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.
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Really fascinating! How I’d love to go to sleep in November and wake up April!
I was very surprised to hear on the news in a January 2016 trip to South Florida, that a woman’s home had been invaded by a Black Bear family, and in the same week, they were in another lady’s yard checking out the trash. This was in Coral Gables Florida, near the beach…not a place you expect to run into bears. I guess they’re pretty much everywhere now?
Black bears are very wide ranging animals. They historically were found in the southern United States (and still are). They have different hibernating habits there, as there is food available year round.
Sometime about January, 2007, I decided to hike to the top of Roundtop Mountain, a low mountain of about 1,100 feet which is about 5 miles west of Hancock, Maryland. There would be a fine view of the Potomac River from the top. As I climbed up through an area which had once been an orchard, I noticed a patch of black fur amongst a thick pile of dead leaves on the leeward side of a large log about 2 feet in diameter. Thinking it a dead animal, I thought of investigating it further with my hiking stick, but suddenly (and fortunately) realized it was a hibernating bear. Fortunately, my dog was busy 75 or a hundred yards away. Shaken, I quietly walked away taking my dog with me, and abandoned my attempt to climb the hill for that day. I actually live in adjoining West Virginia, where State Law has resulted in an inadvertent boon to the local bear population. Our bears feast on the weekly garbage pickup, because the State, in its wisdom, allows only about 8 landfills to serve the entire state, and furthermore each landfill is allowed to accept only so much tonnage per day. This results in a “race to the landfill” among competing collection companies, with the looser being required to drive collected garbage to an alternative landfill maybe 50 or 75 miles away, as no garbage is allowed to remain in a truck overnight. The collection companies require garbage to be put out at 6AM, and since most customers do not want to get up before 6AM to put garbage out, they put it out the night before. This gives the bears about 8 hours to raid the choicest garbage cans. I don’t think the State could arrange a better bear feeding system. As a result of this shortsighted system we have an abundance of bears living in deep-suburban conditions, and bears are sometimes sighted during daylight hours. Furthermore they seem to be slowly loosing their fear of humans.
I have heard that upon emergence from torpor, Ursus will search for a salad of skunk cabbage to purge itself of both liquid & solid waste produced during its winter sleep. Any truth to that?
Nice article! I found it while I am searching information about months when bears go to sleep and wake up. Currently what I found is that usualy they are going to sleep on november/december and they wake up in april, maybe you have better information?
I thought bears do not hibernate, but go into a deep sleep ?