How Black Bears Hibernate

Dec 28, 2017

Black bears are easily one of our most interesting wildlife species. In this episode of Wildlife Matters, the Masked Biologist discusses some of what is going on in the life of a bear in the midst of Winter.

Black bears are a continual source of questions for folks who contact me for information. The calls tend to taper off somewhat in the winter, naturally, as bear activity slows to a crawl. However, any winter observation of a bear or a bear den gets a lot of attention and will trigger a number of calls.

Black bears enter their dens in early winter.  They drop into a deep sleep, which slows their metabolism and allows them to live off their body fat stored through the summer and fall months.  True hibernation by mammals like chipmunks and groundhogs includes lowering body temperature to near freezing. Bears do not enter true hibernation, but more of a winter sleep, or torpor.  Torpor allows them to rouse more easily than true hibernation, so they reposition, groom themselves, give birth, and if necessary escape danger readily.  However, as researchers discovered that bears can lower their heart rate down to 8 beats per minute, and breathe once every 45 seconds, they have decided hibernation is still an accurate description of their overwinter condition. Their den may be completely underground, partially exposed until covered with snow, or out in the open (I heard one story about a bear denning in an eagle’s nest). If you are out in the woods, you can sometimes find a bear den by spotting a melted exhaust hole in the snow.

During hibernation, bears form what is referred to as a fecal plug at the far lower end of their intestines, which was long thought to serve the purpose of “stopping up” the digestive system to cause the bear to stop eating. I have had a few people express concern about this, stating that once a bear flushes its fecal plug, it begins to metabolize and will need to find food to survive the rest of the winter. As research science has improved, though, we have learned that the fecal plug is simply a product of bears sleeping while their digestive system very slowly operates normally. Humans are not much different; if we reduce what we eat and stop drinking, we would quickly become constipated. Consequently, bears nearing the end of their sleep may get up and defecate, then return to their sleep. Bears oftentimes will chew the callused pads off their paws in the winter, which also will turn up in their late wintertime excrement.

Right now, bear cubs are being born—they weigh less than a pound and fit into the palm of your hand. They have a covering of fine stiff hairs, but for all practical purposes they are essentially naked and their eyes are closed. They nurse from their mother through the winter, growing rapidly. In less than a month, the eyes are open and they have a covering of short fuzzy fur. While the mother may snooze off and on during this time, they are very attentive to their cubs; if the den is too significantly disturbed she may even select a new den and move the cubs one at a time to finish waiting out the winter. Wildlife rehabilitators on occasion will ask if anyone has reported a mother bear’s den location. They will get reports of cubs orphaned or abandoned for one reason or another, and if they can find a drowsy mother bear with cubs, they oftentimes can add a cub or two to her litter. Bears are great mothers, but they can’t count, and will readily adopt cubs without risk of rejection. In late March, the cubs will emerge from their dens weighing about six pounds, still nursing and completely dependent on their mother. While it was once basically considered a fact that female bears have two cubs, we now know that three to five cubs are common. Success of the pregnancy depends heavily on how much fat the sow (female bear) puts on during her foraging before hibernating.

There is a lot of information out there on the web about black bears, which is great for biologists and researchers as well as nature enthusiasts.  If you are interested in learning more, I recommend you check out the North American Bear Center website, which provided a lot of background material for this article.  Find them at www.bear.org and check out the articles, or see the live bear den camera they post online every winter.

Striving to make new things familiar and familiar things new, this is the Masked Biologist coming to you from the heart of Wisconsin’s great Northwoods.