Reproduction, Denning, and Cub Rearing

Key Insight

Female polar bears undertake an intensive reproductive cycle centered around denning, typically from late October or early November. Prior to this, a heavy layer of fat is critical to sustain herself and her cubs through the winter, with denning potentially skipped if food is scarce. Conception occurs in April or May, but the fertilized eggs undergo delayed implantation, attaching to the uterine wall only when the female commits to the extended denning period, which is exclusively undertaken by females carrying fertilized eggs.

Den selection is meticulous, favoring sites with early autumn snowdrifts, often on the leeward side of a ridge to prevent exposure or burial by avalanches. Dens exhibit a consistent architecture: an upward-sloping entrance tunnel 5 to 10 feet long and 24 to 28 inches wide and high, leading to a small inner room, and a ventilation hole. Through precise control of snow thickness and radiating approximately 200 watts of heat, the female maintains the den at about 32°F, ensuring fresh air circulation while trapping warmth. She is not truly hibernating; her heart rate and respiration are reduced, but her temperature falls only slightly, allowing her to awaken quickly to adjust den conditions like scraping ice or digging new chambers. Older bears, learning from past mistakes, often construct more economical and successful dens.

Cubs, usually two (sometimes one or three, rarely four), are born blind, deaf, poorly insulated, and unable to walk or smell in December or early January, weighing barely a pound. Their survival depends entirely on the den's protection, the mother's warmth, and her exceptionally rich milk, which has the consistency of cream and is richer than whale milk. Cubs develop rapidly: they can hear at about 24 days, see a week later, and walk and smell several more weeks after that. By late March or early April, weighing approximately 25 pounds, they emerge from the den, initially staying close to a sheltered 'sun porch' for nursing and vigorous exercise. During these first critical weeks, the mother balances her need to hunt with investing in the cubs' learning and development of strength, coordination, social habits, and fighting techniques, before the family unit disperses at 24 to 28 months, a transitional stage with the highest mortality rate for young bears.