Climate change disrupts bear hibernation schedules

This article was originally published by Knowable magazine.

Each spring, as the days in the north get longer and melting snow trickles into streams, sleepy animals ranging from grizzly bears to ground squirrels begin to emerge from hibernation. It’s tempting to say they “wake up,” but hibernation is more complicated and mysterious than just a long sleep: any animal that can spend months underground without eating or drinking and coming out ready to face the world has clearly mastered an amazing trick. of biology.

The list of hibernating animals includes all sorts 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 grizzlies and black bears outwit even the greatest American football players and have the energy and curiosity of preschoolers, but they have no trouble squatting for months of play. in a row. According to Elena Gracheva, a neurophysiologist at Yale University in New Haven, Connecticut, the choreography needed to stop a creature of this size defies easy explanation. “Hybernation is so complex that it requires adaptations on many levels,” she says.

Bear hibernation offers important insights into how large mammals function, especially us, says Gracheva, co-author of a 2020 exploration of the physiology of hibernation. Annual Journal of Cellular 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 disease.

Bears, too, will have to rethink their concept of hibernation as the climate warms and winters get shorter. Their reaction will speak volumes about their commitment to winter naps and the deep interconnections between climate and animal behavior.


Bears take a very different approach to hibernation than other sleepers. Arctic ground squirrels can temporarily drop their body temperature to -3 degrees Celsius (27 degrees Fahrenheit) without freezing solidly. Bears, on the other hand, lose virtually no heat in their winter dens, but they are still considered hibernators, the scientists I spoke with said, because their metabolism slows down. The process is one Brian Barnes, a zoologist at the University of Alaska Fairbanks, and his colleagues carefully tracked over a decade ago when studying black bears hibernating in man-made dens.

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

Yet hibernating bears don’t just rest in the winter, says Barnes. They really shut down, completely resetting the parameters of their daily lives. “They come in, turn around two or three times, lie down and stay like that for six months,” he says; they get up to switch sides every few days. “Hybernation sets the outer limits of what is possible in terms of mammalian function.” Barnes notes that cubs often nurse twins or triplets during hibernation without eating or drinking, drawing on their own fat and water stores for the sake of their young.

For humans, this level of lethargy would come at a cost. Whether we were recovering in a hospital bed or aboard a rocket to Mars, our muscles would wither and our bones would thin after months of inactivity. Bears don’t have this problem. Part of the secret of their strong bones is being revealed. In 2021, Barnes and his colleagues published a study showing that hibernating bears are able to shut down genes involved in bone breakdown.

The researchers suggest that it may one day be possible to manipulate the same process in people to prevent osteoporosis. Barnes adds that such an approach could be particularly useful for people confined to prolonged bed rest, the humans currently closest to hibernation.

The slow metabolism of hibernating bears is an incredible feat in itself. A bear can slow its breathing and heart rate by about 75% for months while maintaining a relatively high body temperature. Although no one knows exactly how they put the metabolic brakes on, Gracheva says the strategy makes sense. She suspects that bears don’t relax like ground squirrels because it would take far too much energy to warm up their big bodies in the spring. Instead, they curl up, letting their fat and fur keep them warm with only the occasional shiver to help get the blood flowing.

The mastery of near-suspended animation by human-sized (or larger) animals has caught the attention of science fiction writers and others who dream of one day being able to send astronauts around the solar system as they “hibernate” for 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 dramatically reduce the supply of oxygen and nutrients to the brain. This lack of supply would be much less detrimental if doctors could quickly reduce demand by putting a patient into a state of hibernation, or something like that. Barnes notes that stroke victims are more likely to benefit from treatment within 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 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 hibernators from the animal world, proposed a study that would use a combination of the sedative drug promethazine and the antipsychotic chlorpromazine to temporarily create a “state of hibernation” in stroke patients, with the ultimate goal of preserving brain function.


Bear hibernation is unique in other ways. Some rodents and other animals hibernate according to strict schedules governed by day length. Bears, however, decide for themselves when to die out and when to recover, says Heather Johnson, a wildlife research biologist at the US Geological Survey in Anchorage, Alaska. This time is determined by a number of cues, including food supply and, most importantly, temperature.

In a study published in 2017, Johnson and colleagues tracked the hibernation of 51 black bears for an average of three winters each in the vicinity of Durango, Colorado. The total duration of hibernation varied widely, from less than four months to more than seven months, depending on age and parental status. Older bears and mothers with cubs tended to squat longer than younger bears that were alone. But all of the bears were clearly paying attention to the weather as they prepared to return to an active life.

On average, bears left their dens three and a half days earlier for every 1 degree Celsius (1.8 degrees Fahrenheit) increase in mean spring minimum temperature. “When a temperature hits a certain level, that’s their signal that it’s time to come out of hibernation,” Johnson says.

Similarly, Scandinavian brown bears appear to wait for the temperature to reach a threshold before emerging from their winter dens, says Alina Evans, wildlife veterinarian at Inland Norway University of Applied Sciences in Evenstad. In a study published in 2016, Evans and colleagues tracked the hibernation routines of 14 bears that had been fitted with satellite collars and monitors that tracked heart rate and body temperature.

The bears chose different times and places (tree roots, caves, dug anthills) to start making their dens, but they all left their dens when the average daytime temperature approached 5 degrees Celsius (about 40 degrees Fahrenheit) . Evans thinks the weather is pushing them out, but adds there are other possibilities. Bears can also pay attention to unpleasant humidity due to melting snow, for example.

As winters get milder in a warming climate, bears will undoubtedly start coming out of their dens earlier, Johnson says. It’s hard to say if such changes have ever happened, but, anecdotally, there are many stories of bears appearing in cities or on cabin porches during a warm winter time. She worries that shorter hibernation periods will give bears more time to get into trouble. Bears that emerge from dens early are more likely to knock over trash cans, be hit by cars, or find themselves in the crosshairs of a hunter. “Bears have about a 100% survival rate while they hibernate,” she says. “Otherwise it’s a dangerous world for them.”

Bears typically time their hibernation so that they are lying down when food is scarce, but active during times of plenty, Evans says. She fears that temperature changes will alter this schedule. In theory, an early warm spell could drive the bears out of the den sooner than usual, only for them to be thrust back into the cold when they have already started to lose weight in the winter. “They may miss an opportunity to save energy in a harsh environment,” she says.

In some cases, a sudden change in climate could temporarily turn bears into binge-eaters, Gracheva says. Typically, a bear loses up to 30-40% of its body weight, mostly fat, during hibernation. If a bear wakes up early several years in a row, all those spring meals could add extra pounds. “The bear could become obese,” she says. “They could become susceptible to diabetes like we are.”

But in the long term, bears may eventually adjust their feeding and hibernation schedules to adapt to a warming world, Evans says. After all, she notes, black bears thrive in the swamps of Florida and the forests of Mexico, and brown bears have gained a foothold in countries in southern Europe where harsh winters have generally subsided with the period. glacial. Some bears in warmer climates have decided to skip hibernation altogether. Bears in Greece and Croatia may not take shelter at all unless they are pregnant, says Evans. Hibernation may be less urgent in a warming world, but scientists are no less interested in what the process can teach us.

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