Climate Science: How Marmots Are Getting Fat on Global Warming

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Oh, you can't tell a marmot from a gopher anyway

Burn carbon—it’s good for the marmots. Not a slogan you’re likely to see at the next climate change rally, but according to a new study published in the July 21 Nature, it might just be true—at least for a little while.

Scientists led by Arpat Ozgul, an ecologist at University Imperial College London, examined more than three decades worth of data to look at growth and body size patterns among yellow-bellied marmots—a chubby ground squirrel also known as a rock chuck—in Colorado’s Upper East River Vally. (You know the groundhog in Caddyshack? Marmots look a lot like that.) They already knew that the marmot population had been thriving in the area—their numbers have tripled over the past decade. But Ozgul and his colleagues found that the marmots in Upper East River Valley also got steadily heavier—from an average of about 6.83 lbs. (3.1 kg) in the first half of the study to 7.5 lbs. (3.4 kg) in the second half of the study. (You can read a summary here—the full paper requires a Nature subscription.)

The reason, the team hypothesizes, could be climate change, which has resulted in longer summers and growing seasons. Yellow-bellied marmots hibernate for seven to eight months a year—they’re only awake during the warmer seasons. The longer those seasons are, the less time the marmots have to spend burning up their stored fat—and the fatter they can therefore become, as Ozgul said in a statement:

They have to eat and gain weight, get pregnant, produce offspring and get ready to hibernate again. Since the summers have become longer, marmots have had more time to do all these things and grow before winter, so they are more likely to succeed and survive.

Simple enough—and it’s not the first time a study has raised the possibility that a changing climate could impact the average body size of a species. An article that appeared last July in Science found that the wild sheep of Soay Island in Scotland had actually shrunk over the past 25 years, thanks to the shorter and milder winters—which are in turn, largely due to climate change. (When the summer is longer and food is more abundant, it allows younger and more vulnerable sheep to survive long enough to have offspring which themselves tend to be smaller.)

But the Nature study goes further. Not only do warmer temperatures allow the marmots to hibernate for a shorter time and therefore retain more weight, but the easier winters allow more marmots to survive the season at all body sizes, as the ecologist Michael Visser points out in a commentary accompanying the study. (Subscription required.) The change in the marmots is a phenotypic one—they’re not changing genetically, but the altered environmental conditions allow them to thrive with larger body sizes:

In the case of the marmots, the altered ecological processes change the way in which the demographic rates are affected by hibernation mass. The evolutionary processes select for phenotypic plasticity — that is, how the environment influences hibernation mass.

The Nature study still leaves plenty of unanswered questions. Marmots have gradually increased in average size over the past three decades—but it’s only been since 2000 that the population size has suddenly spiked. Why didn’t both changes occur at the same gradual rate? (Ozgul told Lucas Larsen of Nature that it’s possible the marmots passed a threshold that allowed for some kind of population explosion.) And further research will be needed to examine the impact of any number of other environmental factors, including more precise figures on how temperature, snow cover or humidity had changed in this area of Colorado over the past 33 years. Visser points out in his commentary that the marmots may be changing their diet, even as climate change gives them more time to eat, and that might impact body size as well.

Determining the impact of climate change on different populations of wildlife will be one of the most important challenges facing climate science in the coming years. “It is only by this route that biologists will be able to forecast the implications of various climate scenarios for population viability, and ultimately, biodiversity,” Visser wrote. We’ll also need much longer-term studies. But we can guess, though, that climate change probably won’t be a positive—even for big, fat marmots. “Will populations thrive in the changing climate?” said Ozgul. “We suspect that this population increase is a short-term response.” Sorry, marmot.