Secrets of Black Bear Hibernation

This is one of the better articles out there on the science and marvels of black bear hibernation. You can also read the article as it originally appeared on PBS NOVA Online here. Hibernating bear photos courtesy of Paul Cyr.

Secrets of Hibernation

by Peter Tyson

Certain mammals have what many people might consider the good fortune to be able to sleep through the winter—to hibernate. They bed down in the fall and, for all intents and purposes, don't arise again until the spring. Raccoons and skunks do it. So do woodchucks and chipmunks, hamsters and hedgehogs, bats and bears. Some, particularly rodents, sleep very deeply, while others, such as bears, slumber more lightly.*

*This has led some biologists to differentiate between the hibernation of, say, jumping mice and the "winter lethargy" of bears. Ours is not to quibble, however, and for the purposes of this article, sleeping through the winter—to whatever degree—will be referred to as hibernation.

One of the most celebrated hibernators is the American black bear (Ursus americanus). It can go for as long as 100 days without eating, drinking, urinating, defecating, or exercising. Biologists have long acknowledged that hibernating black bears may have something to teach us, and they are now studying the animals with an eye to aiding everything from organ preservation to kidney disorders, from human hibernation to long-distance space travel.

With all this at stake, it's worth taking a closer look at the black bear and its stunning physiological feats. How can it survive for so long without drinking? Why doesn't hunger force it to wake up and seek a meal in midwinter? What triggers it to enter and leave its den?

The bear facts: One of nine species of bear in the world, the American black bear is found throughout North America, from the frozen tundras of the Yukon to the steaming bayous of Louisiana. Unlike, say, the panda, it is thriving as a species, with an estimated 600,000 individuals roaming the continent. Its sheer numbers have forced it into ever closer contact with people, with black bears regularly seen these days in and around urban housing developments and other man-made environments.

Nesting instinct: Preparations for over-wintering begin in the summer, when bears begin gorging carbohydrate-rich berries and other foods to put on weight. During this period, they can gain as much as 30 pounds per week. In early autumn, a bear (and its cubs, if any) will rake leaves, twigs, and other plant materials into the den to form a nest. Throughout the fall its activity level steadily drops until it ends completely when the bear enters its den.

Bears make dens in burrows, caves, hollowed-out trees, and rock crevices. Dens of the bears Lynn Rogers studies in Minnesota typically feature entrances just large enough for a bear to squeeze through; interior chambers measure two-and-a-half to five feet wide and two to three feet high.

It's cramped for a single bear, much less for a mother and her cubs. But that's the way bears like it: Black bears do no exercising of any sort during the winter months, prefering to lie rolled into a tight ball, with their heads between their forepaws and their heavily furred backs exposed to the worst of the cold. Dens themselves offer little insulation. In Minnesota, dens with open entrances are about as warm inside as outside, where the temperature, Rogers says, often plummets to as low as -28°F. Bears keep warm using their great bulk, their inches-deep layer of fat, and their fur, which more than doubles its insulative value during the fall.

Missing beats: Once a black bear begins hibernating, it can doze for many months with a body temperature of 88°F or higher, which is within 12°F of summer levels. By contrast, the body temperature of smaller hibernators such as marmots, chipmunks, and ground squirrels may drop below 40°F. These daintier creatures must awaken every few days, raise their body temperatures to summer levels, eat stored food, and pass wastes.

Bears can go on slumbering because their warm pelts and lower surface-to-mass ratio allow them to better retain body heat. This, in turn, enables them to cut their metabolic rate in half. Using telemetry, Edgar Folk of the University of Iowa monitored the heart rate of a captive bear in Alaska as it slept. In the early fall, its heart beat 40 to 50 times a minute for most of each night. By December, when the bear was deep in hibernation, its sleeping heart rate had slowed to as few as eight beats a minute.

Black bears keep their heads and torsos warm enough that they can wake if disturbed, though some may take awhile to do so. In a 1981 article in Natural History, Rogers told of the time he accidentally fell onto a six-year-old female in her den. Even though her cub bawled, she didn't wake up for at least eight minutes. On the other hand, some individuals can revive disconcertingly quickly. Rogers again:

On January 8, 1972, I tried to hear the heartbeat of a soundly sleeping five-year-old female by pressing my ear against her chest. I could hear nothing. Either the heart was beating so weakly that I could not hear it, or it was beating so slowly I didn't recognize it. After about two minutes, though, I suddenly heard a strong, rapid heartbeat. The bear was waking up. Within a few seconds she lifted her head as I tried to squeeze backward through the den entrance. Outside, I could still hear the heartbeat, which I timed (after checking to make sure it wasn't my own) at approximately 175 beats per minute.

Bear essentials: Over-wintering black bears do other extraordinary things—things that might someday benefit humans. For starters, snoozing bears are able to gain all the sustenance they need entirely from within their own bodies. Fat tissues break down and supply water and up to 4,000 calories a day; muscle and organ tissues break down and supply protein.

Our bodies do the same thing when we're starving—with one crucial difference: Our bodies can't restore muscle and organ tissue, which those of hibernating bears can. Bears' bodies are somehow able to take urea—a chief component of urine that is produced during tissue breakdown and that, if left to build up, becomes toxic—and use the nitrogen in it to build new protein.

Even though a hibernating bear drinks no water, it does not become dehydrated. In a 1973 study published in the American Journal of Physiology, hibernation expert Ralph Nelson and colleagues at the Mayo Clinic and Mayo Foundation found that the three hibernating bears they studied were in "almost perfect water balance" after about 100 days of hibernation, during which they swallowed not a single drop of water. Nelson's team and other researchers want to learn how bears accomplish this metabolic feat, during which the amount of urine entering the kidneys drops by 95 percent, in hopes of using the information to help treat people suffering from chronic kidney failure.

Hibernating bears also have what would seem to be dangerously high cholesterol levels. Because they live off their own fat, their cholesterol levels are more than twice what they are in summer (and more than two times higher than those of most people). But bears evince no signs of hardening of the arteries or the formation of cholesterol gallstones. Research has shown that hibernating bears generate a form of bile acid that, when administered to people, dissolves gallstones, eliminating the need for surgery. Despite being cooped up in a space about the size of a doghouse, hibernating black bears also appear to avoid muscle cramping and degenerative bone loss. How they accomplish this remains a mystery.

Another mystery goes by the name "delayed implantation." A female will carry a fertilized egg in her womb for many months. The egg is ready to attach itself to the uterine wall and begin developing into a fetus. But it doesn't do so until the female's body gives some unknown signal. This adaptation allows bears to time the birth of their cubs, so they're not born too early or too late. It also gives the mother a way out if food is scarce: If she has not accumulated enough fat by the time she settles into her den to hibernate, the egg will spontaneously abort. Some biologists see this neat trick as a natural mechanism to control population.

Evidence is mounting that hormone-like substances in hibernating bears may control all these physiological tricks. When injected into other species, both those that hibernate and those that don't, these substances engender hibernation-like effects. Who knows? If people can be made to hibernate, perhaps sufferers of seasonal-affective disorder, or SAD, will find the ultimate relief: a winter-long snooze.

Want to hibernate? Human hibernation may not be as far-fetched as it sounds. In December 2000, Gerhard Heldmaier, a professor at the University of Marburg in Germany and chairman of the International Hibernation Society, announced the discovery of two genes that are thought to trigger hibernation. These genes direct enzymes to burn fat rather than carbohydrates, thereby equipping the body for hibernation. "There is no real reason," Heldmaier told London's Independent on December 3rd, "to say that humans are so different from other mammals that they are unable to enter hibernation."

Although it's possible that people may one day be able to nod off for the winter, the most likely applications of human hibernation involve medicine and perhaps space travel. Doctors might be able to preserve transplant organs longer if those organs could go into hibernation, as a true hibernator's organs do. The U.S. Army is reportedly eager to look into the potential of using hibernation to preserve wounded soldiers during transport from battlefields to hospitals. And NASA has sponsored research on using hibernation for long-distance space travel.

Spring break: In January, a pregnant black bear wakes up long enough to give birth in the den to one or more cubs. She then slumbers anew, rousing herself every now and then to lick the cubs and otherwise tend to them. The cubs, meanwhile, do not hibernate but suckle their mother, safely warmed by her sparsely furred belly. At some unknown cue, mom and cubs, which are now about three months old and weigh four to eight pounds, leave the den behind and get on with their lives in the world at large.

Weight loss is extreme among those leaving the nest. Between early fall and late spring, male black bears will typically drop between 15 and 30 percent of their body weight, while lactating mothers can lose up to 40 percent. Despite this grave weight loss, over 99 percent of black bears survive the winter. Most that do succumb do so because of den flooding or predators and not from starvation. A slumbering bear has all it needs within.

- Peter Tyson is editor in chief of NOVA Online.