Polar bears are often thought to hibernate during the harsh Arctic winter, similar to other bear species. However, most polar bears remain active, hunting seals on the sea ice. Only pregnant female polar bears enter a specialized winter den to give birth and nurse their cubs in a sheltered environment. This denning period is important for the survival of their offspring.
Polar Bear Denning: A Unique Adaptation
The denning behavior of pregnant polar bears differs significantly from the true hibernation seen in many other bear species. While black and brown bears undergo a deep, prolonged dormancy with a substantial drop in body temperature and metabolic rate, pregnant polar bears experience a less extreme physiological change. Their body temperature and heart rate do not decrease as dramatically, which is essential to keep their newborn cubs warm. Females prepare for denning by accumulating substantial fat reserves, sometimes gaining over 200 kilograms, which they rely on for sustenance. This process involves delayed implantation, where a fertilized egg does not immediately implant in the uterus but remains dormant until the female has sufficient fat stores to support pregnancy.
Prime Locations for Maternity Dens
Pregnant polar bears select sites for their maternity dens, seeking insulation and protection from the severe Arctic weather. Most dens are dug into snowdrifts that accumulate on the leeward side of hills, valleys, or coastal bluffs. While the majority of denning occurs on land, typically within 16 kilometers of the coast, some females in northern populations with year-round sea ice may construct dens directly on the ice or peat banks.
Specific regions have high concentrations of maternity dens. These include Wapusk National Park on the Hudson Bay Plain in Manitoba, Canada, and parts of Alaska, particularly the Beaufort and Chukchi Seas. Svalbard is another important denning area. These locations provide deep snow cover or stable ground that can be excavated, offering a secure chamber for the mother and cubs.
Life Within the Den
Pregnant polar bears enter their dens in late fall, between October and November. Cubs are born in mid-winter, in December or January, inside the insulated den. At birth, cubs are remarkably small, weighing 0.5 to 1 kilogram, and are blind, toothless, and covered with a thin layer of fur.
The mother does not eat, drink, or defecate while in the den, relying on her fat reserves to nourish herself and her cubs. The cubs grow rapidly on their mother’s rich milk, which contains over 30% fat. This high-fat diet allows them to increase their birth weight by up to 25 times in just two months. The den itself provides a warm environment, with temperatures inside reaching 25°C (45°F) warmer than the outside air due to the mother’s body heat and the snow’s insulating properties.
Life After the Den
The polar bear family emerges from the maternity den in late March or April, when the cubs are stronger and better equipped for the Arctic environment. Upon emergence, the cubs, now weighing between 10 to 15 kilograms, are still somewhat wobbly. For the first few days to weeks, the mother and cubs remain near the den site, allowing the cubs to acclimate to the cold air and strengthen their muscles through play and exploration.
This initial period outside the den is important for cub development and survival. After this acclimatization phase, the family travels to the sea ice. There, the mother, who has lost much of her body weight during denning, can begin hunting seals to replenish her fat reserves and continue nursing her cubs.