The polar bear, Ursus maritimus, is the apex predator of the Arctic, an environment defined by extreme cold and shifting sea ice. Survival requires specialized biological mechanisms and behaviors unlike those of any other bear species. This combination of physical traits and survival strategies allows the polar bear to thrive where other large mammals would perish.
Physical and Physiological Adaptations to Extreme Cold
The polar bear maintains its internal temperature through a multi-layered insulation system designed to prevent heat loss. A thick layer of subcutaneous fat, or blubber, can measure up to 10 centimeters (four inches) deep, providing insulation and an energy reserve. This fat layer is complemented by a dense double-layered coat of fur, consisting of a woolly undercoat and long guard hairs that cover the entire body, except for the nose tip. The outer guard hairs are hollow and water-repellent, helping the bear shed moisture quickly after swimming and preventing chilling.
The bear’s immense body size, reaching nearly 700 kilograms in large males, contributes to heat retention, following Bergmann’s rule (larger bodies have a lower surface-area-to-volume ratio). Small, rounded ears and a short tail further limit heat loss by reducing exposed surface area. Specialized paws are large, measuring up to 30 centimeters (12 inches) across, which helps distribute the bear’s weight across thin ice. The undersides of the paws feature small, soft bumps called papillae, which provide traction on slick ice, working with the fur between the toes for warmth and grip.
Specialized Hunting and Dietary Strategies
The polar bear’s existence is linked to the sea ice, which serves as a mobile platform for hunting their primary prey: ringed and bearded seals. These marine mammals constitute over 95% of the bear’s diet, providing the high-fat content necessary to sustain its body and blubber stores. The high-calorie blubber is the preferred part of the seal, and a single successful hunt can provide energy to last for many days.
The most common hunting method is still-hunting, where a bear uses its acute sense of smell to locate a seal’s breathing hole, or aglu, in the ice. The bear waits patiently until a seal surfaces for air, then pounces, pulling the seal onto the ice. Another technique involves stalking seals basking on the ice, approaching unseen during the seal’s brief sleep-wake cycles. When the sea ice melts during summer, polar bears are forced onto land, entering seasonal fasting. While they may consume alternative terrestrial foods like bird eggs, vegetation, or carrion, these sources lack the fat content needed to sustain the bear’s long-term energy needs.
Life Cycle and Maternal Denning Behavior
The reproductive cycle involves specialized denning behavior that protects newborn cubs. Mating occurs on the sea ice between April and June, but the fertilized egg undergoes delayed implantation, embedding only in the fall if the female has sufficient fat reserves. Pregnant females seek maternity den sites, often excavating a chamber in a snowdrift on land or stable ice between October and November.
Cubs, typically one to three (twins are common), are born blind and helpless in December or January, weighing about half a kilogram (one pound). The mother enters a state similar to walking hibernation, fasting completely for the entire denning period, which can last for several months. She relies entirely on her fat reserves to produce rich milk (approximately 31 to 36% fat), allowing the cubs to grow rapidly. When the family emerges in March or April, the mother is thin, having lost nearly 44% of her pre-denning body mass. The cubs remain with their mother for approximately two to two-and-a-half years, learning the hunting and survival skills necessary to navigate the perilous Arctic environment.
Current Conservation Status and Habitat Reliance
The polar bear is classified as Vulnerable on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. This status reflects the primary threat: the loss of its sea ice habitat caused by global climate change. The sea ice is the fundamental platform upon which the bear’s survival strategy depends.
The bear relies on persistent sea ice to effectively hunt seals, build up necessary fat reserves, and travel between feeding and denning areas. Declines in the duration and extent of sea ice force bears to fast for longer periods or attempt to find less nutritious food sources on land. This environmental stress impacts their body condition, reproductive success, and the survival rate of cubs. The continued availability of stable sea ice is the most important factor for the long-term persistence of the species.