Polar bear

The polar bear (Ursus maritimus) is a hypercarnivorous species of bear. Its native range lies largely within the Arctic Circle, encompassing the Arctic Ocean and its surrounding seas and landmasses, which includes the northernmost regions of North America and Eurasia. It is the largest extant bear species, as well as the largest extant land carnivore. A boar (adult male) weighs around 350–700 kg (770–1,540 lb),[8] while a sow (adult female) is about half that size. Although it is the sister species of the brown bear,[9] 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 seals, which make up most of its diet.[10] Although most polar bears are born on land, they spend most of their time on the sea ice. 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. Because of their dependence on the sea ice, polar bears are classified as marine mammals.[11]

Because of expected habitat loss caused by climate change, the polar bear is classified as a vulnerable species. 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 circumpolar peoples, and polar bears remain important in their cultures. Historically, the polar bear has also been known as the "white bear". It is sometimes referred to as the "nanook", based on the Inuit term nanuq.

Naming and etymology

Constantine John Phipps was the first to describe the polar bear as a distinct species in 1774 in his report about his 1773 expedition towards the North Pole.[4][2] He chose the scientific name Ursus maritimus, the Latin for "maritime bear",[15] due to the animal's native habitat. The Inuit refer to the animal as nanook (transliterated as nanuq in the Inupiat language).[16][14] The Yupik also refer to the bear as nanuuk in Siberian Yupik.[17] In the Chukotko-Kamchatkan languages of Alyutor and Koryak, the name of the polar bear is umqa, while in the related Chukchi, it is umqə.[18] In Russian, it is usually called бе́лый медве́дь (bély medvéd', 'white bear'), though an older word still in use is ошку́й (Oshkúj, which comes from the Komi oski, "bear").[19] In Quebec, the polar bear is referred to by the french terms ours blanc ('white bear') or ours polaire ('polar bear').[20] In Norwegian, one of the primary languages of the Svalbard archipelago,[21] the polar bear is referred to as isbjørn ('ice bear') or kvitbjørn ('white bear').

The polar bear was previously considered to be in its own genus, Thalarctos. 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.

Taxonomy and evolution

The bear family, Ursidae, is thought to have split from other carnivorans about 38 million years ago.[24] The subfamily Ursinae originated approximately 4.2 million years ago.[25] The oldest known polar bear fossil is a 130,000 to 110,000-year-old jaw bone, found on Prince Charles Foreland in 2004.[26] Fossils show that between 10,000 and 20,000 years ago, the polar bear's molar teeth changed significantly from those of the brown bear.[27] Polar bears are thought to have diverged from a population of brown bears that became isolated during a period of glaciation in the Pleistocene[5] from the eastern part of Siberia (from Kamchatka and the Kolym Peninsula).[27]

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.[26] Further, some clades of brown bear, as assessed by their mtDNA, were thought to be more closely related to polar bears than to other brown bears,[28] meaning that the brown bear might not be considered a species under some species concepts, but paraphyletic.[29] The mtDNA of extinct Irish brown bears is particularly close to polar bears.[30] 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,[31] although the latest research is based on analysis of the complete genomes (rather than just the mitochondria or partial nuclear genomes) of polar and brown bears, and establishes the divergence of polar and brown bears at 400,000 years ago.[32]

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, the ABC Islands bears, has between 5 percent and 10 percent polar bear genes, indicating more frequent and recent mating.[33] Polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids;[5][34] 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.[33] However, because neither species can survive long in the other's ecological niche, and because they have different morphology, metabolism, social and feeding behaviours, and other phenotypic characteristics, the two bears are generally classified as separate species.

When the polar bear was originally documented, two subspecies were identified: the American polar bear (Ursus maritimus maritimus) by Constantine J. Phipps in 1774, and the Siberian polar bear (Ursus maritimus marinus) by Peter Simon Pallas in 1776.This distinction has since been invalidated.One alleged fossil subspecies has been identified: Ursus maritimus tyrannus, which became extinct during the Pleistocene. U.m. tyrannus was significantly larger than the living subspecies.However, recent reanalysis of the fossil suggests that it was actually a brown bear.

Population and distribution

The polar bear is found in the Arctic Circle and adjacent land masses as far south as Newfoundland. Due to the absence of human development in its remote habitat, it retains more of its original range than any other extant carnivore.[40] 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 20–25,000 or 22–31,000 polar bears worldwide.

With the discovery of a southeast Greenland population in 2022, there are 20 generally recognized, discrete subpopulations of polar bears.The subpopulations display seasonal fidelity to particular areas, but DNA studies show that they are not reproductively isolated.[39] The 14 North American subpopulations range from the Beaufort Sea south to Hudson Bay and east to eastern Greenland and account for about 54% of the global population.

The usual 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 conservation efforts throughout the polar bear's range.[47] Bears sometimes swim to Iceland from Greenland—about 600 sightings since the country's settlement in the 9th century, and five in the 21st century as of 2016—and are always killed because of their danger, as well as the cost and difficulty of repatriation.

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. 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. 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.[49] The Polar Bear Specialist Group of the IUCN Species Survival Commission 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."

Of the 19 polar bear subpopulations recognized in 2017, one was in decline, two were increasing, seven were stable, and nine had insufficient data.[51]

Habitat

The polar bear is a marine mammal because it spends many months of the year at sea.[52] However, it is the only living marine mammal with powerful, large limbs and feet that allow them to cover kilometres on foot and run on land.[53] 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.[40][54] 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.[55] Freshwater is limited in these environments because it is either locked up in snow or saline. Polar bears are able to produce water through the metabolism of fats found in seal blubber,[56] and 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.[57]

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. 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.[54] In the Chukchi and Beaufort seas, most polar bears retreat each summer to the ice further north that remains frozen year-round, though a large portion of the population (15–40%) has been observed spending all summer on land every year in these areas since the 1980s.

Physical characteristics

The only other bear of comparable size to the polar bear is the Kodiak bear, which is a subspecies of brown bear.[59] Adult male polar bears weigh 350–700 kg (770–1,500 lb) and measure 2.4–3 metres (7 ft 10 in – 9 ft 10 in) in total length.[60] Around the Beaufort Sea, however, mature males reportedly average 450 kg (1,000 lb).[61] 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.[62] When pregnant, however, females can weigh as much as 500 kg (1,100 lb).[60] The polar bear is among the most sexually dimorphic of mammals, surpassed only by the pinnipeds such as elephant seals.[63] 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. This specimen, when mounted, stood 3.39 m (11 ft 1 in) tall on its hindlegs. The shoulder height of an adult polar bear is 122 to 160 cm (4 ft 0 in to 5 ft 3 in).[64][65] 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.[66]

Compared with its closest relative, the brown bear, the polar bear has a more elongated body build and a longer skull and nose.[35] As predicted by Allen's rule for a northerly animal, the legs are stocky and the ears and tail are small.[35] 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.[67] The pads of the paws are covered with small, soft papillae (dermal bumps), which provide traction on the ice. 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.[35] The claws are deeply scooped on the underside which assists 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.[68] 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.[citation needed]

The 42 teeth of a polar bear reflect its highly carnivorous diet. The cheek teeth are smaller and more jagged than in the brown bear, and the canines are larger and sharper. The dental formula is

3.1.4.2

3.1.4.3

.[35]

Polar bears are superbly insulated by up to 10 cm (4 in) of adipose tissue,[67] their hide and their fur. 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.[67] Two genes that are known to influence melanin production, LYST and AIM1, are both mutated in polar bears, possibly leading to the absence on this pigment in their fur.[69] The guard hair is 5–15 cm (2–6 in) over most of the body.[70] Polar bears gradually moult from May to August,[71] but, unlike other Arctic mammals, they do not shed their coat for a darker shade to provide camouflage in summer conditions.[72] 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 hypothesis was disproved by a study in 1998.[73]

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.[74] 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.[75]

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. Its hearing is about as acute as that of a human, and its vision is also good at long distances.[76]

The polar bear is an excellent swimmer and often will swim for days.[77] One bear swam continuously for 9 days in the frigid Bering Sea for 700 km (400 mi) to reach ice far from land. She then travelled another 1,800 km (1,100 mi). During the swim, the bear lost 22% of her body mass and her yearling cub died.[78] With its body fat providing buoyancy, the bear swims in a dog paddle fashion using its large forepaws for propulsion. Polar bears can swim at 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). When sprinting, they can reach up to 40 km/h (25 mph).[80] Researches have engineered synthetic textiles that mimic the fur's ability to transmit solar radiation toward the bears’ skin and insulate heat from escaping

Life history and behaviors'

Unlike brown 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.[82] Satiated polar bears rarely attack humans unless severely provoked.[83][84] 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.[85] 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. Whereas brown bears often maul a person and then leave, polar bear attacks are more likely to be predatory and are almost always fatal.[86] 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.[87]

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,[85] and polar bear zoologist Nikita Ovsianikov has described adult males as having "well-developed friendships."[82] 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.[88] Polar bears are usually quiet but do communicate with various sounds and vocalizations. Females communicate with their young with moans and chuffs, and the distress calls of both cubs and subadults consists of bleats.[89] Cubs may hum while nursing.[90] When nervous, bears produce huffs, chuffs and snorts while hisses, growls and roars are signs of aggression.[89] Chemical communication can also be important: bears leave behind their scent in their tracks which allow individuals to keep track of one another in the vast Arctic wilderness.[91]

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 familiaris) a tenth of its size.[92][93] The pair wrestled harmlessly together each afternoon for 10 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. This kind of social interaction is uncommon; it is far more typical for polar bears to behave aggressively towards dogs