Polar bears, the world’s largest land carnivores, are one of the slowest reproducing mammals on earth. Their reproductive cycle is lengthy, complex and susceptible to human activity.
Conception Occurs in Spring
Female polar bears reproduce at most, on a three year cycle. They conceive in April or May during the peak of their estrus. However the fertilized egg is not implanted until fall. US Fish and Wildlife biologists estimate pregnant females need to gain approximately 200 kilograms of fat to sustain a successful pregnancy, so summer hunting is critical to the survival of both the female, who will not eat during 5 months of winter confinement, and her vulnerable cubs.
Females Return to Preferred Den Sites in Late Fall, Early Winter
By late October or early November pregnant polar bears leave the sea ice to construct maternity dens. The majority of the dens observed in Alaska were on coastal banks, though a small percentage were found as much as ten kilometers inland, and others were found on the sea ice. Polar bear researchers G.M. Durner, and Steve Amstrup of the U.S. Geological Survey state that polar bears have a strong fidelity to denning sites, and could be easily disturbed by human activity along the Arctic coast.
Den Structure
Though den structures vary widely from region to region, some consistent features are:
- Similarity of the snow--neither hard packed or too soft
- Built with an upward sloping entry tunnel
- Having a small oval main chamber where birth occurs
- Having adequate ventilation for thermoregulation and gas exchanges
Cubs Born Mid-Winter
While the mother bear conserves her energy in a state of semi-hibernation, she gives birth to two cubs sometime in December or January. At birth the cubs weigh about half a kilogram each, and are blind, deaf, and nearly hairless. Over the next ninety days or so the cubs nurse and nestle into the warm folds of their mother’s body while developing rapidly.
Leaving the Den
By March or April, depending on the weather, the female bear breaks out of the den, and the cubs emerge a few days later. The family stays on shore while the cubs develop their strength and coordination. During this time, the mother bear feeds on vegetation and small mammals such as lemmings and ground squirrels. After several weeks, the female takes her cubs onto the ice. The cubs will remain with their mother and not be fully weaned until they are approximately 2.5 years of age.
Considerations for Oil Drilling and Other Arctic Development
In a report on the Arctic National Wildlife Refuge (ANWR) presented to the US Congress in 1987, US Fish and Wildlife Service researchers concluded that maternal den habitat is essential to polar bear survival and should be considered when developing a management plan for Arctic resources. G. M. Durner, in a 2002 paper titlled Habitat Characteristics of Polar Bears and Terrestrial Maternal Den Sites in Northern Alaska states that one purpose of on-going polar bear research is to provide data on denning habitats to decision making agencies.
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