What really happens in a winter den
BY THE MASKED BIOLOGIST
Special to the Star Journal
Black bears enter their dens in late November or early December. 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 deep winter sleep, or torpor. Torpor allows bears to rouse more easily than if they were in 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 eight beats per minute, and breathe once every 45 seconds, they have decided hibernation is still an accurate description of their dormant or semi-dormant overwinter condition.
Most people think of a cave as a standard bear den, but in fact a black bear can choose any number of different options. Bears select a den just large enough to fit them; an adult male will fit into a hole that might be only four feet in diameter. A sow that had cubs last winter may allow the yearlings to den with her for their first winter, but a pregnant sow will choose a site where she can protect and tend her cubs when they are born. When I am trying to find a likely bear den location, I look for an overturned tree with a large root wad; bears love to dig in beneath the roots. They might also choose a hollow tree or a small dense stand of conifers. Their den may be completely underground, partially exposed until covered with snow, or out in the open.
I heard one story about a bear that denned by lying on the ground and letting the snow cover it and another seen denning up in an eagle’s nest. Bears usually line their den with leaves, moss and soft bark. If you are out in the woods this time of year, you can sometimes find a bear den by spotting a melted breath hole in the snow. While most bears are very drowsy or difficult to rouse this time of year, I recommend you not try to get an up-close wildlife experience with a hibernating bear. Professionals still use drugs to immobilize bears in the den before handling animals for any reason. These are wild animals and they deserve to be given space.
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 its fat too quickly 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 fluids, we would quickly become constipated. It is not a cause for concern if bears nearing the end of their sleep may get up and defecate, then return to their sleep. It may well be true that some bears awake early because they did not put on enough fat, but with all the food options available for bears it seems very unlikely unless the bear is injured or has a debilitating deformity of some kind.
It appears that one of the last things the bears consume before going into the den is dry grass, for some reason; maybe because other foods are in short supply in fall, maybe because it helps them feel full or settles their stomach, or maybe they just swallow some while lining their dens. While in the den, bears oftentimes will chew the dry callused heel pads off their paws in the winter, as the new softer pads grow in underneath. Consequently, fecal plugs tend to have pieces of dry grass and chewed foot pads in them.
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.
The Masked Biologist earned a Bachelor of Science degree in wildlife biology. His work in natural resource agencies across the country provided opportunities to gain experience with a variety of common and rare fish, plant and wildlife species. Follow The Masked Biologist on Facebook. Email questions to MaskedBiologist@charter.net.