Post by vodmeister on May 11, 2014 7:04:39 GMT
Polar bear
The polar bear (Ursus maritimus) is a carnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is a large bear, approximately the same size as the omnivorous Kodiak bear (Ursus arctos middendorffi).[3] A boar (adult male) weighs around 350–700 kg (770–1,540 lb),[4] while a sow (adult female) is about half that size. Although it is the sister species of the brown bear,[5] it has evolved to occupy a narrower 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.[6] Although most polar bears are born on land, they spend most of their time at sea. Their scientific name means "maritime bear", and derives from this fact. Polar bears hunt their preferred food of seals from the edge of sea ice, often living off fat reserves when no sea ice is present.
The polar bear is classified as a vulnerable species, with eight of the nineteen polar bear subpopulations in decline.[7] For decades, large scale hunting raised international concern for the future of the species but populations 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 polar bears remain important in their cultures.
Constantine John Phipps was the first to describe the polar bear as a distinct species in 1774.[1] He chose the scientific name Ursus maritimus, the Latin for 'maritime bear',[8] due to the animal's native habitat. The Inuit refer to the animal as nanook[9] (transliterated as nanuq in the Inupiat language).[10] The Yupik also refer to the bear as nanuuk in Siberian Yupik.[11] The bear is umka in the Chukchi language. In Russian, it is usually called бе́лый медве́дь (bélyj medvédj, the white bear), though an older word still in use is ошку́й (Oshkúj, which comes from the Komi oski, "bear").[12] In French, the polar bear is referred to as ours blanc ("white bear") or ours polaire ("polar bear").[13] In the Norwegian-administered Svalbard archipelago, the polar bear is referred to as Isbjørn ("ice bear").
The polar bear was previously considered to be in its own genus, Thalarctos.[14] However, evidence of hybrids between polar bears and brown bears, and of the 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. The oldest known polar bear fossil is a 130,000 to 110,000-year-old jaw bone, found on Prince Charles Foreland in 2004.[15] Fossils show that between ten to twenty 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.[16]
The evidence from DNA analysis is more complex. The mitochondrial DNA (mtDNA) of the polar bear diverged from the brown bear, Ursus arctos, roughly 150,000 years ago.[15] Further, some clades of brown bear, as assessed by their mtDNA, are more closely related to polar bears than to other brown bears,[17] meaning that the polar bear would not be a true species according to some species concepts.[18] The mtDNA of Irish brown bears is particularly close to polar bears.[19] A comparison of the nuclear genome of polar bears with that of brown bears revealed a different pattern, the two forming genetically distinct clades that diverged approximately 603,000 years ago,[20] although the latest research is based on analysis of the complete genomes (rather than just the mitochondria or partial nuclear genomes) of polar, brown and black bears, and establishes the divergence of polar and brown bears at 4-5 million years ago.[21]
However, the two species have mated intermittently for all that time, most likely coming into contact with each other during warming periods, when polar bears were driven onto land and brown bears migrated northward. Most brown bears have about 2 percent genetic material from polar bears, but one population residing in the Alexander Archipelago has between 5 percent and 10 percent polar bear genes, indicating more frequent and recent mating.[22] Polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids,[16][23] rather than indicating that they have only recently diverged, the new evidence suggests more frequent mating has continued over a longer period of time, and thus the two bears remain genetically similar.[22] However, because neither species can survive long in the other's ecological niche, and because they have different morphology, metabolism, social and feeding behaviors, and other phenotypic characteristics, the two bears are generally classified as separate species.[24]
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.[25] 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.[16]
The polar bear is found in the Arctic Circle and adjacent land masses as far south as Newfoundland Island. Due to the absence of human development in its remote habitat, it retains more of its original range than any other extant carnivore.[26] 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. Their southernmost range is near the boundary between the subarctic and humid continental climate zones. 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; however, biologists use a working estimate of about 20,000 to 25,000 polar bears worldwide.[1][27]
There are 19 generally recognized, discrete subpopulations.[27][28] The subpopulations display seasonal fidelity to particular areas, but DNA studies show that they are not reproductively isolated.[29] 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 eastern 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, the United States (Alaska) and Canada. These five nations are the signatories of the International Agreement on 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.[30] 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.[30] 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.[30] 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."[31]
Of the 19 recognized polar bear subpopulations, eight are declining, three are stable, one is increasing, and seven have insufficient data, as of 2009.[7][27]
The polar bear is often regarded as a marine mammal because it spends many months of the year at sea.[32] However, it is the only living "marine mammal" with powerful, large limbs and feet that allow them to cover miles on foot and run on land.[33] Its preferred habitat is the annual sea ice covering the waters over the continental shelf and the Arctic inter-island archipelagos. These areas, known as the "Arctic ring of life", have high biological productivity in comparison to the deep waters of the high Arctic.[26][34] The polar bear tends to frequent areas where sea ice meets water, such as polynyas and leads (temporary stretches of open water in Arctic ice), to hunt the seals that make up most of its diet.[35] 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.[36]
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.[34] 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.[34] In the Chukchi and Beaufort seas, polar bears retreat each summer to the ice further north that remains frozen year-round.
The polar bear is the largest living species of terrestrial predator.[37] The only other bear of a similar size is the Kodiak bear, which is a subspecies of brown bear.[38] Adult male polar bears weigh 350–700 kg (770–1,540 lb) and measure 2.4–3 metres (7 ft 10 in–9 ft 10 in) in total length.[39] The Guinness Book of World Records listed the average male as having a body mass of 385 to 410 kg (849 to 904 lb) and a shoulder height of 133 cm (4 ft 4 in), slightly smaller than the average cited for male Kodiak bears.[40] Around the Beaufort Sea, however, mature males reportedly average 450 kg (1,000 lb).[41] Adult females are roughly half the size of males and normally weigh 150–250 kg (330–550 lb), measuring 1.8–2.4 metres (5 ft 11 in–7 ft 10 in) in length. Elsewhere, a slightly larger estimated average weight of 260 kg (570 lb) was claimed for adult females.[42] When pregnant, however, females can weigh as much as 500 kg (1,100 lb).[39] The polar bear is among the most sexually dimorphic of mammals, surpassed only by the pinnipeds such as elephant seals.[43] The largest polar bear on record, reportedly weighing 1,002 kg (2,209 lb), was a male shot at Kotzebue Sound in northwestern Alaska in 1960.[40] This specimen, when mounted, stood 3.39 m (11 ft 1 in) tall on its hindlegs.[40] The shoulder height of an adult polar bear is 122 to 160 cm (4 ft 0 in to 5 ft 3 in).[40][44] While all bears are short-tailed, the polar bear's tail is relatively the shortest amongst living bears, ranging from 7 to 13 cm (2.8 to 5.1 in) in length.[45]
Compared with its closest relative, the brown bear, the polar bear has a more elongated body build and a longer skull and nose.[24] As predicted by Allen's rule for a northerly animal, the legs are stocky and the ears and tail are small.[24] 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 30 cm (12 in) across in an adult.[46] The pads of the paws are covered with small, soft papillae (dermal bumps) which provide traction on the ice.[24] 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.[24] The claws are deeply scooped on the underside to assist in digging in the ice of the natural habitat. Research of injury patterns in polar bear forelimbs found injuries to the right forelimb to be more frequent than those to the left, suggesting, perhaps, right-handedness.[47] Unlike the brown bear, polar bears in captivity are rarely overweight or particularly large, possibly as a reaction to the warm conditions of most zoos.
Polar bears are superbly insulated by up to 10 cm (4 in) of blubber,[46] their hide and their fur; they overheat at temperatures above 10 °C (50 °F), and are nearly invisible under infrared photography.[48] 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 transparent.[46] The guard hair is 5–15 cm (2–6 in) over most of the body.[49] Polar bears gradually moult from May to August,[50] 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 disproven by recent studies.[51]
The white coat usually yellows with age. When kept in captivity in warm, humid conditions, the fur may turn a pale shade of green due to algae growing inside the guard hairs.[52] Males have significantly longer hairs on their forelegs, which 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.[53]
The polar bear has an extremely well developed sense of smell, being able to detect seals nearly 1.6 km (1 mi) away and buried under 1 m (3 ft) of snow.[54] Its hearing is about as acute as that of a human, and its vision is also good at long distances.[54]
The polar bear is an excellent swimmer and individuals have been seen in open Arctic waters as far as 300 km (200 mi) from land. With its body fat providing buoyancy, it swims in a dog paddle fashion using its large forepaws for propulsion.[55] Polar bears can swim 10 km/h (6 mph). When walking, the polar bear tends to have a lumbering gait and maintains an average speed of around 5.6 km/h (3.5 mph).[55] When sprinting, they can reach up to 40 km/h (25 mph).[56]
The polar bear is the most carnivorous member of the bear family, and throughout most of its range, its diet primarily consists of ringed (Pusa hispida) and bearded seals (Erignathus barbatus).[58] 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.[57] Polar bears hunt primarily at the interface between ice, water, and air; they only rarely catch seals on land or in open water.[59]
The polar bear's most common hunting method is called still-hunting:[60] The bear uses its excellent sense of smell to locate a seal breathing hole, and crouches nearby in silence for a seal to appear. The bear may lay in wait for several hours. When the seal exhales, the bear smells its breath, reaches into the hole with a forepaw, and drags it out onto the ice. The polar bear kills the seal by biting its head to crush its skull. The polar bear also hunts by stalking seals resting on the ice: Upon spotting a seal, it walks to within 90 m (100 yd), and then crouches. If the seal does not notice, the bear creeps to within 9 to 12 m (30 to 40 ft) of the seal and then suddenly rushes forth to attack.[57] A third hunting method is to raid the birth lairs that female seals create in the snow.[60]
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.[55]
Mature bears tend to eat only the calorie-rich skin and blubber of the seal, whereas younger bears consume the protein-rich red meat.[57] Studies have also photographed polar bears scaling near-vertical cliffs, to eat birds' chicks and eggs.[61] 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.[55]
The polar bear is perhaps unequaled by any other living land predator in its enormous physical power. However, its primary prey species, the ringed seal, is much smaller than itself, and many of the seals hunted are pups rather than adults. Ringed seals are born weighing 5.4 kg (12 lb) and grown to an estimated average weight of only 60 kg (130 lb).[62][63] The bearded seal, on the other hand, can be nearly the same size as the bear itself, averaging 270 kg (600 lb).[63] Adult male bearded seals, at 350 to 500 kg (770 to 1,100 lb) are believed to be too large for a female bear to overtake, and so are potential prey only for mature male bears.[64] Enormously powerful large males also occasionally attempt to hunt and kill even larger prey items.[65] It can kill an adult walrus (Odobenus rosmarus), although this is rarely attempted. At up to 2,000 kg (4,400 lb) and a typical adult mass range of 600 to 1,500 kg (1,300 to 3,300 lb), a walrus can be more than twice the bear's weight,[66] and has up to 1-metre (3 ft)-long ivory tusks that can be used as formidable weapons. Most attacks on walruses occur when the bear charges a group and either targets the slower moving walruses, usually either young or infirm ones, or a walrus that is injured in the rush of walruses trying to escape. They will also attack even adult walruses when their diving holes have frozen over or intercept them before they can get back to the diving hole in the ice. Yet, polar bears very seldom attack full-grown adult walruses, with the largest male walrus probably invulnerable unless otherwise injured or incapacitated. Since an attack on a walrus tends to be an extremely protracted and exhausting venture, bears have been known to back down from the attack after making the initial injury to the walrus.[67] Polar bears have also been seen to prey on beluga whales (Delphinapterus leucas) and narwhals (Monodon monoceros), by swiping at them at breathing holes. The whales are of similar size to the walrus and nearly as difficult for the bear to subdue.[68][69] Most terrestrial animals in the Arctic can outrun the polar bear on land as polar bears overheat quickly, and most marine animals the bear encounters can outswim it. In some areas, the polar bear's diet is supplemented by walrus calves and by the carcasses of dead adult walruses or whales, whose blubber is readily devoured even when rotten.[70]
With the exception of pregnant females, polar bears are active year-round,[71] although they have a vestigial hibernation induction trigger in their blood. Unlike brown and black bears, polar bears are capable of fasting for up to several months during late summer and early fall, when they cannot hunt for seals because the sea is unfrozen.[71] When sea ice is unavailable during summer and early autumn, some populations live off fat reserves for months at a time.[48] Polar bears have also been observed to eat a wide variety of other wild foods, including muskox (Ovibos moschatus), reindeer (Rangifer tarandus), birds, eggs, rodents, crabs, other crustaceans 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.[66] When stalking land animals, such as muskox, reindeer and even willow ptarmigan (Lagopus lagopus), polar bears appear to make use of vegetative cover and wind direction to bring them as close to their prey as possible before attacking. Polar bears have been observed to hunt the small Svalbard reindeer (R. t. platyrhynchus), which weigh only 40 to 60 kg (90 to 130 lb) as adults, as well as the Barren-ground caribou (R. t. groenlandicus), which is about twice as heavy as that.[72][73] Adult muskox, which can weigh 450 kg (1,000 lb) or more, are a more formidable quarry.[74] Although ungulates are not typical prey, the killing of one during the summer months can exponentially increase the odds of survival during that lean period. Like the brown bear, most ungulate prey of polar bears is likely to be young, sickly or injured specimens rather than healthy adults.[73] 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.[75][76]
Being both curious animals and scavengers,[66][77] polar bears investigate and consume garbage where they come into contact with humans.[66] 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.[66][77] The dump in Churchill, Manitoba was closed in 2006 to protect bears, and waste is now recycled or transported to Thompson, Manitoba.[78][79]
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.[80] Satiated polar bears rarely attack humans unless severely provoked. However, due to their lack of prior human interaction, hungry polar bears are extremely unpredictable, fearless towards people and are known to kill and sometimes eat humans.[70] Many attacks by brown bears are the result of surprising the animal, which is not the case with the polar bear. Polar bears are stealth hunters, and the victim is often unaware of the bear's presence until the attack is underway.[81] Whereas brown bears often maul a person and then leave, polar bear attacks are more likely to be predatory and are almost always fatal.[81] However, due to the very small human population around the Arctic, such attacks are rare. Michio Hoshino, a Japanese wildlife photographer, was once pursued briefly by a hungry male polar bear in northern Alaska. According to Hoshino, the bear started running but Hoshino made it to his truck. The bear was able to reach the truck and tore one of the doors off the truck before Hoshino was able to drive off.[82] Hoshino was later killed in his camp by a Brown Bear while on assignment in Russia.[83]
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,[70] and polar bear zoologist Nikita Ovsianikov has described adult males as having "well-developed friendships."[80] 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.[84] Polar bears have a wide range of vocalisations, including bellows, roars, growls, chuffs and purrs.[85]
In 1992, a photographer near Churchill took a now widely circulated set of photographs of a polar bear playing with a Canadian Eskimo Dog (Canis lupus familiaris) a tenth of its size.[86][87] 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.[86] This kind of social interaction is uncommon; it is far more typical for polar bears to behave aggressively towards dogs.[86]
Courtship and mating take place on the sea ice in April and May, when polar bears congregate in the best seal hunting areas.[88] A male may follow the tracks of a breeding female for 100 km (60 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.[88] Polar bears have a generally polygynous mating system; recent genetic testing of mothers and cubs, however, has uncovered cases of litters in which cubs have different fathers.[89] Partners stay together and mate repeatedly for an entire week; the mating ritual induces ovulation in the female.[90]
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 her body weight.[88]
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.[88] Most maternity dens are in snowdrifts, but may also be made underground in permafrost if it is not sufficiently cold yet for snow.[88] 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.[26] 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.[91] Her body temperature does not decrease during this period as it would for a typical mammal in hibernation.[48][92]
Between November and February, cubs are born blind, covered with a light down fur, and weighing less than 0.9 kg (2.0 lb),[90] but in captivity they might be delivered in the earlier months. The earliest recorded birth of polar bears in captivity was on 11 October 2011 in the Toronto Zoo.[93] On average, each litter has two cubs.[88] 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.[88] By the time the mother breaks open the entrance to the den, her cubs weigh about 10 to 15 kilograms (22 to 33 lb).[88] 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.[88] Then they begin the long walk from the denning area to the sea ice, where the mother can once again catch seals.[88] Depending on the timing of ice-floe breakup in the fall, she may have fasted for up to eight months.[88]
Cubs may fall prey to wolves 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.[89] Adult male bears occasionally kill and eat polar bear cubs,[94] for reasons that are unclear.[95] As of 2006, in Alaska, 42% of cubs now reach 12 months of age, down from 65% 15 years ago.[96] In most areas, cubs are weaned at two and a half years of age,[88] 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.[88] This was the case for 40% of cubs there in the early 1980s; however by the 1990s, fewer than 20% of cubs were weaned this young.[97] After the mother leaves, sibling cubs sometimes travel and share food together for weeks or months.[70]
Females begin to breed at the age of four years in most areas, and five years in the Beaufort Sea area.[88] 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.[88] A study in Hudson Bay indicated that both the reproductive success and the maternal weight of females peaked in their mid-teens.[98]
Polar bears appear to be less affected by infectious diseases and parasites than most terrestrial mammals.[95] Polar bears are especially susceptible to Trichinella, a parasitic roundworm they contract through cannibalism,[99] although infections are usually not fatal.[95] Only one case of a polar bear with rabies has been documented, even though polar bears frequently interact with Arctic foxes, which often carry rabies.[95] Bacterial Leptospirosis and Morbillivirus have been recorded. 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.[100] The oldest wild bears on record died at age 32, whereas the oldest captive was a female who died in 1991, age 43.[101] The causes of death in wild adult polar bears are poorly understood, as carcasses are rarely found in the species's frigid habitat.[95] In the wild, old polar bears eventually become too weak to catch food, and gradually starve to death. Polar bears injured in fights or accidents may either die from their injuries or become unable to hunt effectively, leading to starvation.[95]
The polar bear is the apex predator within its range. Several animal species, particularly Arctic Foxes (Vulpes lagopus) and Glaucous Gulls (Larus hyperboreus), routinely scavenge polar bear kills.[55]
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.[59] 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.[55] 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.[55]
Polar bears rarely enter conflict with other predators, though recent brown bear encroachments into polar bear territories have led to antagonistic encounters. Brown bears tend to dominate polar bears in disputes over carcasses,[102] and dead polar bear cubs have been found in brown bear dens.[103] Wolves are rarely encountered by polar bears, though there are two records of Arctic wolf (Canis lupus arctos) packs killing polar bear cubs.[104] A rather unlikely killer of a grown polar bear has reportedly included a wolverine (Gulo gulo), anecedotely reported to have suffocated a bear with a bite to the throat during a conflict over food.[105] Polar bears are sometimes the host of arctic mites such as Alaskozetes antarcticus.[55]
The Canadian Journal of Zoology tracked 52 sows in the southern Beaufort Sea off Alaska with GPS system collars; no boars were involved in the study due to males' necks being too thick for the GPS-equipped collars. Fifty long-distance swims were recorded; the longest at 354 kilometres (220 mi), with an average of 155 kilometres (96 mi). The length of these swims ranged from most of a day to ten days. Ten of the sows had a cub swim with them and after a year six cubs survived. The study did not determine if the others lost their cubs before, during, or some time after their long swims. Researchers do not know whether or not this is a new behavior; before polar ice shrinkage, they opined that there was probably neither the need nor opportunity to swim such long distances.[106]
The polar bear (Ursus maritimus) is a carnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is a large bear, approximately the same size as the omnivorous Kodiak bear (Ursus arctos middendorffi).[3] A boar (adult male) weighs around 350–700 kg (770–1,540 lb),[4] while a sow (adult female) is about half that size. Although it is the sister species of the brown bear,[5] it has evolved to occupy a narrower 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.[6] Although most polar bears are born on land, they spend most of their time at sea. Their scientific name means "maritime bear", and derives from this fact. Polar bears hunt their preferred food of seals from the edge of sea ice, often living off fat reserves when no sea ice is present.
The polar bear is classified as a vulnerable species, with eight of the nineteen polar bear subpopulations in decline.[7] For decades, large scale hunting raised international concern for the future of the species but populations 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 polar bears remain important in their cultures.
Constantine John Phipps was the first to describe the polar bear as a distinct species in 1774.[1] He chose the scientific name Ursus maritimus, the Latin for 'maritime bear',[8] due to the animal's native habitat. The Inuit refer to the animal as nanook[9] (transliterated as nanuq in the Inupiat language).[10] The Yupik also refer to the bear as nanuuk in Siberian Yupik.[11] The bear is umka in the Chukchi language. In Russian, it is usually called бе́лый медве́дь (bélyj medvédj, the white bear), though an older word still in use is ошку́й (Oshkúj, which comes from the Komi oski, "bear").[12] In French, the polar bear is referred to as ours blanc ("white bear") or ours polaire ("polar bear").[13] In the Norwegian-administered Svalbard archipelago, the polar bear is referred to as Isbjørn ("ice bear").
The polar bear was previously considered to be in its own genus, Thalarctos.[14] However, evidence of hybrids between polar bears and brown bears, and of the 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. The oldest known polar bear fossil is a 130,000 to 110,000-year-old jaw bone, found on Prince Charles Foreland in 2004.[15] Fossils show that between ten to twenty 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.[16]
The evidence from DNA analysis is more complex. The mitochondrial DNA (mtDNA) of the polar bear diverged from the brown bear, Ursus arctos, roughly 150,000 years ago.[15] Further, some clades of brown bear, as assessed by their mtDNA, are more closely related to polar bears than to other brown bears,[17] meaning that the polar bear would not be a true species according to some species concepts.[18] The mtDNA of Irish brown bears is particularly close to polar bears.[19] A comparison of the nuclear genome of polar bears with that of brown bears revealed a different pattern, the two forming genetically distinct clades that diverged approximately 603,000 years ago,[20] although the latest research is based on analysis of the complete genomes (rather than just the mitochondria or partial nuclear genomes) of polar, brown and black bears, and establishes the divergence of polar and brown bears at 4-5 million years ago.[21]
However, the two species have mated intermittently for all that time, most likely coming into contact with each other during warming periods, when polar bears were driven onto land and brown bears migrated northward. Most brown bears have about 2 percent genetic material from polar bears, but one population residing in the Alexander Archipelago has between 5 percent and 10 percent polar bear genes, indicating more frequent and recent mating.[22] Polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids,[16][23] rather than indicating that they have only recently diverged, the new evidence suggests more frequent mating has continued over a longer period of time, and thus the two bears remain genetically similar.[22] However, because neither species can survive long in the other's ecological niche, and because they have different morphology, metabolism, social and feeding behaviors, and other phenotypic characteristics, the two bears are generally classified as separate species.[24]
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.[25] 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.[16]
The polar bear is found in the Arctic Circle and adjacent land masses as far south as Newfoundland Island. Due to the absence of human development in its remote habitat, it retains more of its original range than any other extant carnivore.[26] 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. Their southernmost range is near the boundary between the subarctic and humid continental climate zones. 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; however, biologists use a working estimate of about 20,000 to 25,000 polar bears worldwide.[1][27]
There are 19 generally recognized, discrete subpopulations.[27][28] The subpopulations display seasonal fidelity to particular areas, but DNA studies show that they are not reproductively isolated.[29] 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 eastern 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, the United States (Alaska) and Canada. These five nations are the signatories of the International Agreement on 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.[30] 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.[30] 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.[30] 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."[31]
Of the 19 recognized polar bear subpopulations, eight are declining, three are stable, one is increasing, and seven have insufficient data, as of 2009.[7][27]
The polar bear is often regarded as a marine mammal because it spends many months of the year at sea.[32] However, it is the only living "marine mammal" with powerful, large limbs and feet that allow them to cover miles on foot and run on land.[33] Its preferred habitat is the annual sea ice covering the waters over the continental shelf and the Arctic inter-island archipelagos. These areas, known as the "Arctic ring of life", have high biological productivity in comparison to the deep waters of the high Arctic.[26][34] The polar bear tends to frequent areas where sea ice meets water, such as polynyas and leads (temporary stretches of open water in Arctic ice), to hunt the seals that make up most of its diet.[35] 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.[36]
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.[34] 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.[34] In the Chukchi and Beaufort seas, polar bears retreat each summer to the ice further north that remains frozen year-round.
The polar bear is the largest living species of terrestrial predator.[37] The only other bear of a similar size is the Kodiak bear, which is a subspecies of brown bear.[38] Adult male polar bears weigh 350–700 kg (770–1,540 lb) and measure 2.4–3 metres (7 ft 10 in–9 ft 10 in) in total length.[39] The Guinness Book of World Records listed the average male as having a body mass of 385 to 410 kg (849 to 904 lb) and a shoulder height of 133 cm (4 ft 4 in), slightly smaller than the average cited for male Kodiak bears.[40] Around the Beaufort Sea, however, mature males reportedly average 450 kg (1,000 lb).[41] Adult females are roughly half the size of males and normally weigh 150–250 kg (330–550 lb), measuring 1.8–2.4 metres (5 ft 11 in–7 ft 10 in) in length. Elsewhere, a slightly larger estimated average weight of 260 kg (570 lb) was claimed for adult females.[42] When pregnant, however, females can weigh as much as 500 kg (1,100 lb).[39] The polar bear is among the most sexually dimorphic of mammals, surpassed only by the pinnipeds such as elephant seals.[43] The largest polar bear on record, reportedly weighing 1,002 kg (2,209 lb), was a male shot at Kotzebue Sound in northwestern Alaska in 1960.[40] This specimen, when mounted, stood 3.39 m (11 ft 1 in) tall on its hindlegs.[40] The shoulder height of an adult polar bear is 122 to 160 cm (4 ft 0 in to 5 ft 3 in).[40][44] While all bears are short-tailed, the polar bear's tail is relatively the shortest amongst living bears, ranging from 7 to 13 cm (2.8 to 5.1 in) in length.[45]
Compared with its closest relative, the brown bear, the polar bear has a more elongated body build and a longer skull and nose.[24] As predicted by Allen's rule for a northerly animal, the legs are stocky and the ears and tail are small.[24] 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 30 cm (12 in) across in an adult.[46] The pads of the paws are covered with small, soft papillae (dermal bumps) which provide traction on the ice.[24] 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.[24] The claws are deeply scooped on the underside to assist in digging in the ice of the natural habitat. Research of injury patterns in polar bear forelimbs found injuries to the right forelimb to be more frequent than those to the left, suggesting, perhaps, right-handedness.[47] Unlike the brown bear, polar bears in captivity are rarely overweight or particularly large, possibly as a reaction to the warm conditions of most zoos.
Polar bears are superbly insulated by up to 10 cm (4 in) of blubber,[46] their hide and their fur; they overheat at temperatures above 10 °C (50 °F), and are nearly invisible under infrared photography.[48] 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 transparent.[46] The guard hair is 5–15 cm (2–6 in) over most of the body.[49] Polar bears gradually moult from May to August,[50] 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 disproven by recent studies.[51]
The white coat usually yellows with age. When kept in captivity in warm, humid conditions, the fur may turn a pale shade of green due to algae growing inside the guard hairs.[52] Males have significantly longer hairs on their forelegs, which 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.[53]
The polar bear has an extremely well developed sense of smell, being able to detect seals nearly 1.6 km (1 mi) away and buried under 1 m (3 ft) of snow.[54] Its hearing is about as acute as that of a human, and its vision is also good at long distances.[54]
The polar bear is an excellent swimmer and individuals have been seen in open Arctic waters as far as 300 km (200 mi) from land. With its body fat providing buoyancy, it swims in a dog paddle fashion using its large forepaws for propulsion.[55] Polar bears can swim 10 km/h (6 mph). When walking, the polar bear tends to have a lumbering gait and maintains an average speed of around 5.6 km/h (3.5 mph).[55] When sprinting, they can reach up to 40 km/h (25 mph).[56]
The polar bear is the most carnivorous member of the bear family, and throughout most of its range, its diet primarily consists of ringed (Pusa hispida) and bearded seals (Erignathus barbatus).[58] 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.[57] Polar bears hunt primarily at the interface between ice, water, and air; they only rarely catch seals on land or in open water.[59]
The polar bear's most common hunting method is called still-hunting:[60] The bear uses its excellent sense of smell to locate a seal breathing hole, and crouches nearby in silence for a seal to appear. The bear may lay in wait for several hours. When the seal exhales, the bear smells its breath, reaches into the hole with a forepaw, and drags it out onto the ice. The polar bear kills the seal by biting its head to crush its skull. The polar bear also hunts by stalking seals resting on the ice: Upon spotting a seal, it walks to within 90 m (100 yd), and then crouches. If the seal does not notice, the bear creeps to within 9 to 12 m (30 to 40 ft) of the seal and then suddenly rushes forth to attack.[57] A third hunting method is to raid the birth lairs that female seals create in the snow.[60]
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.[55]
Mature bears tend to eat only the calorie-rich skin and blubber of the seal, whereas younger bears consume the protein-rich red meat.[57] Studies have also photographed polar bears scaling near-vertical cliffs, to eat birds' chicks and eggs.[61] 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.[55]
The polar bear is perhaps unequaled by any other living land predator in its enormous physical power. However, its primary prey species, the ringed seal, is much smaller than itself, and many of the seals hunted are pups rather than adults. Ringed seals are born weighing 5.4 kg (12 lb) and grown to an estimated average weight of only 60 kg (130 lb).[62][63] The bearded seal, on the other hand, can be nearly the same size as the bear itself, averaging 270 kg (600 lb).[63] Adult male bearded seals, at 350 to 500 kg (770 to 1,100 lb) are believed to be too large for a female bear to overtake, and so are potential prey only for mature male bears.[64] Enormously powerful large males also occasionally attempt to hunt and kill even larger prey items.[65] It can kill an adult walrus (Odobenus rosmarus), although this is rarely attempted. At up to 2,000 kg (4,400 lb) and a typical adult mass range of 600 to 1,500 kg (1,300 to 3,300 lb), a walrus can be more than twice the bear's weight,[66] and has up to 1-metre (3 ft)-long ivory tusks that can be used as formidable weapons. Most attacks on walruses occur when the bear charges a group and either targets the slower moving walruses, usually either young or infirm ones, or a walrus that is injured in the rush of walruses trying to escape. They will also attack even adult walruses when their diving holes have frozen over or intercept them before they can get back to the diving hole in the ice. Yet, polar bears very seldom attack full-grown adult walruses, with the largest male walrus probably invulnerable unless otherwise injured or incapacitated. Since an attack on a walrus tends to be an extremely protracted and exhausting venture, bears have been known to back down from the attack after making the initial injury to the walrus.[67] Polar bears have also been seen to prey on beluga whales (Delphinapterus leucas) and narwhals (Monodon monoceros), by swiping at them at breathing holes. The whales are of similar size to the walrus and nearly as difficult for the bear to subdue.[68][69] Most terrestrial animals in the Arctic can outrun the polar bear on land as polar bears overheat quickly, and most marine animals the bear encounters can outswim it. In some areas, the polar bear's diet is supplemented by walrus calves and by the carcasses of dead adult walruses or whales, whose blubber is readily devoured even when rotten.[70]
With the exception of pregnant females, polar bears are active year-round,[71] although they have a vestigial hibernation induction trigger in their blood. Unlike brown and black bears, polar bears are capable of fasting for up to several months during late summer and early fall, when they cannot hunt for seals because the sea is unfrozen.[71] When sea ice is unavailable during summer and early autumn, some populations live off fat reserves for months at a time.[48] Polar bears have also been observed to eat a wide variety of other wild foods, including muskox (Ovibos moschatus), reindeer (Rangifer tarandus), birds, eggs, rodents, crabs, other crustaceans 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.[66] When stalking land animals, such as muskox, reindeer and even willow ptarmigan (Lagopus lagopus), polar bears appear to make use of vegetative cover and wind direction to bring them as close to their prey as possible before attacking. Polar bears have been observed to hunt the small Svalbard reindeer (R. t. platyrhynchus), which weigh only 40 to 60 kg (90 to 130 lb) as adults, as well as the Barren-ground caribou (R. t. groenlandicus), which is about twice as heavy as that.[72][73] Adult muskox, which can weigh 450 kg (1,000 lb) or more, are a more formidable quarry.[74] Although ungulates are not typical prey, the killing of one during the summer months can exponentially increase the odds of survival during that lean period. Like the brown bear, most ungulate prey of polar bears is likely to be young, sickly or injured specimens rather than healthy adults.[73] 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.[75][76]
Being both curious animals and scavengers,[66][77] polar bears investigate and consume garbage where they come into contact with humans.[66] 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.[66][77] The dump in Churchill, Manitoba was closed in 2006 to protect bears, and waste is now recycled or transported to Thompson, Manitoba.[78][79]
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.[80] Satiated polar bears rarely attack humans unless severely provoked. However, due to their lack of prior human interaction, hungry polar bears are extremely unpredictable, fearless towards people and are known to kill and sometimes eat humans.[70] Many attacks by brown bears are the result of surprising the animal, which is not the case with the polar bear. Polar bears are stealth hunters, and the victim is often unaware of the bear's presence until the attack is underway.[81] Whereas brown bears often maul a person and then leave, polar bear attacks are more likely to be predatory and are almost always fatal.[81] However, due to the very small human population around the Arctic, such attacks are rare. Michio Hoshino, a Japanese wildlife photographer, was once pursued briefly by a hungry male polar bear in northern Alaska. According to Hoshino, the bear started running but Hoshino made it to his truck. The bear was able to reach the truck and tore one of the doors off the truck before Hoshino was able to drive off.[82] Hoshino was later killed in his camp by a Brown Bear while on assignment in Russia.[83]
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,[70] and polar bear zoologist Nikita Ovsianikov has described adult males as having "well-developed friendships."[80] 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.[84] Polar bears have a wide range of vocalisations, including bellows, roars, growls, chuffs and purrs.[85]
In 1992, a photographer near Churchill took a now widely circulated set of photographs of a polar bear playing with a Canadian Eskimo Dog (Canis lupus familiaris) a tenth of its size.[86][87] 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.[86] This kind of social interaction is uncommon; it is far more typical for polar bears to behave aggressively towards dogs.[86]
Courtship and mating take place on the sea ice in April and May, when polar bears congregate in the best seal hunting areas.[88] A male may follow the tracks of a breeding female for 100 km (60 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.[88] Polar bears have a generally polygynous mating system; recent genetic testing of mothers and cubs, however, has uncovered cases of litters in which cubs have different fathers.[89] Partners stay together and mate repeatedly for an entire week; the mating ritual induces ovulation in the female.[90]
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 her body weight.[88]
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.[88] Most maternity dens are in snowdrifts, but may also be made underground in permafrost if it is not sufficiently cold yet for snow.[88] 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.[26] 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.[91] Her body temperature does not decrease during this period as it would for a typical mammal in hibernation.[48][92]
Between November and February, cubs are born blind, covered with a light down fur, and weighing less than 0.9 kg (2.0 lb),[90] but in captivity they might be delivered in the earlier months. The earliest recorded birth of polar bears in captivity was on 11 October 2011 in the Toronto Zoo.[93] On average, each litter has two cubs.[88] 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.[88] By the time the mother breaks open the entrance to the den, her cubs weigh about 10 to 15 kilograms (22 to 33 lb).[88] 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.[88] Then they begin the long walk from the denning area to the sea ice, where the mother can once again catch seals.[88] Depending on the timing of ice-floe breakup in the fall, she may have fasted for up to eight months.[88]
Cubs may fall prey to wolves 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.[89] Adult male bears occasionally kill and eat polar bear cubs,[94] for reasons that are unclear.[95] As of 2006, in Alaska, 42% of cubs now reach 12 months of age, down from 65% 15 years ago.[96] In most areas, cubs are weaned at two and a half years of age,[88] 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.[88] This was the case for 40% of cubs there in the early 1980s; however by the 1990s, fewer than 20% of cubs were weaned this young.[97] After the mother leaves, sibling cubs sometimes travel and share food together for weeks or months.[70]
Females begin to breed at the age of four years in most areas, and five years in the Beaufort Sea area.[88] 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.[88] A study in Hudson Bay indicated that both the reproductive success and the maternal weight of females peaked in their mid-teens.[98]
Polar bears appear to be less affected by infectious diseases and parasites than most terrestrial mammals.[95] Polar bears are especially susceptible to Trichinella, a parasitic roundworm they contract through cannibalism,[99] although infections are usually not fatal.[95] Only one case of a polar bear with rabies has been documented, even though polar bears frequently interact with Arctic foxes, which often carry rabies.[95] Bacterial Leptospirosis and Morbillivirus have been recorded. 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.[100] The oldest wild bears on record died at age 32, whereas the oldest captive was a female who died in 1991, age 43.[101] The causes of death in wild adult polar bears are poorly understood, as carcasses are rarely found in the species's frigid habitat.[95] In the wild, old polar bears eventually become too weak to catch food, and gradually starve to death. Polar bears injured in fights or accidents may either die from their injuries or become unable to hunt effectively, leading to starvation.[95]
The polar bear is the apex predator within its range. Several animal species, particularly Arctic Foxes (Vulpes lagopus) and Glaucous Gulls (Larus hyperboreus), routinely scavenge polar bear kills.[55]
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.[59] 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.[55] 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.[55]
Polar bears rarely enter conflict with other predators, though recent brown bear encroachments into polar bear territories have led to antagonistic encounters. Brown bears tend to dominate polar bears in disputes over carcasses,[102] and dead polar bear cubs have been found in brown bear dens.[103] Wolves are rarely encountered by polar bears, though there are two records of Arctic wolf (Canis lupus arctos) packs killing polar bear cubs.[104] A rather unlikely killer of a grown polar bear has reportedly included a wolverine (Gulo gulo), anecedotely reported to have suffocated a bear with a bite to the throat during a conflict over food.[105] Polar bears are sometimes the host of arctic mites such as Alaskozetes antarcticus.[55]
The Canadian Journal of Zoology tracked 52 sows in the southern Beaufort Sea off Alaska with GPS system collars; no boars were involved in the study due to males' necks being too thick for the GPS-equipped collars. Fifty long-distance swims were recorded; the longest at 354 kilometres (220 mi), with an average of 155 kilometres (96 mi). The length of these swims ranged from most of a day to ten days. Ten of the sows had a cub swim with them and after a year six cubs survived. The study did not determine if the others lost their cubs before, during, or some time after their long swims. Researchers do not know whether or not this is a new behavior; before polar ice shrinkage, they opined that there was probably neither the need nor opportunity to swim such long distances.[106]