Posted by Janet Harriett on Jan.06, 2010

Photo Credit: Ellizabeth Labunski/ USFWS

Polar bears are the largest carnivorous land animal, though they spend so much time out at sea that they are sometimes classified as a marine animal. In fact, their Latin name, Ursus maritimus, means “maritime bear” or “sea bear.” In other languages, polar bears are called “white bear” or “ice bear.” They range throughout the Arctic, half a world away from the penguins with which they are often grouped on greeting cards, generally keeping to the edges of pack ice where they hunt.

Diet: Polar Bears on Atkins

Polar bears’ primary food is the ringed seal, the most numerous seal in the arctic. The bears preferentially pick off the high-calorie seal blubber when hunting is good, leaving the meat for polar scavengers, notably the arctic fox. When seals are less plentiful, bears eat both the blubber and the meat. Seal blubber contains high levels of omega-3 fatty acids; polar bear researchers found that the cholesterol level of bears eating seal fat was lower than for fasting bears.

In addition to seals, polar bears eat beluga whales, walruses and the occasional reindeer. They scavenge beached carcasses of other types of whales and marine animals, but don’t hunt much on land, preferring to ambush their prey by waiting near an air hole in the ice. While they eat kelp and lichen, vegetables aren’t a large part of their diet.

The high levels of seal fat in a polar bear’s diet has also contributed to the observed decline in polar bear health. Fat accumulates environmental toxins throughout the food web, and as the top predator in the arctic, polar bears consume the toxins accumulated in the fats of its prey, and the prey’s prey, down the line.

Polar Bear Family Life

Polar bears have one of the largest size differences between males and females of any animal. Male polar bears get to be 8-10 feet long and weigh 770-1,500 pounds each. Females only get to be about 6-8 feet long and weigh less than half of the males, at 330-500 pounds. When she’s pregnant, a female polar bear gains weight to reach about 1100 pounds, since she retreats to a maternity den and fasts for 4-8 months while gestating, giving birth and nursing her cubs until springtime. Though holed up in a den, the mother polar bear isn’t hibernating.

Female polar bears typically give birth to two cubs per litter, who may have been fathered by different males. Female polar bears “hold on” to their fertilized ova in a process called delayed implantation. Mating season occurs in spring, but the fertilized ova don’t implant until closer to the time that the female digs her maternity den, after she has gained the weight to sustain her through a pregnancy and the cubs’ early months.

In the southern part of polar bears’ range, cubs stay with their mother for two years after they emerge from the maternity den, while more northern-ranging bears tend to their cubs for three years. Though the mother teaches the cubs to hunt during the 2-3 year period, the cubs aren’t fully weaned until they leave her care. Males don’t play a role in caring for the cubs.

A female doesn’t have another litter until after the cubs leave. A female polar bear averages about five 2-cub litters during her life. Typically, fewer than half of the cubs survive to reproductive age themselves.

Polar Bear Populations

Technical challenges prevent an accurate census of polar bears, but estimates put the worldwide population at 20,000 - 25,000 polar bears. These are grouped into 19 subpopulations, but unlike penguins and many other animals, polar bears don’t have any subspecies. The 19 subpopulations consist of groups of bears who, though solitary, have overlapping ranges, and there is some genetic mixing among the subpopulations. Bears keep to themselves but are not territorial and will share scavenged carcasses with other polar bears. Of the 19 subpopulations, five are declining and seven don’t have enough information to know whether their population is growing or declining.

Stay tuned later in January for winterizing lessons from polar bears and polar bears’ role as a signal species for environmental health.

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Posted under Living, Nature and Environment.