Hibernation Means Different Things to Different Animals
by Mark D. Jones
Agricultural Liaison Biologist, former Black Bear Biologist
“Do bears really hibernate”? This is a question I often hear from the general public and even other wildlife biologists. When I answer yes, many people next ask, “do they really hibernate in North Carolina, or is this just a northern behavior”? Well, the answer is that bears hibernate throughout North America and certainly in North Carolina. However, the process is very different from what is considered “normal” hibernation exhibited by rodents and bats.
To understand this issue, we first need to define the process of hibernation. In its simplest definition, hibernation is a specialized reduction in metabolism brought about by low food availability and/or low temperatures. Several body changes can occur during hibernation. These include lower heart rates, constriction of blood vessels, suppressed shivering, reduced breathing, and lower oxygen consumption. Another process common during hibernation is known as adaptive hypothermia. This is a process by which an animal lowers its body temperature in order to conserve energy. Different species may utilize different degrees and combinations of the previously mentioned metabolic processes. Many species of rodents and bats drop their body temperatures very dramatically, some to near freezing. Bears on the other hand only drop their body temperatures by 10-15 degrees in most cases. As a result, bears are somewhat wakeful sleepers and are capable of abandoning a den if seriously disturbed whereas many rodents are so deep in hibernation that they can be handled without waking.
Most hibernating mammals, such as woodchucks (ground hogs to most North Carolinians), must arouse from hibernation periodically to feed, urinate, and defecate. Herein lies the main difference in bear and rodent hibernation. Bears have the ability to remain stationary for longer periods than rodents without feeding or eliminating waste. In northern areas of the U.S. and Canada, bears hibernate as long as 8 months without moving from their den. In the South, bears exhibit the same characteristics, only for shorter time periods. The build-up of waste that bears somehow process would kill most animals if they did not arouse from hibernation to handle normal bodily functions. Furthermore, the long periods of inactivity would result in degradation of bones and muscles for non-hibernating animals. Bears however exit their winter dens strong and healthy after long periods of inactivity. Whatever you call the process in bears, it is a truly remarkable physiological achievement. Medical doctors are attempting to understand bear hibernation better in order to help patients suffering from kidney dysfunction and bone degradation.
What about bears in North Carolina? Based on hundreds of radio-collared black bears studied across the state, we know that the vast majority of our bears hibernate. Females typically hibernate longer than males. North Carolina’s bears just do it for shorter time periods than their northern cousins. Bears studied in eastern North Carolina by radio-telemetry entered dens as early as November and as late as January. These same bears exited dens as early as February and as late as April. This results in the possibility of bear sightings and roadkills in all months and the misconception that coastal bears do not hibernate. Only human disturbance interrupts these periods of hibernation in North Carolina’s bears.
Female bears give birth during hibernation and nurse their cubs through a period of helplessness. The female “hibernates” throughout the entire process, and unless disturbed, she will not leave the den until she brings her cubs out of the den in the spring. By that time, the cubs can walk and follow the mother as she feeds.
The next time someone says black bears do not “hibernate”, please tell them that whatever you wish to call the process, bears do enter a long period of physical inactivity and exhibit some amazing physiological responses to low food availability and temperatures. In many ways, their ability to hibernate for long periods without feeding or eliminating waste is more advanced and remarkable than what has been considered “normal” hibernation involving periodic arousal to move about and feed.