How Do Bears Survive the Winter? A Detailed Explanation of Hibernation Habits

Bears hibernate to get through the winter. When hibernating, a bear chooses a suitable den, usually on a sunny, wind-sheltered hillside or inside a hollow dead tree. Hibernation can last 4 to 5 months. During hibernation, if it is disturbed, it will wake up immediately, and occasionally it will come out of the den to move around.

During hibernation a bear is in a state of torpor, but its body temperature does not drop, or drops only slightly. It goes without eating for a long time and remains in a sleep-like state, yet it wakes easily and can respond immediately to outside stimuli. It is known as a semi-hibernating animal.

Lack of food is the main reason animals hibernate. If food is plentiful, many bears will not hibernate and will instead hunt all winter long. But when food is scarce, bears will hide in a den to get through the winter. Small mammals' body temperature drops sharply during hibernation, but a bear's body temperature drops only about 4 degrees, though its heart rate slows by 75%. Once a bear begins hibernating, its source of energy shifts from food to the fat stored in its body.

Tom Evans, a field biologist working on the U.S. Fish and Wildlife Service's polar bear program in Alaska, says this change in chemistry is dramatic. When fat burns, metabolism produces toxins. But during hibernation a bear's cells break these toxins down into harmless substances and recycle them. This biochemical process also lets bears recycle water within their bodies, so they do not urinate during hibernation, and polar bears can also use the mechanism of fat burning. This waking-style hibernation allows polar bears to stay active all winter without hiding in a den.