Hibernation and Denning

During late fall, bears spend less time feeding and more time preparing to den. One of the primary factors that influence the time a bear will den is its fat reserves. When inadequate, the bear will be compelled to remain active in search of food. The timing of den entrance is also influenced by the age and sex of the bear. As a rule, a pregnant female or a mother with cubs will enter the den first, followed by subadults, while adult males are generally the last to enter their dens. Den entry further depends on geographical latitude - some southern bears do not den at all.

As bears travel their home range during the summer, they may make note of potential denning sites and return in fall. During the last days before entering the den, bears often appear lethargic. Bears will rest in day beds before they actually bed down for the winter in a den. Day beds are constructed by raking together leaves, conifer needles, and small branches into a pile. Bears may have several day beds spread throughout their territory.

Hibernation is a method to conserve energy when weather conditions are unfavourable and food is scarce. Most B.C. black and brown bears den for four to six months every year, entering the den in November or December and emerging in March or April. Of course there are always stragglers going in and coming out.

Dens are usually located in sites that are dry and well drained and where snow cover is deep. Bears like to use cavities in the base of old growth trees, small caves or rock cavities; often with remarkably small entrances. Bears will excavate suitable cavities in the ground, digging a small hole big enough for the bear to twist around in and that's all. Many dens have a tunnel leading to a larger chamber, while some dens have been found with multiple chambers accommodating several bears of the same family. Dens may be used again and again or a bear may choose a different site each winter. Some dens are dug below the roots of trees using the root mass as a ceiling. Bears usually line their dens with leaves, conifer needles and bits of branches or wood shavings. Some bears have even used surface dens, open to the elements. Bears generally prefer secluded sites for denning, although they've been known to den in culverts under busy streets.

Denning generally consumes more time in a bear's life than any other activity. Hibernation is a method to conserve energy when weather conditions are unfavourable and food is scarce.

Bears are not true hibernators. If you look inside a bear den in mid-winter, the animal may lift its head and look back at you, and if the bear feels threatened it may get up and run away, although its movements would initially be slow and stiff.

Bears actually enter a state of dormancy where their heart rate drops from 40-70 beats/min. to 8-12 beats/min.; their metabolism slows down by half; and their body temperature drops by 3° to 7°C. Bears do not eat or drink during this time, nor do they eliminate waste. In fact they actually recycle the build up of urea, using its constituents to manufacture new proteins. While hibernating, the bear lives off its fat reserves, and may lose a quarter to a third of its body weight.