Polar bears, iconic inhabitants of the Arctic, are formidable predators adapted to some of the planet’s harshest conditions. Despite their large adult size, their offspring begin life as small and vulnerable creatures. These tiny cubs are born in secluded snow dens, starting a journey of rapid growth and development in an extreme environment.
Weight at Birth
Newborn polar bear cubs are remarkably small and helpless, typically weighing between 450 to 700 grams (approximately 1 to 1.5 pounds) and measuring 30 to 35 centimeters (12 to 14 inches) in length. At birth, they are blind, toothless, and covered in fine, almost hairless white fur. Cubs are born in maternity dens, dug into snowdrifts, during winter months, most commonly in December. This protective den provides an insulated environment, shielding them from frigid Arctic temperatures.
Rapid Early Growth
Inside the den, the cubs experience an extraordinary period of rapid growth, fueled by their mother’s exceptionally rich milk. Polar bear milk is high in fat (31% to 33%), significantly higher than most other mammals. This nutrient-dense milk allows the cubs to gain weight quickly.
Within their first month, their eyes open. By two months, their teeth erupt, fur thickens, and they begin to walk within the den. When the family emerges from the den in March or April, cubs weigh 10 to 15 kilograms (22 to 33 pounds), an increase of up to 20 times their birth weight. During this denning period, the mother does not eat or drink, relying on stored fat reserves to sustain herself and nurse her litter.
Factors Affecting Cub Weight
Several elements influence both the initial weight of polar bear cubs and their subsequent growth trajectory. The mother’s health and nutritional status before and during denning play a significant role, directly impacting milk quality and quantity. Pregnant females must accumulate substantial fat reserves (often over 200 kilograms or 441 pounds) to sustain themselves and their offspring throughout the denning period. Insufficient maternal fat stores can compromise cub survival.
The size of the litter also affects individual cub weight and growth. While mothers typically give birth to twins, litters range from one to three cubs (four are rare). Single cubs tend to be larger, weighing 25% to 35% more than cubs from twin litters, as resources are distributed among fewer individuals. Environmental conditions within the den, such as insulation, also contribute to cub well-being and growth by providing a stable, warm microclimate.
Life Beyond the Den and Continued Growth
Around March or April, the polar bear family emerges from the maternity den, though they typically remain nearby for one to four weeks (averaging 12 days). This period allows cubs to acclimate to colder outside temperatures and strengthen their muscles. After this, the mother leads her cubs to the sea ice, their primary hunting ground.
Cubs continue to rely on milk but begin consuming solid food, primarily seal blubber, as early as three to four months, learning hunting techniques by observing their mother. Growth remains rapid; by eight months, they can weigh over 45 kilograms (99 pounds). Cubs generally stay with their mother for two to three years, acquiring essential survival skills like hunting and navigating the Arctic, before becoming self-sufficient.