Every spring, as days in the north stretch longer and melting snow trickles into streams, drowsy animals ranging from grizzlies to ground squirrels start to rally from hibernation. It’s tempting to say that that they are “waking up,” but hibernation is more complicated and mysterious than a simple long sleep: Any animal that can spend months underground without eating or drinking and still emerge ready to face the world has clearly mastered an amazing trick of biology.

The roster of animals that hibernate includes all manner of rodents, some amphibians and even a few primates (several species of dwarf lemurs), but bears are literally the biggest hibernators of them all. Adult grizzly and black bears weigh as much as American football players, or more, with the energy and curiosity of preschoolers, but they have no trouble hunkering down for months at time. The choreography that goes into shutting down a creature this big defies easy explanation, says Elena Gracheva, a neurophysiologist at Yale University in New Haven, Connecticut. “Hibernation is so complex it requires adaptations at multiple levels,” she says.

Bear hibernation offers important insights into the workings of large mammals, especially us, explains Gracheva, who coauthored an exploration of the physiology of hibernation in the 2020 Annual Review of Cell and Developmental Biology. A better understanding of the process could potentially change our approach to a wide range of human conditions, including stroke, osteoporosis, Parkinson’s disease and Alzheimer’s (see sidebar).

Bears, too, will have to rethink their concept of hibernation as the climate warms and winters grow shorter. How they respond will say much about their commitment to winter naps, and about the deep interconnections between climate and animal behavior.

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Bears take an approach to hibernation that’s far different from other slumberers. Arctic ground squirrels can temporarily drop their body temperature to -3°C (27°F) without freezing solid. Bears, in contrast, hardly lose any heat at all in their winter dens, but they still qualify as hibernators because their metabolism slows to a crawl. It’s a process that Brian Barnes, a zoologist at the University of Alaska Fairbanks, and his colleagues carefully tracked more than a decade ago by studying black bears hibernating in artificial dens.

The winter quarters were actually chambers that could gauge oxygen intake and carbon dioxide production, important measures of metabolism, while sensors tracked body temperature. It was the first study to definitively show that animals could hibernate without cooling down.

Still, hibernating bears aren’t just resting away the winter, Barnes says. They truly shut down, completely resetting the parameters of their daily lives. “They go in, turn around two or three times, lie down, and they stay that way for six months,” he says, and they only get up to switch sides every few days. “Hibernation defines the outer limits of what’s possible in terms of mammalian function.” Barnes notes that sow bears often nurse twins or triplets during hibernation without eating or drinking, tapping into their own fat and water stores for the sake of their cubs.

For humans, that level of lethargy would come at a cost. Whether we were recovering in a hospital bed or riding a rocket to Mars, our muscles would wither and our bones would thin after months of inactivity. Bears have no such problem. Part of the secret to their strong bones is just now coming to light. In 2021, Barnes and colleagues published a study showing that hibernating bears are able to shut down genes involved with the breakdown of bone.

The researchers suggest that it might someday be possible to manipulate the same process in people to prevent osteoporosis. Barnes adds that such an approach could be especially helpful for people confined to extended bed rest, the closest humans currently get to hibernating.

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The sluggish metabolism of hibernating bears is an amazing feat in itself. A bear can slow its breathing and heart rate by about 75 percent for months at a time while maintaining a comparatively high body temperature. While nobody knows exactly how they put on the metabolic brakes, Gracheva says the strategy makes good sense. She suspects that bears don’t chill out like ground squirrels because it would take far too much energy to rewarm their large bodies in the spring. Instead, they curl up, letting their fat and fur keep them warm with just a few occasional shivers to help keep blood moving.

The mastery of near suspended animation by human-sized (or bigger) animals has of course caught the attention of science fiction writers and others who dream of someday being able to send astronauts around the solar system as they “hibernate” away the months or years on limited oxygen, food and exercise. More immediately, it might be possible to use the lessons of hibernation to protect people in intensive care.

As Barnes explains, heart attacks and strokes greatly reduce the supply of oxygen and nutrients to the brain. That lack of supply would be much less damaging if doctors could rapidly reduce the demand by putting a patient in a state of hibernation, or something like it. Barnes notes that stroke victims are most likely to benefit from treatment in the first hour after the stroke. Doctors call their window of opportunity to restore blood flow the “golden hour.” If doctors could replicate hibernation to a point where the brain’s needs don’t outstrip supply, “that golden hour could be a golden week or three weeks,” Barnes says.

The speculation may soon be over: In March 2021, researchers in the United States and China, inspired by the hibernators of the animal world, proposed a study that would use a combination of the sedative drug promethazine and the antipsychotic drug chlorpromazine to temporarily create a “hibernation-like state” in stroke patients with the ultimate goal of preserving brain function.