Black Bear, Black Bear How Do You Spend Winter?

We continue to receive lots of questions about hibernation.  We'd like to reach back to the Woodprints archives from the winter of 2015 and share the cover article.

Written by Ruth Forsgren, BCR Naturalist

When I was taking animal behavior and other like classes at UW Steven’s Point, I was taught that black bears were not true hibernators. Animals like chipmunks and woodchucks were true hibernators while bears were loosely defined as deep sleepers. Under the old definition of hibernation that looked solely at the lowering of body temperature to near freezing, bears were not hibernators. Scientists have redefined hibernation to include continuous periods of inactivity and/or reductions in heart and metabolic rates. With these broadened parameters, bears are one of the best-of-the-best when it comes to hibernation. Bears cut their metabolic rate by 50 to 75%, breathing only once every 45 seconds, their heart rate drops from about 100 beats per minute (bpm) to between 8 and 20 bpm, and blood flow to skeletal muscles is reduced by 45%.

                                                                   

Black Bear by Hope Rutledge

There are differences in how animals hibernate. Small animals like chipmunks will store food in the den while black bears store body fat. The den of the chipmunk will usually be below the frost line and sealed. Temperatures in the burrow will not usually drop below freezing. A black bear den is open and will roughly match outside temperatures. A hibernating chipmunk’s body temperature will drop to about 40°F while the black bear’s temperature will only drop about 12° to between 88 to 90°F. Chipmunks will periodically raise body temperatures, awaken, eat stored foods and produce wastes throughout the winter. Bears will go through periods of being somewhat awake but can go up to 7 months without eating, drinking or producing wastes.

During hibernation, a male bear will lose 15 to 30% of their weight, while pregnant females can lose up to 40%. The weight they lose is mostly fat, with little loss in muscle or bone mass. The bears also emerge from the dens without being severely dehydrated or having any loss of organ function.  How they do this is still a mystery, but it is being heavily studied by doctors and scientists that can see many applications benefiting humans, from keeping bed-ridden patients from losing muscle to an induced hibernation state for space exploration. (Click here to learn more about recent research)

Surprisingly it is estimated that 99% of bears that safely enter hibernation will survive the winter. Those that do not survive usually have died from predation or flooding of the den and not starvation. A bear must be ready when winter comes, which means in late summer to early fall they go through a stage known as hyperphagia, or a period of overeating. A bear with unlimited resources can eat 15,000 to 20,000 kcals (1 kcal = 1000 calories) and drink several gallons of water. Later in the fall, the bears will transition by eating less but continuing to drink water to purge the system of wastes.

Photo Courtesy of Joe Motto, BCR Photo Club

Personally, I am a tad bit jealous of the bears. When our winters are exceptionally cold or long I wish I too could put my head between my paws and sleep, at least through a few of the worst days.

Want to learn more about the black bears of Wisconsin?  Come to a special program taught by Dave Schmidt on Saturday, November 4th from 10:00 - 11:00 am at the Wise Nature Center. Click here for more information.