Bears hibernate to survive winter. This biological process allows them to endure food scarcity and cold temperatures by altering bodily functions, conserving energy.
The Duration of Bear Hibernation
The length of bear hibernation varies, typically from two to seven months. Black bears may hibernate for up to seven and a half months, while grizzly bears generally spend five to seven months in their dens. Bears in northern regions with prolonged, harsh winters, such as parts of Alaska, tend to hibernate for longer periods, sometimes up to seven months. Conversely, bears in warmer, coastal areas or southern states might hibernate for only two to five months, or occasionally not at all, especially if food remains available.
Several factors influence the duration of a bear’s hibernation. The availability of food in the preceding autumn plays a role; if food is scarce, bears may enter their dens earlier and remain there longer. Weather severity also dictates hibernation length, with more severe winters necessitating extended periods of dormancy. A bear’s health and accumulated fat reserves are also important, as these sustain the bear through winter. Pregnant female bears typically begin denning earlier and stay in their dens for a longer duration, sometimes up to eight months, as they give birth and nurse their cubs during this period.
Physiological Adaptations During Hibernation
During hibernation, bears undergo physiological changes that enable survival without food or water. Their heart rate drops from a typical 40-50 beats per minute (bpm) to 8-19 bpm. Their metabolic rate also decreases, often slowing to about 25% of its normal active rate, efficiently conserving energy.
Unlike many other hibernating animals, a bear’s body temperature only experiences a slight reduction. It typically drops from 37-38°C (100-101°F) to 30-35°C (86.7-91.4°F), a decrease of 8-12 degrees Fahrenheit.
Bears avoid urinating or defecating throughout hibernation. Instead of expelling waste, they recycle urea, a waste product normally found in urine, converting it into proteins that help maintain muscle mass and organ tissues. This internal recycling, along with relying on stored fat for energy, prevents muscle atrophy and bone loss, allowing bears to emerge in good physical condition.
Hibernation Versus True Sleep
The term “hibernation” for bears can be misleading, as their winter dormancy differs from the deep, comatose state of “true” hibernators. Unlike animals such as groundhogs, chipmunks, or bats, whose body temperatures plummet to near freezing and who require a long period to rewarm, bears maintain a relatively higher body temperature.
This physiological difference means bears can be roused quickly from their dens, which is important for defending themselves or their cubs, particularly when newborn cubs are present. For this reason, some scientists prefer to describe a bear’s winter state as “winter lethargy,” “denning,” or “torpor,” rather than true hibernation.
Bears can shift positions within their dens and may even emerge briefly if disturbed or if weather conditions become unseasonably mild. This dormancy allows them to conserve energy and survive harsh winters without the extreme physiological shutdown seen in smaller true hibernators. Polar bears, for example, typically do not hibernate unless they are pregnant females in need of a den to give birth.