A Long Winter’s Nap: Bear Hibernation

October 26, 2016
bear hibernation
S.H. LeeBear hibernation is a complex process that differs from other animals’ winter naps.

While many of us are enjoying Thanksgiving dinner, Northeastern black bears will be finishing up their own autumn feasts before bedding down for the winter. After entering their dens in late November or early December (if food is abundant), or as early as October (if food is scarce), black bears in our region usually hibernate until April.

You might be surprised how nearby these animals remain to us and how much we could benefit from a better understanding of their winter activity. “There’s been a lot of interest [in hibernation] in the medical field,” says Jennifer Vashon, a wildlife biologist with the Maine Department of Inland Fisheries and Wildlife who has been studying bears for more than 20 years. “When I started out, we were sending blood samples to NASA,” because the space agency was exploring the idea of astronauts “hibernating” during long voyages—or at least avoiding muscle atrophy while inactive, as bears do.

Although hibernation seems like a long nap in a fur coat, it’s a complex process governed by a specific set of rules. During their time in their dens, black bears do not eat, drink, exercise, defecate, or urinate. Their heart rate drops and breathing slows much more than it would in ordinary sleep, and their body temperature decreases modestly. By contrast, other hibernators, such as bats and rodents, experience a much larger temperature drop. This difference used to fuel debate about which animals are true hibernators, but now hibernation is defined by the change in metabolism rather than by temperature. So bears qualify.

Hibernating bears are not comatose. They can rouse themselves if necessary—if their den is swamped by rainwater, for example—and move to a better spot, Vashon says. In January, pregnant black bears give birth to a litter of one to four cubs then actively care for their young, sleeping only when the cubs sleep. The nursing cubs will grow from less than 1 pound each to as much as 10 pounds by the time they leave the den in spring. No wonder the mothers are hungry.

By slowing their metabolism, black bears save energy at a critical time, Vashon says. “They do it to get through a period when food is not available.” That’s why bears will start hibernating earlier if they can’t find much to eat, even though they won’t have as much extra fat to tide them over. “The cost of foraging is so high, it’s better to conserve their resources,” Vashon explains.

How the bears survive is still something of a mystery. They get energy and water from their fat, and protein from muscles and organ tissues. Their cholesterol levels spike, but their arteries do not harden. They also avoid the muscle cramping and bone loss you might expect from staying curled up for months. Understanding all this could help us better treat human gallstones, cholesterol, kidney disease, and osteoporosis. And while NASA has not implemented deep-space sleep yet, scientists are exploring ways to induce hibernation to save the lives of seriously injured people until the patients can receive treatment.

Meanwhile, some 30,000 bears will be slumbering this winter in Maine alone. Vashon, who often joins teams that check on radio-collared female bears in their dens, has seen young bears in hollow cedars and other bears in rock cavities, root systems, downed logs, and even brush piles. “I grew up in Maine tooling around in the woods,” she says. “I wonder how many dens I went right by.” Maybe you’ll pass one, too, as you hike off that second helping of Thanksgiving turkey.



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Heather Stephenson

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