The idea of a bear retreating to a cave for a long winter sleep is deeply ingrained in popular culture, yet the term “hibernation” as it applies to these large mammals is a scientific misnomer. To understand which bears enter a winter rest, it is necessary to move beyond the traditional definition of hibernation. Most bear species living in temperate or arctic zones undergo a period of winter dormancy. However, the physiological state they achieve is remarkably different from the deep slumber of smaller animals, a distinction fundamental to grasping the diversity of survival strategies across the Ursidae family.
Understanding the Difference Between Hibernation and Torpor
True hibernation, scientifically known as deep torpor, is characterized by massive drops in body temperature and metabolic rate. Small mammals like ground squirrels and marmots exemplify this state, allowing their core body temperature to plummet to near-ambient levels, sometimes as low as 2 to 5 degrees Celsius. This drastic reduction in energy expenditure can slow their metabolism by as much as 95%, making them extremely difficult to arouse. True hibernators must also periodically wake up to raise their body temperature and perform biological functions before re-entering a deep sleep.
The winter dormancy practiced by most bears is a specialized form of torpor that avoids such a dramatic physiological collapse. A bear’s large body mass, combined with metabolic suppression, allows its core temperature to drop only slightly, typically from 37 degrees Celsius down to a range of 30 to 36 degrees Celsius. This moderate temperature decrease enables a metabolic slowdown of about 50 to 75%, sufficient to conserve fat reserves for months. Because their body temperature remains relatively high, bears can be easily roused, allowing them to react to threats or disturbances, which differs significantly from deep hibernators.
The Bears That Enter Winter Dormancy
The most well-known bears that enter winter dormancy are the North American species, including the American Black Bear (Ursus americanus) and the Brown Bear, or Grizzly (Ursus arctos). Their decision to enter a den is driven by the lack of available food during the colder months, a period known as seasonal resource scarcity. Once denned, these bears sustain themselves entirely on fat accumulated during autumn, often losing between 15% and 40% of their body weight over the winter.
The physiological control during this period is exceptional, as the bears do not eat, drink, urinate, or defecate for up to seven months. They have evolved specialized biochemical pathways that recycle metabolic waste products, such as urea, converting them into amino acids used to build new proteins. This process allows them to maintain muscle mass and bone density, avoiding the severe atrophy that would occur in other mammals after prolonged inactivity.
Pregnant female bears are an exception, modifying their dormancy to ensure the survival of their unborn and newborn cubs. Throughout gestation and birth, which occurs mid-winter, a pregnant female maintains a higher and more stable body temperature, closer to 37 to 38 degrees Celsius, than non-pregnant bears. This increased thermoregulation requires a greater expenditure of energy but is necessary for the proper development of the helpless cubs and for the mother to produce the rich milk required for lactation in the den.
Bears That Do Not Participate in Winter Rest
Not all bear species are subject to the environmental pressures that necessitate a long period of winter dormancy. The primary factor influencing year-round activity is the constant availability of food, which is why tropical bears generally remain active. Species such as the Sun Bear (Helarctos malayanus), the Sloth Bear (Melursus ursinus), and the Spectacled Bear (Tremarctos ornatus) inhabit regions where seasonal changes are not severe enough to cause prolonged food scarcity.
In these equatorial and subtropical climates, food sources like fruits, insects, and vegetation are available throughout the year, removing the pressure for these bears to develop a multi-month dormant state. They may exhibit short periods of lethargy or rest during harsh weather or dry seasons, but this is not the prolonged physiological shutdown seen in their temperate cousins. Their continuous activity reflects an adaptation to a steady, non-seasonal food supply.
The Polar Bear (Ursus maritimus) represents an opposite case of year-round activity in an extreme environment. For non-pregnant males and females, winter is the peak hunting season, as the extensive sea ice provides the ideal platform for hunting their primary prey: ringed and bearded seals. Therefore, the Polar Bear remains actively hunting and roaming the ice floes throughout the winter to build the fat reserves necessary for survival; only pregnant females enter a den to give birth to their cubs in a sheltered, insulated environment.