While the image of a bear hibernating through winter is common, female polar bears do not undergo true hibernation. Instead, they engage in a specialized biological process known as maternity denning. This distinct behavior is essential for the survival of their offspring in the harsh Arctic environment.
The Nuance of “Hibernation” in Polar Bears
True hibernation involves a deep state of dormancy characterized by significant reductions in body temperature, heart rate, and metabolic activity. Animals like ground squirrels enter a profound hypothermia, with body temperatures dropping close to 0°C (32°F) and metabolic rates decreasing dramatically. Polar bears, however, do not exhibit such extreme physiological changes. Unlike brown and black bears, which hibernate to conserve energy when food is scarce, polar bears can hunt seals on sea ice throughout the year.
Pregnant female polar bears enter a state of reduced activity within their dens, but their body temperature remains relatively high, typically near normal, around 35-37°C (95-98.6°F). This allows them to remain alert enough to care for their cubs, a marked difference from the near-freezing body temperatures seen in true hibernators. Non-pregnant polar bears, including males, typically remain active on the sea ice throughout the winter, continuing to hunt and forage.
The Maternity Denning Process
Pregnant female polar bears typically begin seeking out and preparing their den sites in late autumn. These dens are most often excavated in deep snowdrifts on land, or may utilize dens dug into permafrost banks; previously used dens can also be re-occupied. The female constructs a small snow cave, just large enough for her to turn around comfortably, and falling snow often conceals the entrance.
Inside this insulated chamber, cubs are born in mid-winter. The mother and her cubs remain within the den for several months, isolated from the extreme Arctic conditions outside.
Physiological Adaptations During Denning
During maternity denning, female polar bears undergo specific physiological adjustments. Their metabolic rate significantly reduces, and their heart rate slows considerably, dropping from a normal resting rate of about 46 beats per minute to around 27 beats per minute. This reduction in activity allows them to conserve energy, relying entirely on substantial fat reserves accumulated during productive hunting seasons.
Despite these changes, the mother’s body temperature stays close to normal levels. This sustained warmth is essential for the demands of pregnancy, birth, and nursing her cubs. Unlike true hibernators, which experience profound hypothermia, the polar bear’s ability to maintain a relatively high core temperature is crucial for the care of her vulnerable offspring. Moreover, denning females can suppress urination and defecation for the entire denning period, which can last up to eight months. During this time, mothers can lose nearly half their body weight while providing rich milk for their growing cubs.
Significance for Cub Survival
Maternity denning is a survival strategy for polar bear cubs, which are born in a highly undeveloped state, blind, nearly hairless, and weighing only about 0.5 to 0.9 kilograms (1 to 2 pounds). The den provides a sheltered and insulated environment, offering protection from the extreme Arctic cold and potential predators. The temperature inside the den can be as much as 25°C (45°F) warmer than the outside air.
This period spent within the den allows the cubs to grow and develop. They rapidly gain weight, reaching approximately 10 to 15 kilograms (22 to 33 pounds) by the time they emerge in the spring, largely due to their mother’s milk, which is rich with about 31% fat. They also grow a thicker coat of fur and build sufficient strength. This essential early development and acclimatization within the den significantly increases their chances of survival when they eventually venture out into the harsh Arctic landscape. Cub survival rates to adulthood are estimated to be around 50%.