Animals

Bear dens vary by locale and conditions. A cave, fallen trees, brush piles, culverts, the base of tree roots, and under buildings all may house the wintering bear. Current research points to reduced thyroid gland activity as the primary motivator for black bears to hibernate, but this is still under investigation. Early researchers believed that the onset of hibernation was due to the snow, temperature changes, decreased atmospheric pressure or a decrease in daylight hours. But a skinny bear will continue to forage in the winter, despite the “rule” that it should be hibernating. That bear might not survive the winter burning up its limited fat supply.
Black bears are not true hibernators like chipmunks and ground squirrels, for example. True hibernators lower their heart and breathing rates to about 10 percent of their normal activity. Black bears lower their body temperature about 10 degrees and their breathing and heart rates decrease slightly.