Secrets of a Black Bear's Half-year Hideaway

After six months of hibernation, two black bears recently emerged to spring sunshine. The bears, a three-year old female and a yearling male, spent the winter in the same state as thousands of other black bears in Alaska-slumbering in a cozy nest, occasionally rising, stretching, and plopping back down. The major difference was the location of their dens; they wintered in a spruce forest on the University of Alaska Fairbanks campus.

Brian Barnes and Oivind Toien set up the hibernating quarters for the sow and young boar, problem bears trapped in the fall at Elmendorf Air Force Base near Anchorage. Barnes, a professor of zoology at the Institute of Arctic Biology, and Toien, a Fulbright scholar from Norway, watched the bears closely over the winter and got a fresh look at what happens to black bears during hibernation.

The bears wintered in padded boxes built so Barnes and Toien could detect each breath of the bears. By installing probes on the bears, they also knew the bears' heartbeats, shivering, and brain wave patterns. Monitoring the bears continually on computers, they found the bears perform physiological feats quite different from other hibernating mammals, such as ground squirrels.

While ground squirrels can save energy by cooling their bodies at times below the freezing point, the UAF black bears stayed relatively warm. The bears' body temperatures never went below about 85 degrees Fahrenheit. Their body temperature when not hibernating is about the same as a human's--99 degrees. How did the bears stay so warm all winter but still conserve enough fat to make it though to spring?

The answer might be related to an incredibly efficient heartbeat pattern. Barnes and Toien noticed the bears' hearts beat once every 10 seconds during the deepest state of hibernation. When the bears inhaled, their hearts quickly ticked off a few beats, enabling their blood to gain oxygen and their breath to pick up carbon dioxide and water. They also shivered when inhaling, a function that may keep their muscles in working shape during months of relative inactivity. After the bears exhaled, their heart rates dropped back down. About 40 seconds later, the bears inhaled again, repeating the slow process.

During the winter, the bears would, during a moment that happened every one to five days, consume more oxygen than normal and become warmer. Though Barnes and Toien could not see the bears, they assumed the bears were moving ever so slightly, maybe lifting their heads or even getting up to circle like a dog. This agrees with the hypothesis that bears are sometimes quite aware of their surroundings while hibernating, as may have been the case when a bear attacked a seismologist early this spring.

By monitoring the bears' brain waves, Barnes and Toien were able to tell that the bears sleep almost all the time. Since sleep is thought to be necessary in humans to repair the brain, the bears' ability to sleep for much of the winter begs the question of what sleep is doing for them.

As the bears' mid-April exit from their dens approached, their body temperature steadily increased, as did their brain activity and the patterns of their heartbeats. The bears emerged from their dens in good shape. The female lost 19 percent of its body fat, a bit less than was found in other studies of hibernating bears. The male lost 28 percent of its fat.

After just two weeks of eating apples, carrots and dog food, the bears were back to their pre-hibernation weight. The female leapt around, showing no signs of muscle loss during the long period of relative inactivity. Both bears appeared ready for the summer only a few weeks after rising from a half year of dormancy.