Polar Bear Reproduction

Female polar bears usually breed for the first time at five or six years of age. Mating takes place out on the pack ice during a rather long breeding season, extending from late March to mid-July. An eager throng of fiercely competitive male suitors often follows a female during her time of estrus, which lasts for about three weeks. After fertilization, the tiny embryo will divide several times and then free float in the uterus, not to resume development until September, when it finally implants in the uterine wall.

Just prior to denning, pregnant females become particularly fat. They enter their over-wintering maternity dens in October or November, and later give birth to between one and four cubs (twins are most common) sometime during December or January. The igloo-like den is an important barrier against the Arctic cold, as at birth, the cubs are blind and helpless and are not much bigger than guinea pigs, weighing only about 20 ounces each. The sex ratio of newborn cubs appears to be about equal. They remain in the den until late March or April, growing quickly on their mother’s milk, which is very rich in fat.

By the time increasing daylight encourages the family to surface from the den, the cubs may weigh between 22 and 33 pounds. In only two months, the cubs have gained 25 times their birth weight. Initially, for up to two weeks, the cubs stay at the den entrance, playing and acclimating to the outside. At the slightest sign of danger, the entire family withdraws to the den. When they finally do leave, the cubs are large enough to travel with their mother across the frozen tundra and out onto the sea ice. Having lived only on her fat reserves, the mother is now extremely thin and hungry for seals. After a kill, she consumes the energy-rich blubber and skin first.

Female polar bears have four functional mammary glands, and the cubs continue to nurse, suckling for about 15 minutes, six or seven times a day. Before taking a nursing position, the mother usually digs a shallow pit in which she lies down. She often nurses while lying on her back, with the cubs on her abdomen, but will sometimes sit upright and lean forward.

A polar bear family waiting out the fall by the Hudson Bay coast in Churchill Wildlife Management Area in Churchill, Manitoba.

Polar bear cubs with mother by the coast of Hudson Bay. Kurt Johnson photo.

Polar bear cubs begin receiving hunting lessons almost immediately. Although they are still nursing, cubs can participate in eating seal meat on their mother’s first hunt out of the den. At this stage, they follow their mother everywhere, even riding piggyback when she goes swimming. By August, the cubs weigh over 100 pounds, but they are still dependent on their mother; they will continue to den with her for one or two more winters. When the family finally does break up, the cubs are usually between 24 and 28 months old. However, some families may remain denning together for a third or fourth winter.

When abandoned by their mother, the half-grown cubs wander about on their own. They may stay together for a short time, but eventually each goes on its own way. Now alone, the young bears face a high mortality rate during the next year or two; however, in the food-rich Hudson Bay area, cubs weaned at the age of two have a good chance of survival. Surviving females reach adult weight by their fifth year and males between ages eight and ten.

The usual time span between litters is three to four years, but in the ideal conditions found on Hudson Bay’s west coast, about 40 percent of the bears breed every second year. Recent studies indicate that some mothers, living under the difficult conditions found in other Arctic areas, may produce only one or two litters in their lifetime.

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