Bears undergo hibernation to survive periods of food scarcity and challenging environmental conditions. This state of inactivity and metabolic slowdown conserves energy during colder months, ensuring their survival until warmer temperatures and food sources return.
The Winter Den
When winter approaches, bears seek out sheltered locations to establish their dens. These winter homes include natural shelters like caves, rock crevices, and hollow logs. Bears also use brush piles or excavate dens into hillsides or beneath the root systems of large trees. Sometimes, they may even choose unconventional spots like road culverts or spaces under human structures such as porches.
A bear’s primary considerations for a den site are seclusion, protection from the elements, and insulation. Dens are often just large enough for the bear to fit inside, with a small entry point to minimize cold air intrusion. While bears often select a new den each year, some natural rock cavities or older dens may be reused by different bears over time.
Preparation for denning involves bears lining their dens with materials such as leaves, grass, moss, or branches. This bedding helps to trap body heat and provide insulation against the cold. Pregnant females are often among the first to den, sometimes as early as October, to ensure a safe and warm environment for giving birth to their cubs in mid-winter.
The Science of Bear Hibernation
During hibernation, bears undergo significant physiological changes to conserve energy. Their metabolic rate decreases significantly, allowing them to survive for months without food or water. This metabolic suppression is accompanied by a reduced heart rate, dropping significantly from summer rates. Their respiration also slows considerably.
While their body temperature decreases, it remains relatively high compared to other hibernators, staying above 88°F (31°C). This relatively higher body temperature allows bears to wake up and become alert faster than animals that enter a deeper state of torpor. Bears primarily rely on stored fat reserves for energy during this period, metabolizing fat to produce both energy and water.
Bears also exhibit adaptations in waste management. They do not eat, drink, urinate, or defecate while in the den. Instead, their bodies recycle urea, a waste product, into protein, which helps them maintain muscle mass and organ tissues despite prolonged inactivity. This ability means bears emerge from their dens strong, with minimal muscle or bone loss.
How Hibernation Differs Among Bears
The duration and intensity of hibernation can vary among different bear species and geographical locations. For instance, bears in colder, northern regions may hibernate for longer periods, up to seven months. In contrast, bears in warmer climates, where food might be available for more of the year, may hibernate for shorter durations or even skip it entirely.
Black bears can hibernate for up to seven and a half months in northern areas, while some in more temperate regions may hibernate for only a few weeks. Grizzly bears hibernate for five to seven months, often choosing high-elevation, north-facing slopes for their dens. Environmental factors like snow depth and the availability of food sources influence when bears enter and exit their dens.
Polar bears differ; adult males and non-pregnant females do not hibernate. Winter is their primary hunting season for seals on the sea ice, making hibernation impractical for them. However, pregnant female polar bears do enter maternity dens, usually dug into snowdrifts, where they give birth and nurse their cubs. While denning, these females reduce their activity and live off fat reserves, but they do not enter the deep hibernation state of other bear species.