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Polar bear
The polar bear (Ursus maritimus) is a bear native to the Arctic. Polar bears and Kodiak bears are the world's largest land carnivores, with most adult males weighing 300-600 kg (660-1320 lb); adult females are about half the size of males. Its fur is hollow and translucent, but usually appears as white or cream colored, thus providing the animal with effective camouflage. Its skin is actually black in color. Its thick blubber and fur insulate it against the cold. The bear has a short tail and small ears that help reduce heat loss, as well as a relatively small head and long, tapered body to streamline it for swimming.
A semi-aquatic marine mammal, the polar bear has adapted for life on a combination of land, sea, and ice,[2] and is the apex predator within its range. It feeds mainly on seals, young walruses, and whales, although it will eat anything it can kill. It is the bear species most likely to prey on humans.
The polar bear is a vulnerable species at high risk of extinction. Zoologists and climatologists believe that the projected decreases in the polar sea ice due to global warming will reduce their population by two thirds by mid-century.[3][1][4][5] Local long-term studies show that 7 out of 19 subpopulations are declining or already severely reduced.[6][7] In the USA, the Center for Biological Diversity petitioned to up-list the legal conservation status of polar bears to threatened species in 2005.[8] This petition is still under review.
Physiology
Size and weight
Polar bears rank with the Kodiak bear as among the largest living land carnivores, and male polar bears may weigh twice as much as a Siberian tiger. Most adult males weigh 350–650 kg (770–1500+ lb) and measure 2.5–3.0 m (8.2–9.8 ft) in length. Adult females are roughly half the size of males and normally weigh 150–250 kg (330–550 lb), measuring 2–2.5 m (6.6–8.2 ft), but double their weight during pregnancy.[9][10] The great difference in body size makes the polar bear among the most sexually dimorphic of mammals, surpassed only by the eared seals.[11] At birth, cubs weigh only 600–700 g or about a pound and a half. The largest polar bear on record was a huge male, allegedly weighing 1002 kg (2200 lb) shot at Kotzebue Sound in northwestern Alaska in 1960.[12]
Fur and skin
A polar bear's fur provides camouflage and insulation. Although the fur appears white, in fact the individual hairs are translucent, like the water droplets that make up a cloud; the coat may yellow with age. Stiff hairs on the pads of a bear's paws provide insulation and traction on the ice.
Polar bears gradually molt their hair from May to August;[13] however, unlike other Arctic mammals, polar bears do not shed their coat for a darker shade to camouflage themselves in the summer habitat. It was once conjectured that the hollow guard hairs of a polar bear coat acted as fiber-optic tubes to conduct light to its black skin, where it could be absorbed - a theory disproved by recent studies.[14]
Hunting, diet and feeding
The polar bear is the most carnivorous member of the bear family, and the one that is most likely to prey on humans as food. It feeds mainly on seals, especially ringed seals that poke holes in the ice to breathe,[30] but will eat anything it can kill: birds, rodents, shellfish, crabs, beluga whales, young walruses, occasionally muskox or reindeer, and very occasionally other polar bears. Its biology is specialized to digest fat from marine mammals and cannot derive much nutrition from terrestrial food.[31],[32] Most animals can easily outrun a polar bear on the open land or in the open water, and polar bears overheat quickly: thus the polar bear subsists almost entirely on live seals and walrus calves taken at the edge of sea-ice in the winter and spring, or on the carcasses of dead adult walruses or whales. They live off of their fat reserves through the late summer and early fall when the sea-ice is at a minimum.[9] They are enormously powerful predators, but they rarely kill adult walruses, which are twice the polar bear's weight, although such an adult walrus kill has been recorded on tape. [1] Humans are the only predators of polar bears. As a carnivore which feeds largely upon fish-eating carnivores, the polar bear ingests large amounts of vitamin A, which is stored in their livers. The resulting high concentrations make the liver poisonous to humans, causing Hypervitaminosis A.[33] Though mostly carnivorous, they sometimes eat berries, roots, and kelp in the late summer.
Polar bear diving in a zoo.
Polar bears are excellent swimmers and have been seen in open Arctic waters as far as 60 miles (100 km) from land. In some cases they spend half their time on ice floes. Their 12 cm (5 in) layer of fat adds buoyancy in addition to insulating them from the cold. Recently, polar bears in the Arctic have undertaken longer than usual swims to find prey, resulting in four recorded drownings in the unusually large ice pack regression of 2005.[34]
Like other bear species, they have developed a liking for garbage as a result of human encroachment, and frequented the dump in Churchill, Manitoba prior to its closure.[35] Polar bears eat nearly anything they find, including hazardous substances such as styrofoam, plastic, car batteries, ethylene glycol, hydraulic fluid, and motor oil.[36][37] To protect the bears, the Churchill dump was closed in 2006. Garbage is now recycled or transported to Thompson, Manitoba.[38]
Polar bears accumulate high levels of artificial halocarbons such as PCBs and pesticides because of their diet. Their position at the top of the food pyramid tends to concentrate pollutants, particularly halocarbons because of their lipophilicity: halocarbons are soluble in the blubber which makes up the bulk of the polar bear's diet. Halocarbons are known to be toxic to other animals because they mimic hormone chemistry, and biomarkers such as immunoglobulin G and retinol suggest similar effects on polar bears. The overall significance to population health is uncertain because of unique features of polar bear biology such as summertime fasting. PCBs have received the most study, and they have been associated with birth defects and immune system deficiency.[39] Polar bears in Svalbard have the highest concentrations of PCBs, and biologists suggest this may explain the high incidence of hermaphroditic bears in the area.[40]
The relevant chemicals have been classified as persistant organic pollutants by the UN, with the aim of discouraging their production. The most notorious of these, PCBs, DDT and other, have been banned, but their concentrations in polar bear tissues continued to rise for decades as these chemicals spread upwards on the food pyramid. The most recent data now indicates a decreasing trend.[41]
Breeding
Mother with cub at Svalbard
A mother and cubs in Churchill, Manitoba
Polar bears mate in April/May over a one week period needed to induce ovulation. The fertilized egg then remains in a suspended state until August or September. During this time, the females then eat prodigial amounts in preparation for pregnancy, doubling their body weight or more. In October they dig a maternity den in a snow drift and enter a dormant state similar to hibernation. Cubs are born in December without awakening the mother. She remains dormant while nursing her cubs until the family emerges from the den in March. Cubs are weaned at two or three years of age and are separated from their mother. Sexual maturity typically comes at the age of four, but may be delayed by up to two years.[9]
In the 1990's less than 20% cubs in the Western Hudson Bay were weaned at eighteen months, as opposed to 40% of cubs in the early 1980's.[29]
In Alaska, the United States Geological Survey reports that 42 percent of cubs now reach 12 months of age, down from 65 percent 15 years ago.[42] In other words, less than two of every three cubs that survived 15 years ago are now making it past their first year.
The USGS has also published research which purports that the percentage of Alaskan polar bears that den on sea ice has changed from 62% between the years 1985-1994, to 37% over the years 1998-2004. The Alaskan population thus now more resembles the world population, in that it is more likely to den on land.[43] |
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