the marmot population, researchers caught and tagged as many wild marmots as they could about twice a month from 1976 through 2008. Altogether, they tracked 1,190 individual females and found that their weight on Aug. 1 rose from an average of 6.8 pounds in the first half of the study to 7.6 pounds in the second half. (They didn't bother tracking the males, because they typically set out on their own by the age of 2.)
Not only was it easier for fatter marmots to survive the winter, but the advantage of those extra pounds grew. Before 2000, for example, a marmot that weighed 7.2 pounds had a 71% chance of surviving the winter, Ozgul said. Now those odds have improved to 81%, partly because shorter winters are easier to endure at any weight, he said.
The result was a dramatic change in population dynamics. Before 2001, the colony grew by an average of 0.6 females each year. After 2001, the average increase was 14.2, the researchers reported. Altogether, the number of females surged from 43 to 164 over the 33 years of the study, Ozgul said.
The researchers said they weren't sure why 2001 was a tipping point. In a commentary accompanying the study, Marcel Visser of the Netherlands Institute of Ecology suggested a possible mechanism — perhaps climate change altered the array of plants available for marmots to eat.
Visser noted that a 2008 study showed that alpine marmots were able to reduce their core body temperature during hibernation after a change in their diets enabled them to consume plants with more polyunsaturated fatty acids. That made it easier for them to make it through the winter because they didn't need to burn as many calories.
But the fortunes of the yellow-bellied marmots will probably be reversed as their habitat continues to get hotter, said study coauthor Daniel Blumstein, who heads the ecology and evolutionary biology department at UCLA.
"The summer food is drying up," he said. "Summer droughts really nail the population."
At that point, the yellow-bellied marmots will have few options.
"You can evolve or adapt to those changing conditions, or you can go extinct," McAdam said. Since evolution takes a long time, he added, the odds aren't in their favor.