There is Life Within a Black Bear Den

An expectant female black bear dug this den into a steep roadside bank barely 20 feet from the edge of the pavement.

It’s mid-January and spring is still more than two months away. Chipmunks, woodchucks, and even bats are hibernating now. For wildlife, hibernation is a period of dormancy during which all physical activity ceases and bodily functions go into low gear in order to conserve energy. It’s a time when body temperatures, heart rates, and even breathing are reduced to a state of suspended animation or metabolic depression.

But inside of a black bear’s den, it’s a different story. While they are not true hibernators, bears do spend the winter in something of a lethargic state. But their rate of metabolism slows only slightly and their body temperature drops from a summertime norm of 100 degrees to 95 or so in the den. Adult bears do not eat, urinate, or defecate while denned, but they do give birth—and that will begin to happen any time now.

June and July are the months when black bears mate, but through a curious biological process known as delayed implantation, the fertilized eggs of the female, known as a sow, will remain dormant until she enters her winter den in November. When she finally does give birth, she may have anywhere from two to five cubs. A sow only comes into estrus every other year. She is promiscuous and will mate with more than one male, or boar. When combined with the effects of delayed implantation, that means that each of her cubs could potentially have a different father depending on how many times she was bred.

Cubs weigh less than a pound at birth and are not much bigger than a chipmunk. They are born blind, covered with a very fine coat of fur, and nurse on their mother’s milk. They can barely crawl so their mother guides them into position for nursing. Because most den entrances are open, the temperature inside isn’t much different than it is on the outside. The sow’s elevated body temperature is what helps to keep the newborn cubs warm.

Bear dens can vary from rudimentary shelters to elaborate excavations and females seem to be fussier than males. Sows expecting cubs usually dig a protective burrow of sorts into a side hill or beneath a tree root while boars seem content to just crawl underneath a downed tree top or into a hollow log. Natural shelters like rock crevices and caves make ideal places to den and can be reused for many years—but not by the same bear. They never use the same den twice. I know of one female bear that denned in the basement of a collapsed barn and another that dug into a steep roadside bank barely 20 feet from the edge of the pavement. It must have been unnerving for her and her babies whenever the town snowplow came by.


adamski_profile_Apr21Story and photo by John Adamski