The Bear Snores On: New Science On Bear Hibernation

NIKI WILSON – On Science
December 06, 2012

Photo Credit: Alaska Fish and Game


It’s the end of November, and I don’t know about you, but I’m a little bit sleepy. There are days I feel like I could curl up and doze until the sun starts hanging around a little longer. Some days it’s tempting, but to be honest, I’m only good for about eight hours of lying around before body parts start to ache and I need some pancakes.

Bears don’t have this problem. They shut down their biological business for five to seven months of winter hibernation every year, without eating, drinking, defecating or urinating. Researchers have long been intrigued by these abilities, and have studied hibernation extensively for decades. Despite this, a 2011 study has revealed a few more secrets.

The study, conducted by researchers at the Institute of Arctic Biology at the University of Alaska Fairbanks, was the first to continuously measure the metabolic rates and body temperatures of black bears, day and night, as they hibernated through the winter under natural conditions.

Habituated black bears captured by Alaska Department of Fish and Game officers had sensors implanted under their skin to monitor body condition. They were then transferred to outdoor enclosures in an isolated wooded area that contained artificial dens made of wooden boxes and hay. The bears adopted the dens, equipped with infrared cameras, activity detectors and telemetry-receiving antennas, for their hibernation period.

The cameras revealed that the bears maintained a curled up position that preserves heat and minimizes water loss. But unlike other small mammal hibernators that remain virtually comatose for many days at a time, these bears changed position at least once every two days, sometimes twice a day, getting up to stretch, groom and rearrange the hay in the den.

An interesting and previously undocumented pattern was found in the way the black bears regulated their body temperatures. Said researcher Oivind Tøien: “We knew that bears decrease their body temperatures to some degree during hibernation, but in Alaska we found that these black bears regulated their core temperature in variable cycles over a period of many days.”

Once core body temperatures dropped to around 30 degrees Celsius, researchers observed the bears shivering to bring their body temperatures back up to around 36 degrees. This often took several days, although cycles were shortest during the coldest weather. Once they achieved this maximum temperature, the bears would reduce shivering until their body temperatures fell once more. Tøien said this cycling has not been seen in smaller hibernators, nor had his team seen it in other mammals.

The bear that deviated from this cycle was a female bear that was pregnant during hibernation. It would have been difficult for officials to know the bear was pregnant when she was captured, as bears experience delayed implantation. This means that although bears mate in the spring, fertilized eggs remain dormant in the uterus until late November when the female’s body condition determines whether or not she can maintain a pregnancy through the winter months. If she has enough fat and energy reserves, the egg will implant and gestation will begin.

This pregnant bear maintained a wakeful body temperature during the first part of her hibernation, which researchers suggest might be needed for the development of the embryo. But after she birthed her cub (who unfortunately died due to a congenital defect), the female bear’s body temperature began to fall until it eventually joined the variable cycle that the other hibernating bears experienced.

Another interesting pattern emerged in regards to temperature. One would think that temperature would be linked to metabolism and heart rate. We’ve all heard the stories of people who have been submerged in icy water for long periods of time and then are resuscitated intact because the cold water had slowed their heart rate and body function, and therefore slowed the damage caused by a lack of oxygen to the brain.

However, during hibernation, bears appear to be able to slow their heart rates considerably despite maintaining their bodies at relatively high temperatures. Their heart rates slow from around 55 beats per minute during wakeful periods to about 14 beats per minute during hibernation.

Breathing patterns around heartbeats are also remarkable. Says Tøien: “They have an almost-normal heartbeat when they take a breath. But between breaths the bears’ hearts beat very slowly. Sometimes, there is as much as 20 seconds between beats. Each time the bear takes a breath its heart accelerates for a short time to almost that of a resting bear in summer. When the bear breathes out, the heart slows down again.”

The study held one last important finding that supported earlier researchers’ hypotheses. When the bears emerged from the dens, metabolic levels only reached summer levels after two to three weeks. In other words, it takes these bears a long time to fully wake up.

Of course, some of the big questions remain. “When black bears emerge from hibernation in spring, it has been shown that they have not suffered the losses in muscle and bone mass and function that would be expected to occur in humans over such a long time of immobility and disuse,” said Brian Barnes, the senior author of the study. “If we could discover the genetic and molecular basis for this protection, and for the mechanisms that underlie the reduction in metabolic demand, there is the possibility that we could derive new therapies and medicines to use on humans to prevent osteoporosis, disuse atrophy of muscle, or even to place injured people in a type of suspended or reduced animation until they can be delivered to advanced medical care – extending the golden hour to a golden day or a golden week.”

The potential human benefits of hibernation research explain why the study was funded by U.S. Army Medical Research and Material Command grant, grants from the National Science Foundation, a grant from the National Institutes of Health, gift funds to Stanford University, the American Heart Association and the Fulbright Program. But the research may also help us understand how our changing climate may alter bear hibernation patterns and survival potential in the years to come. Let’s hope there’s something in it for the bears.