The polar bear, Ursus maritimus, is classified as a marine mammal due to its reliance on Arctic sea ice. The frozen ocean serves as its primary hunting platform and travel route. The bear’s life cycle is dictated by the seasonal formation and melting of the sea ice. Successful passage through these stages requires specific adaptations and learned behaviors.
Life in the Maternity Den
The cycle of reproduction begins with delayed implantation. After mating on the sea ice, the fertilized egg does not immediately implant in the female’s uterine wall. Implantation is postponed until the female enters her maternity den, provided she has accumulated sufficient fat reserves. This strategy ensures that birth is synchronized with the most advantageous time for cub survival.
Pregnant females seek denning sites, often in snow drifts or large ice floes, around mid-to-late October. They fast for up to eight months, relying entirely on stored body fat to survive gestation and lactation. The cubs, usually twins, are born in the den between November and January.
Newborn cubs are small and vulnerable, weighing less than one kilogram. They are blind, toothless, and dependent on their mother for warmth and nourishment. The mother’s milk is rich, containing about 31% fat, which fuels rapid growth. The family remains inside the den until March or April, when the cubs are strong enough to face the Arctic environment.
Years of Dependency
Following den emergence, the mother leads her cubs to the sea ice after a few weeks of acclimation. This dependency period lasts for two to two and a half years, during which the mother provides protection and guidance.
The mother’s primary role is teaching her young how to hunt seals, the main prey. Cubs observe and mimic her as she employs techniques like stalking seals or waiting at a breathing hole, known as an aglu. They share her kills, learning the location of reliable food sources.
Mortality rates are high during this phase; over 60% of cubs do not survive their first year. Threats include starvation, accidents, and predation from adult male bears. The mother teaches her cubs to recognize and avoid these dangers while navigating the vast, shifting ice landscape.
The young bears are considered subadults once they separate from their mother, usually between two and three years old. Newly independent bears must rely on the skills they have acquired. The mother, now free from lactation, focuses on rebuilding her body reserves and preparing for her next reproductive cycle.
Prime Life and Mating
Independent bears progress toward adulthood, reaching sexual maturity. Females mature around four to six years of age, while males mature around six years. However, males often do not successfully compete for mating opportunities until they are older and larger, usually between eight and ten years old.
Adult polar bears are largely solitary, covering vast territories as they follow the seasonal patterns of the sea ice and their prey. Their specialized hunting targets the energy-rich blubber of seals, providing necessary fat reserves for survival during scarcity. Successful hunting relies on stealth and knowledge of the ice environment.
The mating season takes place on the sea ice between March and June, peaking in April and May. Males track the scent of breeding females, sometimes covering over 100 kilometers. Competition among males often involves aggressive confrontations that can leave them scarred.
Because raising cubs takes over two years, the female polar bear’s reproductive cycle is slow. Females typically only reproduce every three years. This means there are far more adult males seeking a mate than there are breeding females. Once a pair bonds, they may remain together for a week or more, mating multiple times to induce ovulation.
The Final Years
A polar bear’s lifespan ranges from 15 to 18 years, though some survive into their late twenties. Survival in old age is directly related to the bear’s ability to maintain hunting effectiveness.
Physical signs of aging begin to appear around 15 years old, particularly the wearing down or breaking of teeth. Since bears rely on their teeth to grip and consume seals, damaged dentition compromises their ability to hunt. Decreased mobility and chronic injuries further hinder their success on the ice.
Older, weaker bears become susceptible to starvation, a major cause of natural mortality in the Arctic. The physical challenges of securing blubber-rich prey become insurmountable as the bear’s body declines.