When researchers at the Rocky Mountain Biological Lab in Crested Butte, Colorado, started documenting marmot hibernation patterns in the 1970s, the animals rarely awoke before the third week of May.But these days, the scientists say, marmots regularly end their winter naps a month beforehand—by the third week of April.

These abbreviated hibernations are part of a growing body of evidence suggesting that hibernating animals are waking up earlier—or not going to sleep at all—due to rising temperatures from global warming.

From chipmunks and squirrels in the Rocky Mountains to brown bears in Spain, these altered slumber patterns are putting animals at risk both of starvation and increased predation, researchers say—which could bring many species to the brink of extinction.

“With respect to the marmots, at least, the evidence is convincing that it is connected to warming temperatures,” said David Inouye, a biology professor at the University of Maryland who collaborated with the Rocky Mountain Biological Lab researchers.

Average low temperatures in April in Gothic, Colorado, the site of the marmot study, have climbed 2.5 degrees Fahrenheit (1.4 degrees Celsius) since 1976, the first year researchers began recording marmot hibernation stats.

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