How a Black Bear Wakes Up from a Long Winter’s Nap

Across North America, black bears are rousing after a long winter’s nap. How do they function after essentially not moving for up to five months?

For a number of years, I regularly trained for and ran distance races: half-marathons and marathons. But the current time demands of a toddler, work travel and writing conspire to make it difficult to keep up a regular running regime. The toughest is when I stop running for a few weeks, and then try to get back into it. That makes running even 3 miles feel like a slog, and my muscles feel it the next morning. I have to build up endurance all over again. My legs lose their strength rather quickly.

Anyone who exercises or trains for an athletic event understands this. And we also know that if someone is confined to a bed for weeks or months, their muscles atrophy and bones weaken. When it comes to the human body, it really is “use it or lose it.”

Not so for black bears.

As Bernd Heinrich describes in his excellent book Winter World: The Ingenuity of Animal Survival, black bears can spend five months in a den, without eating, drinking, urinating or defecating and lying almost entirely immobile — and not lose muscle mass or bone strength.

Heinrich is fascinated by the bear’s adaptations; he calls them the “ultimate couch potatoes.” He also happens to be a record-setting ultra-marathoner, so he understands the physiology of exercise.

Bear hibernation is not easy to categorize. Hibernating bears are immobile, but they can awaken easily (a fact that makes studying bear hibernation quite difficult). Whereas hibernators like ground squirrels lower their body temperature to almost freezing, bear body temperatures only drop about 6 degrees Celsius. The bear’s thick coat and fat, coupled with regular shivering, keep the bear warm. But maintaining body heat takes energy. Despite this, bears don’t take in calories or expel waste all winter.

Biologists have found that black bears metabolize their urea into nontoxic creatine, and nitrogen wastes are recycled back into protein. Waste calcium is cycled to the bones to keep them strong. Unlike a bed-ridden human, who loses bone mass, bears emerge from their very long rest with their bones still strong and functional.  They also make their own water, likely by metabolizing fat. As researchers noted in a study in the journal Science, a hibernating bear is essentially a closed system, with no nutrients coming in or out.

That study involved five bears that had wandered too close to civilization in Alaska. After being captured, they were placed in artificial hibernating dens where University of Alaska-Fairbanks researchers “fitted the bears with sensors to record their temperature and heart rate, and the dens with infrared cameras and other sensors that monitored the bears’ movement, oxygen consumption, and even their snoring.”

Despite the relatively low drop in body temperature, their metabolism and oxygen consumption dropped by 75 percent.

The researchers found that, “While sleeping, [the bears] took only one or two breaths per minute. As they inhaled, their hearts did a quick flutter and then stopped until the next breath—resulting in a heart rate of about four beats per minute.”

The researchers also found that bears enter an intermediate state before hibernation. They eat and drink normally, but with their body already experiencing a lower metabolism. The bear’s body is preparing for a long period of rest.

In a National Public Radio story, researcher Brian Barnes called black bears a “metabolic marvel.” And part of that marvel is that there is still a lot we don’t know.

The black bear is the most common and adaptable bear species on earth. While these bears live in the wild country of the Rocky Mountains and Alaska, they also thrive in the woodlands and suburbia of the eastern United States. Their diet is similarly diverse, ranging from salmon to berries to backyard garbage. Even their coat color is highly variable, and can be brown, red, white or blue. (Yes, blue).

Despite the bear’s growing population, we still have much to learn about these animals. What we do know is that, in many parts of the country, the bears are emerging – perhaps a bit groggy, but otherwise ready to run.