Ask the Naturalist

-- by Kyle Waggener

Question:  Do Black Bears Hibernate?

Answer: 

Not really. This is not an easy question to answer because it really depends on how you are using the term “hibernation”. The word hibernate is ambiguous, and there is some scientific disagreement as to the exact definition of the term. In its simplest form hibernation means winter sleep. This definition seems to describe what black bears do in the winter. Some scientists reserve the term hibernation only for warm-blooded animals that are capable of lowering their body temperature down to approximate the ambient temperature. This does not describe the black bear. They are capable of lowering their body temperatures from 100 to 88 degrees F, and true hibernators lower their body temperature to around 40 to 45 degrees F. Some scientists use the term “hibernation” to describe any dormant state that animals intentionally enter during the winter that decreases their body’s metabolism by a lowering of the body temperature. This seems to fit black bears also. We humans would be in a lot of trouble if our body temperature dropped by 12 degrees F.

Animals go into hibernation to avoid food shortages during the winter months. Body temperatures are lowered along with the breathing and heart rates. This lowers their metabolism, or the rate at which they use energy, so that their stored fat reserves will last them through the winter.

People like to categorize nature into definite groups and nature doesn’t always comply. There is a continuum of dormancy strategies that animals go through to survive winter. True or deep hibernators represent one extreme. True hibernators in the Eastern United States include woodchucks, bats and some flying squirrels and chipmunks.

Woodchucks are the textbook hibernators. Their body temperature drops from 98 to about 40 degrees F. Their heart rate decreases from 100 to four beats per minute and their breathing rate drops to one breath every three or four minutes. Some individual chipmunks are capable of true hibernation and some have lost the ability. One main difference is that chipmunks aren’t capable of storing enough body fat to last them through the winter so they store food in their tunnels.

At the other end of the dormancy spectrum are animals that merely sleep through the harshest part of winter. One term used for this strategy is winter lethargy. Skunks, raccoons and opossums are examples of animals that will den communally and sleep with little to no changes in respiration and body temperature. They will live off body fat for up to a month before going out to find food again. This behavior happens more frequently further north because of the extreme winters.

 In the winter dormancy spectrum, black bears fall somewhere between the two extremes. They are capable of lowering their metabolic rate to half of their normal rate. They should be considered shallow hibernators or deep sleepers. Hibernators have to warm themselves back up to normal temperature. This is metabolically expensive and takes many calories. The number of calories it takes is directly proportional to their body weight. There seems to be a limit of around 22 pounds for animals to be able to store enough calories to warm their body back up to around 99 degrees F. It is physically impossible for black bears to store enough calories in fat reserves to warm their large bodies up by 50 degrees F. Also, small animals have a greater surface area relative to their body mass than large animals do. This means that they lose body heat at a faster rate. So large animals like black bears are able to retain heat better than smaller “true” hibernators.

There are advantages to the shallow hibernation of black bears. For example, they can awaken rapidly to escape danger. True hibernators take at least twenty minutes to several hours to awaken from deep hibernation. Black bears are able to recycle urea and neither urinate nor defecate during hibernation. In some cases they are out for seven months straight! You could say they are the world recorder holders in holding it. True hibernators have to awaken every two weeks or so to void waste and get a drink of water. They also need to move their muscles around to avoid muscle atrophy. Black bears have found ways to physically avoid muscle atrophy and dehydration. Scientists still aren’t exactly sure how they do it and studies are under way now to determine this. So even though black bears aren’t considered “true” hibernators, in some ways what they do is even better.