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Bear Hibernation: The Science of Our Furry Neighbors’ Winter Naps

For many bears, winter means hibernate or die. Here's what they do to survive. Plus: how climate change puts newborn cubs in danger.

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We humans have to power our way through the winter, bundling up and worrying about frostbite on the trail. But bears? They just snooze their way through it. Bears have some tried-and-true habits to help them stay cozy (and alive) each winter. It starts with hyperphagia, a period when bears gorge themselves on salmon, berries, or anything else they can find. (There’s even a competition every year at Katmai National Park and Preserve that crowns the fattest bear, and therefore the one most prepared for hibernation.) Then, they find a nice cozy spot, and settle in. If you ask us, this evolutionary response to winter is as complex as it is fascinating. Here’s everything you need to know about the deep sleep that many bears experience every year.

Do Bears Hibernate?

Technically, no. At least, not in the way that chipmunks and mice do. It’s more accurate way to say that brown and black bears enter a state of torpor each winter. This means they enter a deep sleep and experience some metabolic changes without becoming completely dormant.

Their pre-hibernation state of hyperphagia—the bulking period with excessive eating and huge appetites—keeps them from starving to death while sleeping. Even though they eat up to 20,000 calories each day and gain up to 30 pounds each week in the time leading up to hibernation, they have still lost 15-30% of their body weight by the time they emerge in the springtime. Getting the crucial fat stores not only saves the bear’s life, but it also creates life. Fertilized eggs from the summer mating season only implant in the mother’s womb if she has enough body fat by the time she has settled into her den at the beginning of winter.

Why Do Bears Hibernate?

Bears have evolved this way because hibernation is their best chance to survive the food scarcity each winter. In order to make it through the severe calorie deficit that occurs during hibernation, they lower their body temperature, slow down their respiration and heart rate, and break down the stored fats and proteins in their bodies. Luckily, in the weeks leading up to hibernation, bears have bulked up and gained nearly 5 inches of fat on their bodies to burn through in the winter.

bear and two cubs walking around grand teton national park
Don’t separate a bear from its cub. Bears become aggressive when they think humans are going to threaten their young or take their food. (Photo: Chase Dekker Wild-Life Images via Getty Images)

How Do Bears Hibernate?

The act of hibernating is instinctual. There are some clues, though, that help the bear know when it’s time to find a den: shorter days, low food sources, and dropping temperatures. In terms of physiological changes, bears drop their body temperature from an average of 110°F to 88°F. This isn’t as low as other hibernators that can drop their body temperatures close to 40°F. Because bears’ body temperatures are higher than true hibernators’, they can wake to defend themselves from external threats faster. They also drop their respiratory rate from an average of six to 1o breaths per minute to one breath every 45 seconds. Their heart rate also drops from an average of 40 to 50 beats per minute to eight to 19 beats per minute. These physiological changes save energy and keep bears alive longer.

How Long Do Bears Hibernate?

Hibernation can last between just a few weeks to seven months. Temperature, sex, and food availability make all the difference. Grizzlies in frigid British Columbia, Canada, can hibernate for seven months while black bears in a more temperate Mexico only hibernate for three to four months.

In general, bears emerge between March and April. Male bears emerge earlier than female bears who stay with cubs. When the cubs are born, they weight about one pound and tuck under the mother’s fur to nurse and grow. These dens are a crucial aspect to keeping cubs alive until they can withstand conditions outside.

Bears don’t wake from hibernation with a spring in their step. They’re groggy and in a state of walking hibernation for two to three weeks. During this period, their metabolisms haven’t quite kicked back in. It’s a myth that they wake up ravenous; it takes them weeks to find a nearby carcass remotely palatable.

Do Polar Bears Hibernate?

No: Polar bears don’t hibernate. Hibernation is an adaptation to scarce resources, and winter is when polar bears head out onto the ice to hunt seals. “If you are born a male polar bear, you will never den up again because you can eat seals year-round,” bear biologist Tom Smith says. “Pregnant bears, however, enter maternal dens where they can give birth in a relatively warm, safe environment.”

polar bear walks on ice at sunset
Polar bears have adapted to handle cold weather and ice. Only female bears need to hibernate. (Photo: Paul Souders via Getty Images)

When Do Bears Hibernate?

This depends on location and duration of winter. According to the North American Bear Center, bears in Minnesota react to the scarce food each fall, retreat to their dens in September or October, and stay there for six or seven months. Bears in the New England states, however, have access to acorns and nuts each winter, so they don’t have to be in such a hurry to climb into their dens. They crawl in at the end of November or beginning of December every year and stay in for less than five months. Also, while bears in the New England states have the resources and warmer temperatures that allow them to wake up and forage, bears in Minnesota don’t have the same conditions and have to hibernate through the winter.

What’s a Bear Den Like?

Bears like to hibernate in insulated single-chamber dens that are about the size of the bear itself; a small entryway is big enough for the bear to squeeze through. Some bears claim the best den long before hibernation, and some procrastinate until the very last minute to find a decent spot. Brown bears stay in a similar neighborhood year after year; they hibernate less than four miles away from the previous year’s den. In wooded areas, bears will hibernate within hollowed-out trees, beneath piles of brush, or in anything their environment offers that will keep them warm and safe all winter.

Smith says the bear dens he has studied are mostly snow dens that are excavated in drifts of hardened snow.

“Interestingly, these dens have no external openings, no vents, nothing for air exchange,” he says. “The air comes through the interstitial spaces in the snow. We sometimes find the inside often all clawed up, suggesting that perhaps bears claw it to keep air flow going because over time, ice forms on the inside. We also find hairs hanging from the ceiling where their bodies rubbed up against, became frozen in the forming ice and got pulled when the bear moved.”

Brown bear resting with both paws on the log.

As they hibernate, bears drop their body temperature, pulse rate, and respiration levels. Their stored body fat keeps them warm and alive each winter. (Photo: ByronOrtizA via Getty Images)

What Do Bears Dream About During Hibernation?

“It’s been said that if a lion could talk, we’d have no idea what he was talking about,” Smith says. “This is to say, that a lion (and polar bear’s) experience on this planet is so radically different than ours, they would not experience life as we do and hence, if they could talk, it would be the ramblings of an insane person. That said, my guess is that their dreams are dominated by two things: fear of predation and of successful seal hunting. Not all that different than us in that respect!”

Do Bears Go to the Bathroom in Hibernation?

Could you imagine holding it for months and months? Well, bears can, because they do. Black bears and grizzlies typically don’t eat, drink, or use the bathroom while they hibernate. They find a better use for their bodily waste: Rather than eating at their muscle mass and organ tissues, bears turn urea, a nitrogen-based compound found in their urine, into protein and break that down instead. As far as number two goes, they form a hard fecal plug to keep things stopped up all season. (Sorry, but you did ask.)

How Does Climate Change Affect Hibernation?

Bears don’t have calendars or alarm clocks; they rely on natural cues, such as temperature, to know when to wake up and emerge from their den. So, because temperatures are rising due to climate change, bears have been leaving the den up to six days earlier than before, according to a study on climate change’s effects on black bear hibernation. Changing temperatures pose a threat to newborn cubs that rely on each day in the den to gain enough weight to survive the elements outdoors. The longer they stay inside, the better.

Scientists are have been theorizing for years that polar bears could have a better shot at surviving the harmful effects of climate change if they enter a walking hibernation state each winter. Unfortunately, according to Science, polar bears can’t hibernate their way out of starvation.

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