JACKSON, Wyo. — As we enter the darkest, coldest days of winter, bears are denned up and hibernating. But just what is going on in there? When and why do they den? Do they stay asleep the entire winter? How do they go to the bathroom? Do sows have cubs during this time?

So many questions. Let’s take a deep dive into bear hibernation and what exactly is going on.

First off, there has been some debate recently as to whether bears truly hibernate. Technically, the answer is no. Hibernation is often described as the time period where bears sleep during the winter, but that isn’t physiologically accurate. Smaller animals like some squirrels, mice, and bats are true hibernators.

Even chipmunks, which enter a state of trance-like slumber that results in a normal heart rate of 350 beats per minute reduced to a barely detectable four beats, don’t technically hibernate because they wake every few days to eat, urinate, and defecate.

Both black bears and grizzly bears enter what is called torpor. Bears slow their bodies down, way down. It’s like deep sleep on steroids. Deep sleep on barbiturates more accurately.

Catching some zzz’s

Bears enter a state of deep sleep in order to conserve energy, allowing them to survive about five months in their dens when food sources are at their scarcest. Where bears tend to take torpor to the extreme is in their ability to stay asleep for such long periods of inactivity and remain quite healthy.

To learn more, scientists have recently studied the bear’s ability to practically shut down its bodily functions and reemerge in spring no worse for the snore. Implications for humans—particularly concerning space travel over long distances—are especially appealing to NASA, for example.

Bears are able to slow their metabolic and heart rates significantly. In hibernation, a bear’s heart beats about 8-12 beats per minute—about a third of normal. It takes a breath once or twice a minute. Body temp is reduced from 98 to about 90 degrees Fahrenheit.

During hibernation, bears will not drink, urinate, or defecate for up to five months. In fact, bears recycle their pee, converting urea into protein. Further, a bear’s body changes physiologically so its kidneys can process nitrogen waste without poisoning themselves or damaging organs.

Not only do muscles not atrophy but the bear is able to lose weight (about 15-30 percent of its body weight) and add muscle while sleeping for months! If only this could be applied to humans. According to recent discoveries, bears maintain their physique with micro-movements to prevent atrophy. No bone loss is experienced. No fluctuation in blood sugar. The bear wakes in spring groggy but perfectly healthy.

What’s going on in there?

Bears aren’t out cold in the winter. They are not oblivious to their surroundings and will wake up to defend themselves and their young if disturbed by predators or unsuspecting recreationists.

It was once thought bears slipped into a coma-like, unawakenable state of sleep. One myth even included mother bears so deep in hibernation they gave birth in their sleep and woke to be surprised by their own cubs. Boy, were we fooled when Lily the Bear gave birth in her den during the world’s first cyber-broadcast of a cub birth during hibernation in 2010.

Bears shift position, scratch, and scratch, yawn, adjust their bedding and even venture outside the den briefly on warmer winter days. Grizzly bears sleep harder probably because they tend to den at higher elevations where it remains cold.

One local outfitter, Gap Pucci, who hunted and observed bears for more than three decades, says he has noticed black bears prefer to hole up with their den opening to the north. He believes the north-facing dens stay cold better than south-facing openings which tend to get more sunlight. Warmer temps wake bears, periodically, often due to the inside of dens becoming wet with snowmelt.


Male grizzlies come out of hibernation in mid to late March. Females with cubs emerge later, in April to early May. After an unseasonably warm winter, bears may come out of hibernation as early as January or February.

Black bears den in lower elevations and therefore wake earlier, typically in late February. Sometimes, if there is a warm winter and food is available, black bears might emerge out of their dens to eat.

When bears begin to emerge from their dens they are hungry and groggy. Next to hyperphagia (the calorie-packing feeding frenzy bears enter in the fall in preparation for hibernation), spring is a time when you do not want to encounter a bear. They are hangry and will get feisty over an elk or bison carcass, or other winter kills it finds.

During the first few weeks of waking in spring, however, some bears are admittedly simply too sleepy to be troublesome. Kerry Gunther, who leads Yellowstone’s bear management program, told Yellowstone Forever Institute, “When they first come out they don’t eat that much, and they are lethargic. For a week or two they spend more time sleeping on the carcass than eating it. Their metabolisms are not totally kicked in—they are in a kind of walking hibernation.”

The ability of bruins to conk themselves out for months at a time is still one of the more fascinating scientific wonders in the animal kingdom. Even while we are currently unable to enjoy observing bears in the wild, it is intriguing to know a little bit more about what is going on during their long seasonal slumber.