Polar Bear

Polar Bear

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Description

Polar bears have longer necks, and smaller heads and ears, compared to other species of bears.  The body of a polar bear is large and stocky, similar to that of a brown bear, except it lacks the shoulder hump.  Polar bear skin is black and the fur is actually clear, lacking in pigment.  The white appearance is the result of light being refracted from the clear hair strands.  Their coat is made up of a thick outer layer of fur on top of a dense undercoat.  The forepaws are broad and webbed to assist in swimming and walking on thin ice.  The soles of their feet are furred for insulation and traction while walking on ice and snow.

Lifestyle

Polar bears are usually solitary, with the exceptions of mothers with cubs or groups of polar bears feeding on whale carcasses.  Males are females are also found together when paired during mating season.  Bears may also come into competition with one another when a seal kill attracts other bears looking to scavenge.  In instances where bears encounter each other, the smaller bear will tend to run away.  A female with cubs, however, will charge males that are much larger to protect her young or a kill that they are feeding on.  Polar bears conserve their energy as much as possible, either sleeping, lying, or waiting (still hunting).  The rest of their time is spent walking, swimming as they travel across sea ice, stalking prey, or feeding.  Humans and other polar bears are the only predators of the polar bear.  Male polar bears may prey on cubs if they come into contact.  Females with cubs tend to avoid other bears for this reason.  Polar bears do not hibernate like other bears and do not den in winter unless they are pregnant females.   A pregnant female will find a den or excavate her own to give birth to her cubs, going in the den in November or December and emerging with cubs for the first time in March or April.  Polar bears will travel as much as 600 miles north and south, as the ice melts and freezes.  During summer, bears may remain on islands or coastlines with land fast ice, drift on ice flows, or get stranded on land where they are forced to endure warm weather.

Food Habits

Polar bears are carnivores.  In summer, they may consume vegetation out of necessity but gain little nutrition from it.  Their primary prey are ringed seals.  Bears often leave a kill after consuming only the blubber because the high caloric value of blubber relative to meat is important to bears for maintaining an insulating fat layer and storing energy for times when food is scarce.  Polar bears do not store or cache unconsumed meat as other bears do.  Polar bears have two hunting strategies.  Still-hunting is used most often, involving finding a seal's breathing hole in the ice and waiting for the seal to surface to make the kill.  When a bear sees a seal basking out of the water, it will use a stalking technique to get close and then make an attempt to catch it.  One stalking technique is crouching and staying out of sight while creeping up on the seal.  Another technique is to swim through any channels or cracks in the ice until it is close enough to catch the seal. Using this technique, a bear may actually dive under the ice and surface through the breathing hole in order the surprise the seal and eliminate its escape route.  Feeding usually occurs immediately after the kill has been dragged away from the water.  Polar bears consume the skin and blubber first and the rest is often abandoned.  Other polar bears or arctic foxes then scavenge these leftovers.

Lifecycle

Females are sexually mature when they are three to six years old, males reach sexual maturity at four to five years.  The breeding season for polar bears is March through May.  They have delayed implantation, as do many mammal species, which allows females to expel fertilized eggs if their body condition is too poor to support themselves and fetal growth.  If body condition is good, a female will become pregnant in fall.   Cubs are born in November or December in snow caves called maternity dens.  After feeding heavily in April or May, females that have mated dig a den in late October or early November. Most choose den sites in snowdrifts along mountain slopes or hills near the shore. Some dig their dens in snowdrifts on the sea ice.  Twins are common, although single births of one cub are becoming more common as females face increased stress due to loss of sea ice habitat and valuable hunting time. 

When cubs are born, they are 12 to 14 inches long and weigh a little more than one pound.  Their eyes are closed, they have no teeth, and are covered in very short and soft fur.  They depend on their mother completely for food and warmth.  While in the den, a female does not eat or drink.  Cubs nurse on their mother's milk, which is over 30% fat, and grow very quickly.  By March or April, females are ready to begin coming out of the den with their cubs for very short periods.  The do not stay out for long or travel far at this time, due to the cubs' lack of mobility.  They gradually range farther from the den site with their mother, who is eager to get back on sea ice to hunt seals before summer ice melt.   Females usually give birth for the first time after age six, and cubs stay with them for up to three years while they learn their hunting and survival skills from their mothers. This gives polar bears one of the slowest reproductive rates of any mammal, typically producing only five litters in their lifetime.

Population Status, Threats & Conservation

In 1972, the United States Marine Mammal Protection Act prohibited all hunting, except for subsistence, of polar bears in the U.S.  In 1973,  the United States, Russia, Norway, Canada, and Denmark came to an agreement to protect polar bear habitat, limit hunting, and cooperate on research.  Polar bear populations are currently threatened by climate change, which continues to decrease the extent of their habitat (pack ice) and their prey base.  In 2005, the IUCN Polar Bear Specialist Group (PBSG) classified polar bears as vulnerable on the IUCN World Conservation Union's Red List of Threatened Species noting that extinction could occur due to sea ice changes.  In 2008, the U.S. Fish and Wildlife Service listed polar bears as threatened under the Endangered Species Act.  At the 2009 meeting of the PBSG, the world's leading polar bear scientists reported that of the 19 subpopulations of polar bears, eight were declining, three were stable, and one was increasing. They lacked sufficient data about the remaining seven. 

Scientists predict that two-thirds of all polar bears in the wild will be gone by mid-century if we do nothing to slow the pace of climate change.  Other threats beyond rapid sea ice loss include pollution, poaching, and impacts of more open traveling and use by industry in their habitat.  Humans need to act now to slow the pace of climate change by reducing our carbon footprint and our overall emission of carbon back into the atmosphere.

Take action to save polar bears now!  Start by clicking here to view Polar Bears International's page on how we can all act now to save this amazing species and it's habitat.  Let's keep wild polar bears and sea ice on this earth for future generations to enjoy!

Sources:
Polar Bears International, Polar Bear FAQ's
USGS polar bear home page
Gunderson, A. 2009. "Ursus maritimus" (On-line), Animal Diversity Web