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Each spring, as the days in the north lengthen and the melting snow trickles into the streams, sleepy animals ranging from grizzlies to ground squirrels begin to emerge from hibernation. It’s tempting to say they “wake 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 be ready to face the world. clearly has an amazing trick of biology.
The roster of hibernating animals 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 bears and black bears weigh as much as American football players, or more, with the energy and curiosity of preschoolers, but they have no trouble lingering for months. The choreography required to take out such a large creature defies any simple explanation, says Elena Gracheva, a neurophysiologist at Yale University in New Haven, Connecticut. “Hibernation is so complex that it requires adjustments on multiple levels,” she says.
Bear hibernation provides important insights into how large mammals work, especially us, explains Gracheva, who co-authored an exploration of the physiology of hibernation in the 2020s. Annual Review of Cell and Developmental Biology† A better understanding of the process could potentially change our approach to a wide range of human disorders, including stroke, osteoporosis, Parkinson’s disease and Alzheimer’s disease (see box).
Bears, too, will have to rethink their concept of hibernation as the climate warms and winters shorten. How they react will say a lot about their commitment to hibernation and about the deep interlinkages between climate and animal behavior.
Not cool
Bears take a hibernation that is very different from other slumbers. Arctic ground squirrels can temporarily lower their body temperature to -3°C (27°F) without freezing. Bears, on the other hand, hardly lose any heat in their winter dens, but still qualify as hibernators because their metabolism slows to crawling. It’s a process that Brian Barnes, a zoologist at the University of Alaska Fairbanks, and his colleagues carefully followed more than a decade ago by studying black bears that hibernate in artificial burrows.
The winter quarters were actually rooms that could measure oxygen uptake and carbon dioxide production, important measurements 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 don’t just rest in the winter, Barnes says. They really shut down and completely reset the parameters of their daily lives. “They go in, turn two or three times, lie down, and stay that way for six months,” he says, and they only get up every few days to switch sides. “Hibernation defines the outer limits of what is possible in terms of mammalian function.” Barnes notes that during hibernation, sow boars often nurse twins or triplets without eating or drinking, tapping into their own fat and water supplies 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 out after months of inactivity. Bears don’t have that problem. Part of the secret of their strong bones is only now being revealed. In 2021, Barnes and colleagues published a study showing that hibernating bears can disable genes involved in bone breakdown.
The researchers suggest it might one day be possible to manipulate the same process in humans to prevent osteoporosis. Barnes adds that such an approach could be especially helpful for people who are limited to extended bed rest, the people currently closest to hibernation.
The slow metabolism of hibernating bears is a great achievement in itself. A bear can slow its breathing and heart rate by about 75 percent for months, while maintaining a relatively high body temperature. While no one knows exactly how they inhibit metabolism, Gracheva says the strategy makes sense. She suspects that bears don’t chill out like ground squirrels, because it would take way too much energy to warm up their large bodies again in the spring. Instead, they curl up and let their fat and fur keep them warm with just a few shivers to keep the blood moving.
The mastery of near-suspended animation by animals the size of a human (or larger) has naturally caught the attention of science fiction writers and others who dream of one day sending astronauts across the solar system while “hibernating” for months or years. on limited oxygen, food and exercise. More directly, it may be possible to use the lessons of hibernation to protect people in intensive care.
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As Barnes explains, heart attacks and strokes significantly reduce the supply of oxygen and nutrients to the brain. That lack of supply would be much less damaging if doctors could quickly reduce demand by putting a patient into a state of hibernation, or something like that. Barnes notes that stroke victims benefit most from treatment in the first hour after the stroke. Doctors call their chance to restore blood flow the “golden hour.” If doctors could replicate hibernation to a point where the brain’s needs don’t exceed 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 animal world’s hibernations, proposed a study that would use a combination of the sedative promethazine and the antipsychotic drug chlorpromazine to temporarily induce “hibernation.” like state” in stroke patients with the ultimate goal of preserving brain function.
