Waking Up Early

Rising temperatures are nudging animals out of hibernation and into peril

By Alisa Opar

Illustration by Josh Cochran

When the yellow-bellied marmot woke one spring morning in the Colorado Rocky Mountains, he set out on what should’ve been a routine day, rousing his family from their seven-month slumber. He toddled through the sleeping chamber where his mate and children dozed, made his way to the front door, and dug through three feet of snow to get outside. Then he started off across the snow-covered field to uncover the home’s second entrance. That’s when the coyote charged.

The marmot ran to the second burrow entrance, but he couldn’t get inside. He turned and faced his pursuer, fighting valiantly for several minutes. Focused on the battle, he didn’t see the second coyote sneaking up behind him. “There was blood and fur all over the place,” recalls Daniel Blumstein, a behavioral ecologist at the University of California, Los Angeles who found the marmot’s remains at the Rocky Mountain Biological Laboratory in Gothic, Colorado, in 2005.

The trend toward warmer winters is changing hibernation patterns for many species that live at high elevations and  northern latitudes—and the consequences can be deadly. If global temperatures continue to rise as predicted, some animals may become fatally out of sync with their environment, especially those in the Northern Hemisphere and those that live at the lower range of high-altitude ecosystems. If the marmot’s hibernation had ended a month later, he might have survived. Less snow coverage could have made it easier to get into the second burrow, and the vegetation that likely would’ve sprouted by then might have provided him some cover. But ever-earlier emergence has become the norm for the marmot. Thirty years ago, the animals woke up mid-May; now they come out in mid-April. At the lab over that same period, the average low temperature for April has increased by 6.6˚F.

Most research linking changes in hibernation patterns to global warming has focused on marmots, ground squirrels, and chipmunks. But other animals also seem to be responding to early wake-up calls spurred by warmer weather. In Europe, zoos and hunters have reported bears emerging from hibernation sooner than expected. Last January, rattlesnakes slithered about in New York’s Taconic State Park, and tree frogs in Canada that should’ve been frozen were hopping around calling for mates. “We’re seeing dramatic changes,” says David Inouye, a University of Maryland biologist who has worked at the Rocky Mountain lab since 1971. Nonetheless, Inouye adds, “I don’t think you can generalize how all hibernating species will respond to global warming.”

Milder weather might sound pleasant, but for animals that have to hibernate, it can interfere with feeding and breeding, says Gregory Florant, a biologist at Colorado State University. Hibernators fatten up in the summer and then use their energy reserves throughout the winter. But when ambient temperatures rise too quickly, animals may burn off stored fat faster and emerge from hibernation sooner. “Unless the ground has warmed enough that plants are coming up at the same time, the animals might not have anything to eat,” says Florant.  

So far the early wake-up call probably hasn’t hurt yellow-bellied marmots. “Our population is exploding,” says Blumstein. “Everybody is fat and happy,” because vegetation starts growing rapidly after snow melts. But if marmots keep waking up early, there might not be enough food; or males—they arise first to rouse females from sleep and could be picked off by predators, impairing reproduction.

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