Polar Bear
Polar Bear
The polar bear (Ursus maritimus) is a bear native to the Arctic Ocean and its surrounding seas. The world's largest predator found on land, an adult male weighs around 300–600 kg (660–1,300 lb), while an adult female is about half that size. Although it is closely related to the brown bear, it has evolved to occupy a narrow ecological niche, with many body characteristics adapted for cold temperatures, for moving across snow, ice, and open water, and for hunting the seals which make up most of its diet.[2] As it can hunt consistently only from sea ice, the polar bear spends much of the year on the frozen sea, although most polar bears are born on land.
The polar bear is classified as a vulnerable species. Of the 19 recognized polar bear subpopulations, 5 are declining, 5 are stable, 2 are increasing, and 7 have insufficient data.[3][4] For decades, unrestricted hunting raised international concern for the future of the species; populations have rebounded after controls and quotas began to take effect. For thousands of years, the polar bear has been a key figure in the material, spiritual, and cultural life of Arctic indigenous peoples, and the hunting of polar bears remains important in their cultures.
The IUCN now lists global warming as the most significant threat to the polar bear, primarily because the melting of its sea ice habitat reduces its ability to find sufficient food. The IUCN states, "If climatic trends continue polar bears may become extirpated from most of their range within 100 years."[1] On May 14, 2008, the United States Department of the Interior listed the polar bear as a threatened species under the Endangered Species Act.
Constantine John Phipps was the first to describe the polar bear as a distinct species.[1] He chose the scientific name Ursus maritimus, the Latin for 'maritime bear',[5] due to the animal's native habitat. The Inuit refer to the animal as nanuq (occasionally rendered as nanuk nanook, or nanuuq in the Inupiat language).[6][7] Likewise it is Nanuuk in Siberian Yupik, and Umka in the Chukchi language. In Russian, the polar bear is commonly called "Белый Медведь" - (Bely Medved = White Bear), though an older still familiar word is Ошкуй - Oshkuy, which comes from the Komi Oski ("Bear").[8] In Quebec, the polar bear is referred to as Ours polaire.[9]
The polar bear was previously considered to be in its own genus, Thalarctos.[10] However, evidence of hybrids between polar bears and brown bears, and of the relatively recent evolutionary divergence of the two species, does not support the establishment of this separate genus, and the accepted scientific name is now therefore Ursus maritimus, as Phipps originally proposed.[1
The bear family, ursidae, is believed to have split off from other carnivorans about 38 million years ago. The ursinae subfamily originated approximately 4.2 million years ago. According to both fossil and DNA evidence, the polar bear diverged from the brown bear, Ursus arctos, roughly 200 thousand years ago. The oldest known polar bear fossil is less than 100 thousand years old. Fossils show that between 10 and 20 thousand years ago, the polar bear's molar teeth changed significantly from those of the brown bear. Polar bears are thought to have diverged from a population of brown bears that became isolated during a period of glaciation in the Pleistocene.[12]
More recent genetic studies have shown that some clades]of brown bear are more closely related to polar bears than to other brown bears,[13] meaning that the polar bear is not a true species according to some species concepts.[14] In addition, polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids,[12][15] indicating that they have only recently diverged and are genetically similar.[16] However, as neither species can survive long in the other's ecological niche, and with distinctly different morphology, metabolism, social and feeding behaviors, and other phenotypic characteristics, the two bears are generally classified as separate species.[16]
When the polar bear was originally documented, two subspecies were identified: Ursus maritimus maritimus by Constantine J. Phipps in 1774, and Ursus maritimus marinus by Peter Simon Pallas in 1776.[17] This distinction has since been invalidated.
One fossil subspecies has been identified. Ursus maritimus tyrannus—descended from Ursus arctos—became extinct during the Pleistocene. U.m. tyrannus was significantly larger than the living subspecies.[12]
The polar bear is found throughout the Arctic Ocean and adjacent seas. Due to the absence of human development in its remote habitat, it retains more of its original range than any other extant large carnivore.[18] While they are rare north of 88°, there is evidence that they range all the way across the Arctic, and as far south as James Bay in Canada. They can occasionally drift widely with the sea ice, and there have been anecdotal sightings as far south as Berlevåg on the Norwegian mainland and the Kuril Islands in the Sea of Okhotsk. It is difficult to estimate a global population of polar bears as much of the range has been poorly studied, but biologists use a working estimate of about 50,000-100,000 polar bears worldwide.
There are 19 generally recognized discrete subpopulations.[4][3] The subpopulations display seasonal fidelity to particular areas, but DNA studies show that they are not reproductively isolated.[19] The thirteen North American subpopulations range from the Beaufort Sea south to Hudson Bay and east to Baffin Bay in western Greenland and account for about 70% of the global population. The Eurasian population is broken up into the East Greenland, Barents Sea, Kara Sea, Laptev Sea, and Chukchi Sea subpopulations, though there is considerable uncertainty about the structure of these populations due to limited mark and recapture data.
The range includes the territory of five nations: Denmark (Greenland), Norway (Svalbard), Russia, US (Alaska) and Canada. These five nations are the signatories of the 1973 International Agreement for the Conservation of Polar Bears which mandates cooperation on research and conservations efforts throughout the polar bear's range.
Modern methods of tracking polar bear populations have been implemented only since the mid-1980s, and are expensive to perform consistently over a large area.[20] The most accurate counts require flying a helicopter in the Arctic climate to find polar bears, shooting a tranquilizer dart at the bear to sedate it, and then tagging the bear.[20] In Nunavut, some Inuit have reported increases in bear sightings around human settlements in recent years, leading to a belief that populations are increasing. Scientists have responded by noting that hungry bears may be congregating around human settlements, leading to the illusion that populations are higher than they actually are.[20] The Polar Bear Specialist Group of the IUCN takes the position that "estimates of subpopulation size or sustainable harvest levels should not be made solely on the basis of traditional ecological knowledge without supporting scientific studies."[21]
Of the 19 recognized polar bear subpopulations, 5 are declining, 5 are stable, 2 are increasing, and 7 have insufficient data.[3][4]
Habitat
The polar bear is often regarded as a marine mammal because it spends many months of the year at sea.[22] Its preferred habitat is the annual sea ice (that is, ice that melts for part of the year) covering the waters over the continental shelf and the Arctic inter-island archipelagos. These areas have relatively high biological productivity in comparison to the deep waters of the high Arctic.[18] The polar bear tends to frequent areas of open water, such as polynyas and leads (temporary stretchs of open water in Arctic ice) to hunt the seals that make up most of its diet.[23] Polar bears are therefore found primarily along the perimeter of the polar ice pack, rather than in the Polar Basin close to the North Pole where the density of seals is low.[24]
Annual ice contains areas of water that appear and disappear throughout the year as the weather changes. Seals migrate in response to these changes, and polar bears must follow their prey.[25] In Hudson Bay, James Bay, and some other areas, the ice melts completely each summer (an event often referred to as "ice-floe breakup"), forcing polar bears to go onto land and wait through the months until the next freeze-up.[25] In the Chukchi and Beaufort seas, polar bears retreat to the multi-year ice (that is, ice that remains solid year-round) further north each summer.
Biology and behavior
Physical characteristics
The polar bear is the largest predator that lives on land, being twice as big as a lion or tiger.[26] Adult males weigh 352–680 kg (780–1,500 lb) and measure 2.4–3 m (7.9–9.8 ft) in length.[27] Adult females are roughly half the size of males and normally weigh 150–249 kg (330–550 lb), measuring 1.8–2.4 metres (5.9–7.9 ft) in length. When pregnant, however, they can weigh as much as 499 kg (1,100 lb).[27] The polar bear is among the most sexually dimorphic of mammals, surpassed only by the eared seals.[28] The largest polar bear on record, reportedly weighing 1,002 kg (2,210 lb), was a male shot at Kotzebue Sound in northwestern Alaska in 1960.[29]
Compared with its closest relative, the brown bear, the polar bear has a more elongated body build and a longer skull and nose.[16] As predicted by Allen's rule for a northerly animal, the legs are stocky and the ears and tail are small.[16] However, the feet are very large to distribute load when walking on snow or thin ice and to provide propulsion when swimming; they may measure 36 cm (12 in) across in an adult.[30] The pads of the paws are covered with small, soft papillae which provide traction on the ice.[16] The polar bear's claws are short and stocky compared to those of the brown bear, perhaps to serve the former's need to grip heavy prey and ice.[16] Despite a recurring internet meme that all polar bears are left-handed,[31][32] there is no scientific evidence to support this claim.[33]
The 42 teeth of a polar bear reflect its highly carnivorous diet.[16] The cheek teeth are smaller and more jagged than in the brown bear, and the canines are larger and sharper.[16] The dental formula is:[16]
| 3.1.4.2 |
| 3.1.4.3 |
Polar bear fur consists of a layer of dense underfur and an outer layer of guard hairs, which appear white to tan but are actually translucent.[30] The guard hair is 5–15 cm (2.0–5.9 in) over most of the body.[34]
Polar bears are superbly insulated by their 10 cm (3.9 in) of blubber,[30] their hide and their fur; they overheat at temperatures above 10 °C (50 °F), and are nearly invisible under infrared photography.[35]
Polar bears gradually moult from May to August,[36] but, unlike other Arctic mammals, they do not shed their coat for a darker shade to camouflage themselves in the summer conditions. The hollow guard hairs of a polar bear coat were once thought to act as fiber-optic tubes to conduct light to its black skin, where it could be absorbed; however, this theory was disproved by recent studies.[37] The white coat usually yellows with age. When kept in captivity in warm, humid conditions, it is not unknown for the fur to turn a pale shade of green due to algae growing inside the guard hairs.[38] Males have significantly longer hairs on their forelegs, that increase in length until the bear reaches 14 years of age. The male's ornamental foreleg hair is thought to attract females, serving a similar function to the lion's mane.[28]
The polar bear has an extremely well-developed sense of smell, being able to detect seals nearly 1 mi (1.6 km) away and buried under 3 ft (0.91 m) of snow.[39] Its hearing is about as acute as that of a human, and its vision is also good at long distances.[39]
Polar bears are excellent swimmers and have been seen in open Arctic waters as far as 60 mi (97 km) from land. Their 12 cm (4.7 in) layer of fat adds buoyancy and insulates them from the cold.
Polar bears still have a vestigial hibernation induction trigger in their blood, but they do not hibernate in the winter as the brown bear does. Only female polar bears enter a dormant state (referred to as "denning") during pregnancy, though their body temperature does not decrease during this period as it would for a typical mammal in hibernation.[35][40]
Hunting and diet
The polar bear is the most carnivorous member of the bear family, and most of its diet consists of Ringed and Bearded Seals.[42] The Arctic is home to millions of seals, which become prey when they surface in holes in the ice in order to breathe, or when they haul out on the ice to rest.[41] Polar bears hunt primarily at the interface between ice, water, and air; they only rarely catch seals on land or in open water.[43]
The polar bear's most common hunting method is called still-hunting:[44] The bear uses its excellent sense of smell to locate a seal breathing hole, and crouches nearby in silence for a seal to appear.[41] When the seal exhales, the bear smells its breath, reaches into the hole with a forepaw, and drags it out onto the ice.[41] The polar bear kills the seal by biting its head to crush its skull.[41] The polar bear also hunts by stalking seals resting on the ice: Upon spotting a seal, it walks to within 100 yd (91 m), and then crouches. If the seal does not notice, the bear creeps to within 30 to 40 feet (9.1 to 12 m) of the seal and then suddenly rushes forth to attack.[41] A third hunting method is to raid the birth lairs that female seals create in the snow.[44]
A widespread legend tells that polar bears cover their black noses with their paws when hunting. This behavior, if it happens, is rare — although the story exists in native oral history and in accounts by early Arctic explorers, there is no record of an eyewitness account of the behavior in recent decades.[45]
Mature bears tend to eat only the calorie-rich skin and blubber of the seal, whereas younger bears consume the protein-rich red meat.[41] For subadult bears which are independent of their mother but have not yet gained enough experience and body size to successfully hunt seals, scavenging the carcasses from other bears' kills is an important source of nutrition. Subadults may also be forced to accept a half-eaten carcass if they kill a seal but cannot defend it from larger polar bears. After feeding, polar bears wash themselves with water or snow.[45]
The polar bear is an enormously powerful predator. It can kill an adult walrus, although it rarely attempts to as a walrus can be twice the bear's weight.[46] However, most terrestrial animals can outrun the polar bear on land as polar bears overheat quickly, and most marine animals can outswim it. In some areas, the polar bear's diet is supplemented by walrus calves, by whales captured at breathing holes, and by the carcasses of dead adult walruses or whales, whose blubber is readily devoured even when rotten.[47]
When sea ice is unavailable during summer and early fall, some populations live off fat reserves for months at a time.[35] Polar bears have also been observed to eat a wide variety of other wild foods, including muskox, reindeer, birds, eggs, rodents, shellfish, crabs, and other polar bears. They may also eat plants, including berries, roots, and kelp, however none of these are a significant part of their diet.[46] The polar bear's biology is specialized to require large amounts of fat from marine mammals, and it cannot derive sufficient caloric intake from terrestrial food.[48][49]
Being both curious animals and scavengers,[50][46] polar bears investigate and consume garbage where they come into contact with humans.[46] This was documented at the dump in Churchill, Manitoba before its closure.[51] Polar bears may attempt to consume almost anything they can find, including hazardous substances such as styrofoam, plastic, car batteries, ethylene glycol, hydraulic fluid, and motor oil.[46][50] The Churchill dump was closed in 2006 to protect the bears, and waste is now recycled or transported to Thompson, Manitoba.[52]
Behavior
Unlike grizzly bears, polar bears are not territorial. Although stereotyped as being voraciously aggressive, they are normally cautious in confrontations, and often choose to escape rather than fight.[53] Fat polar bears rarely attack humans unless severely provoked, whereas hungry polar bears are extremely unpredictable and are known to kill and sometimes eat humans.[47] Polar bears are stealth hunters, and the victim is often unaware of the bear's presence until the attack is underway.[54] Whereas brown bears often maul a person and then leave, polar bear attacks are almost always fatal.[54]
In general, adult polar bears live solitary lives. Yet, they have often been seen playing together for hours at a time and even sleeping in an embrace,[47] and polar bear zoologist Nikita Ovsianikov has described adult males as having "well-developed friendships."[53] Cubs are especially playful as well. Among young males in particular, play-fighting may be a means of practicing for serious competition during mating seasons later in life.[55]
In 1992, a photographer near Churchill took a now widely-circulated set of photographs of a polar bear playing with a Canadian Eskimo Dog a tenth of its size.[56][57] The pair wrestled harmlessly together each afternoon for ten days in a row for no apparent reason, although the bear may have been trying to demonstrate its friendliness in the hope of sharing the kennel's food.[56] This kind of social interaction is uncommon; it is far more typical for polar bears to behave aggressively towards dogs.[56]
Reproduction and lifecycle
Courtship and mating take place on the sea ice in April and May, when polar bears congregate in the best seal hunting areas.[58] A male may follow the tracks of a breeding female for 100 km (62 mi) or more, and after finding her engage in intense fighting with other males over mating rights, fights which often result in scars and broken teeth.[58] Polar bears have a generally polygynous mating system, however recent genetic testing of mothers and cubs has uncovered cases of litters in which cubs have different fathers.[59] Partners stay together and mate repeatedly for an entire week; the mating ritual induces ovulation in the female.[60]
After mating, the fertilized egg remains in a suspended state until August or September. During these four months, the pregnant female eats prodigious amounts of food, gaining at least 200 kg (440 lb) and often more than doubling their body weight.[58]
Maternity denning and early life
When the ice floes break up in the fall, ending the possibility of hunting, each pregnant female digs a maternity den consisting of a narrow entrance tunnel leading to one to three chambers.[58] Most maternity dens are in snowdrifts, but may also be made underground in permafrost if it is not sufficiently cold yet for snow.[58] In most subpopulations, maternity dens are situated on land a few kilometers from the coast, and the individuals in a subpopulation tend to reuse the same denning areas each year[18] The polar bears that do not den on land make their dens on the sea ice. In the den, she enters a dormant state similar to hibernation. This hibernation-like state does not consist of continuous sleeping, however the bear's heart rate slows from 46 to 27 beats per minute.[61]
Between November and February, cubs are born blind, covered with a light down fur, and weighing less than 0.9 kg (2.0 lb).[60] On average, each litter has two cubs.[58] The family remains in the den until mid-February to mid-April, with the mother maintaining her fast while nursing her cubs on a fat-rich milk.[58] By the time the mother then breaks opens the entrance to the den, her cubs weigh about 10 to 15 kilograms (22 to 33 lb).[58] For about 12 to 15 days, the family spends time outside the den while remaining in its vicinity, the mother grazing on vegetation while the cubs become used to walking and playing.[58] Then they begin the long walk from the denning area to the sea ice, where the mother can once again catch seals.[58] Depending on the timing of ice-floe breakup in the fall, she may have fasted for up to eight months.[58]
Cubs may fall prey to wolves, to adult male polar bears, or to starvation. Female polar bears are noted for both their affection towards their offspring, and their valiance in protecting them. One case of adoption of a wild cub has been confirmed by genetic testing.[59] In Alaska, 42% of cubs now reach 12 months of age, down from 65% 15 years ago.[62] In most areas, cubs are weaned at two and a half years of age,[58] when the mother chases them away or abandons them. The western coast of Hudson Bay is unusual in that its female polar bears sometimes wean their cubs at only one and a half years.[58] This was the case for 40% of cubs in western Hudson Bay in the early 1980s, however by the 1990s, fewer than 20% of cubs were weaned this young.[63] After the mother leaves, sibling cubs sometimes travel and share food together for weeks or months.[47]
Later life
Females begin to breed at the age of four years in most areas, and five years in the Beaufort Sea area.[58] Males usually reach sexual maturity at six years, however as competition for females is fierce, many do not breed until the age of eight or ten.[58] A study in Hudson Bay indicated that both the reproductive success and the maternal weight of females peaked in their mid-teens.[64]
Polar bears are especially susceptible to Trichinella, a parasitic roundworm they contract through cannibalism.[65] Bacterial Leptospirosis, rabies and Morbillivirus have been recorded. The bears are thought by some to be more resistant than other carnivores to viral disease.[citation needed] Polar bears sometimes have problems with various skin diseases which may be caused by mites or other parasites.
Polar bears rarely live beyond 25 years.[66] The oldest wild bears on record died at the age of 32, whereas the oldest captive was a female who died in 1991 at the age of 43.[67] The oldest living polar bear is Debby of the Assiniboine Park Zoo, who was probably born in December, 1966.[67]
Ecological role
The polar bear is the apex predator within its range. Several animal species, particularly Arctic Foxes and Glaucous Gulls, routinely scavenge polar bear kills.[45]
The relationship between ringed seals and polar bears is so close that the abundance of ringed seals in some areas appears to regulate the density of polar bears, while polar bear predation in turn, regulates density and reproductive success of ringed seals.[43] The evolutionary pressure of polar bear predation on seals probably accounts for some significant differences between Arctic and Antarctic seals. Compared to the Antarctic, where there is no major surface predator, Arctic seals use more breathing holes per individual, appear more restless when hauled out on the ice, and rarely defecate on the ice.[45] The baby fur of most Arctic seal species is white, presumably to provide camouflage from predators, whereas Antarctic seals all have dark fur at birth.[45]
Exploitation
Indigenous people
Polar bears have long provided important raw materials for Arctic peoples, including the Inuit, Yupik, Chukchi, Nentets, Russian Pomors and others. Almost all parts of captured animals was used.[68] The fur was used in particular to sew pants and, by the Nenets, to make galoshes-like outer footwear called tobok; the meat is edible, desp
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