The polar bear, Ursus maritimus, is uniquely adapted to the extreme environment of the Arctic and is scientifically classified as a marine mammal due to its dependence on the ocean. This species is found throughout the circumpolar Arctic region, spending most of its life on the sea ice that serves as its primary habitat. The survival of the polar bear is directly linked to this dynamic ice platform, which it uses to hunt its main prey, ringed and bearded seals. The life cycle of this large carnivore is a long, highly specialized trajectory, governed by the seasonal movements and availability of the sea ice.
The Denning Period
The reproductive cycle begins with the pregnant female preparing for the denning period in late autumn, typically entering her maternity den between October and November. She must engage in intensive feeding, known as hyperphagia, to accumulate the fat reserves necessary to sustain her through the next several months. This stored energy must cover her own metabolic needs and the demands of gestation and lactation.
The female excavates a den, usually in a snowdrift on land or stable ice, creating a sheltered space that remains warmer than the outside air. She enters a state of deep rest, but not true hibernation, where her body temperature remains relatively stable, though her metabolic rate drops significantly. The cubs are born in mid-winter, most often in December, and twins are the most common litter size, though one to three cubs are possible.
Newborn cubs weigh little more than a pound, and are blind, deaf, and covered only in thin fur. The mother must fast completely during the denning period, which can last for up to eight months, relying entirely on her fat reserves to produce exceptionally rich milk. The cubs grow rapidly on this high-energy diet, developing the necessary size and insulation to survive the harsh Arctic conditions. The family emerges from the maternity den in late March or early April, with the cubs weighing around 20 to 30 pounds and strong enough to travel.
Raising and Training the Cubs
Upon leaving the den, the mother immediately begins leading her cubs to the sea ice, where she can resume hunting to replenish her fat stores. The first few weeks are spent near the den site, allowing the cubs to acclimate to the cold and strengthen their muscles before undertaking the long trek. The cubs remain dependent on their mother for food, protection, and learning for an extended period.
This long juvenile phase, which typically lasts between two and two-and-a-half years, is an apprenticeship for survival in the Arctic. The mother teaches her young the specialized hunting techniques for seals, such as ambushing them at breathing holes in the ice or stalking them in their snow lairs. Cubs learn how to navigate the complex, ever-shifting landscape of the sea ice and how to swim long distances in frigid water.
The mother protects the cubs from danger, particularly from adult male polar bears, who pose a threat of infanticide. By the age of independence, the young bear has acquired the skills and physical size needed to survive on its own. The mother initiates dispersal by simply leaving her offspring or actively chasing them away, allowing her to re-enter the reproductive cycle.
Adulthood and Reproductive Cycles
The bear enters the sub-adult phase, a challenging period where hunting success is inconsistent, before reaching full maturity. Female polar bears reach sexual maturity between four and six years of age, though they may not produce their first litter until five to seven years old. Males mature later, becoming active breeders between six and ten years of age, when they are large enough to compete for mating rights.
The mating season occurs on the sea ice during the spring, primarily between March and June. After a successful mating, the female exhibits a biological mechanism called delayed implantation, where the fertilized egg does not immediately begin development. The egg only implants in the uterus in the autumn, around September or October, and only if the female has acquired sufficient fat reserves to sustain herself and the developing cubs through the denning period.
This adaptation ensures that the birth of the cubs is timed perfectly for the relative safety of the snow den during mid-winter. The female’s high investment in raising her young, which requires two to three years of care, means that she can only reproduce about once every three years. This slow reproductive rate makes the species particularly vulnerable to population declines.
Lifespan and Mortality
In the wild, the average lifespan of a polar bear is between 15 and 18 years, though some individuals survive into their late 20s or early 30s. Mortality rates are highest for young cubs and newly independent sub-adults, who are still mastering hunting skills and face intense competition from larger, more experienced bears. Starvation is a frequent cause of death, especially for unsuccessful hunters or for older bears whose worn teeth make it difficult to consume prey.
Other natural causes of death include intraspecific aggression, primarily infanticide by adult males, and competition between males during the mating season. However, the most significant factor affecting survival is the loss of their sea ice habitat. Declining sea ice limits the bear’s access to its seal prey, forcing longer fasting periods and increasing the risk of malnutrition and starvation across all age groups.