Gestation

The total gestation period is about eight months.

Gestation includes a period of delayed implantation.

The fertilized egg divides into a hollow ball of cells one layer thick (a blastocyst), then stops growing and lies free-floating in the uterus for about four months. The blastocyst then implants in the uterine wall and continues to develop. Delayed implantation assures that the cub is born during the best time of the year for survival and allows the female to get into good physical condition and use her energy for nursing her newborn cubs. The actual embryonic development is estimated to be four months.



Denning

Once mated, females begin depositing fat in preparation for cubbing. Females need to gain at least 200 kg (441 lb.) for a successful pregnancy.

Some females may seek out maternity dens as early as late August, but most enter dens in mid to late October. Dens protect newborn cubs from winter's temperature extremes.

Females usually dig dens in snowdrifts on southerly facing slopes. Some dig earthen dens that later become covered by snow.

Most dens are on land, within 16 km (10 mi.) of the coast. In some areas, dens are more than 100 km (62 mi.) from the coast. A few polar bears make dens on the sea ice.

Den elevations range from sea ice level to 548 m (1,800 ft.) above sea level.

Most dens consist of a single chamber slightly elevated from a short entrance tunnel, but they can be complex with several chambers. On average, the chamber is 2 m (6.6 ft.) long, 1.5 m (4.9 ft.) wide, and 1 m (3.3 ft.) high, not much bigger than the bear. Polar bears maintain a ventilation hole through the chamber ceiling to provide fresh air.

Because of the bears' body heat and snow insulation, the den stays warmer than the outside air temperature. Scientists are now using this heat to detect bears with forward-looking infrared technology (FLIR).

Birth Seasons

Polar bear cubs are born November through January in a den. Mother and cubs emerge from their den in late March or April.

Frequency of Birth

Most adult females give birth once every three years. In populations with access to abundant food, birth occurs once every two years.

The most frequent litter size is two, followed by litters of one. Litters of three are less common than twins or singles, and litters of four are rare.

Cubs at Birth

At birth, polar bear cubs weigh about 454 to 680 g (16-24 oz.) and are about 30 cm (12 in.) long. Males are born slightly larger than females.

Polar bear cubs are born small and helpless, with their eyes closed.

The fur is very fine at birth, making the cubs look hairless..

Care of Young

Nursing

Female polar bears have four mammary glands. Mothers nurse their cubs in a sitting position, or lying down on their side or back.

During their first few weeks of life, polar bear cubs nurse most of the time and stay close to their mother to keep warm.

For the next three or four months the cubs nurse as often as six times a day. The length and number of nursing bouts gradually decreases as the cubs grow older.

Mother polar bears nurse their cubs for as long as 30 months. Some cubs stop nursing as young as 18 months of age, but remain with their mothers for survival until they are 30 months old.

The average fat content of polar bear milk is 33%, similar to the milkfat of other marine mammals. For comparison, human milk has a 3-5% fat content.



Mother polar bears are extremely protective of their young, even risking their own lives in their cubs' defense.

Cub Growth and Development