Hibernation is a survival strategy many animals employ to endure harsh environmental conditions, particularly during winter when food is scarce. A common misconception is that all bears hibernate in the same way, implying a deep, unresponsive sleep. However, the reality of bear “hibernation” is more nuanced, differing significantly from the profound torpor observed in smaller mammals. This distinction is important for understanding how various bear species adapt to their environments.
Understanding Bear Hibernation
Bear “hibernation” refers to a physiological state often called “denning” or “winter lethargy,” distinct from the deep hibernation of animals like ground squirrels or bats. While true hibernators experience dramatic drops in body temperature, sometimes to near-freezing, bears maintain a body temperature only slightly below their normal range, typically dropping from around 37°C to 30-34°C. This allows them to wake up relatively quickly if disturbed.
During this period, a bear’s metabolic rate significantly decreases, often by about 75%. Their heart rate slows considerably, from an active rate of 40-70 beats per minute to as low as 8-12 beats per minute. Respiration also becomes infrequent, with some bears taking only one breath every 15 to 45 seconds. Bears sustain themselves on stored fat, burning up to 4,000 calories daily without needing to eat, drink, or excrete for extended periods, up to seven and a half months. Their bodies are adapted to recycle nitrogen to maintain muscle mass and bone density, preventing the muscle atrophy and bone loss that would occur in humans during prolonged inactivity.
Bears That Truly Hibernate
Many bear species engage in this form of winter lethargy, often triggered by decreasing food availability and colder temperatures. American black bears, for instance, can den for up to eight months in northern regions, typically in hollow trees, logs, rock crevices, or under tree roots. Pregnant females of many species, including black bears, give birth in the den and nurse their cubs throughout this period.
Brown bears, which include grizzlies, also undergo a denning period. Their denning duration can vary, with some Alaskan brown bears hibernating for five to eight months, particularly in colder climates. Most Asiatic black bears hibernate for a few months during winter, especially in the colder, northern parts of their range, such as Russia, Korea, and Japan.
Bears That Do Not Hibernate
Not all bear species hibernate, or their denning behavior differs significantly due to their environments and food sources. The polar bear is a primary example of a bear that does not hibernate. Unlike other bears, winter is their prime hunting season, as sea ice forms, providing a platform to hunt seals. If they were to hibernate, they would miss feeding opportunities.
However, pregnant female polar bears dig maternity dens in snowdrifts to give birth and protect their cubs from the harsh Arctic conditions. They remain in these dens, living off fat reserves, until their cubs are strong enough to emerge, typically in March or April.
Other species, such as sun bears in tropical Southeast Asian forests, generally do not hibernate because food resources are available year-round. Similarly, spectacled bears, native to South America, do not hibernate due to constant food availability in their diverse habitats. Sloth bears, found in India and Sri Lanka, also remain active year-round and do not hibernate. Even within species that typically hibernate, individuals in milder climates or areas with year-round food abundance may reduce their denning period or skip it entirely.