Polar bear
Species of bear native largely to the Arctic Circle / From Wikipedia, the free encyclopedia
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The polar bear (Ursus maritimus) is a large bear native to the Arctic and nearby areas. It is closely related to the brown bear, and the two species can interbreed. The polar bear is the largest extant species of bear and land carnivore, with adult males weighing 300–800 kg (660–1,760 lb). The species is sexually dimorphic, as adult females are much smaller. The polar bear is white- or yellowish-furred with black skin and a thick layer of fat. It is more slender than the brown bear, with a narrower skull, longer neck and lower shoulder hump. Its teeth are sharper and more adapted to cutting meat. The paws are large and allow the bear to walk on ice and paddle in the water.
Polar bear Temporal range: Pleistocene–recent[1] | |
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Female near Kaktovik, Barter Island, Alaska, United States | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Mammalia |
Order: | Carnivora |
Family: | Ursidae |
Genus: | Ursus |
Species: | U. maritimus |
Binomial name | |
Ursus maritimus | |
Polar bear range | |
Synonyms | |
Ursus eogroenlandicus |
Polar bears are both terrestrial and pagophilic (ice-living) and are considered to be marine mammals due to their dependence on marine ecosystems. They prefer the annual sea ice but live on land when the ice melts in the summer. They are mostly carnivorous and specialized for preying on seals, particularly ringed seals. Such prey is typically taken by ambush; the bear may stalk its prey on the ice or in the water, but also will stay at a breathing hole or ice edge to wait for prey to swim by. The bear primarily feeds on the seal's energy-rich blubber. Other prey include walruses, beluga whales and some terrestrial animals. Polar bears are usually solitary but can be found in groups when on land. During the breeding season, male bears guard females and defend them from rivals. Mothers give birth to cubs in maternity dens during the winter. Young stay with their mother for up to two and a half years.
The polar bear is considered to be a vulnerable species by the International Union for Conservation of Nature (IUCN) with an estimated total population of 22,000 to 31,000 individuals. Its biggest threats are climate change, pollution and energy development. Climate change has caused a decline in sea ice, giving the polar bear less access to its favoured prey and increasing the risk of malnutrition and starvation. Less sea ice also means that the bears must spend more time on land, increasing conflicts with people. Polar bears have been hunted, both by native and non-native peoples, for their coats, meat and other items. They have been kept in captivity in zoos and circuses and are prevalent in art, folklore, religion and modern culture.
The polar bear was given its common name by Thomas Pennant in A Synopsis of Quadrupeds (1771). It was known as the "white bear" in Europe between the 13th and 18th centuries, as well as "ice bear", "sea bear" and "Greenland bear". The Norse referred to it as isbjørn 'ice bear' and hvitebjørn 'white bear'. The bear is called nanook by the Inuit. The Netsilik cultures additionally have different names for bears based on certain factors, such as sex and age: these include adult males (anguraq), single adult females (tattaq), gestating females (arnaluk), newborns (hagliaqtug), large adolescents (namiaq) and dormant bears (apitiliit).[5] The scientific name Ursus maritimus is Latin for 'sea bear'.[6][7]
Carl Linnaeus classified the polar bear as a type of brown bear (Ursus arctos), labelling it as Ursus maritimus albus-major, articus in the 1758 edition of his work Systema Naturae.[8] Constantine John Phipps formally described the polar bear as a distinct species, Ursus maritimus in 1774, following his 1773 voyage towards the North Pole.[4][9] Due to its adaptations to a marine environment, some taxonomists like Theodore Knottnerus-Meyer have placed the polar bear in its genus Thalarctos.[10][11] However Ursus is widely considered to be the valid genus for the species based on the fossil record and the fact that it can breed with the brown bear.[11][12]
Different subspecies have been proposed including Ursus maritimus maritimus and U. m. marinus.[lower-alpha 1][13] However these are not supported and the polar bear is considered to be monotypic.[14] One possible fossil subspecies, U. m. tyrannus, was posited in 1964 by Björn Kurtén, who reconstructed the subspecies from a single fragment of an ulna which was approximately 20 percent larger than expected for a polar bear.[12] However, re-evaluation in the 21st century has indicated that the fragment likely comes from a giant brown bear.[15][16]
Evolution
The polar bear is one of eight extant species in the bear family, Ursidae, and of six extant species in the subfamily Ursinae. A possible phylogeny of extant bear species is shown in a cladogram based on complete mitochondrial DNA sequences from Yu et al. (2007).[17] The polar bear and the brown bear form a close grouping, while the relationships of the other species are not very well resolved.[18]
Ursidae |
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A more recent phylogeny below is based on a 2017 genetic study. The study concludes that Ursine bears originated around 5 million years ago and show extensive hybridization of species in their lineage.[19]
Ursidae |
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Fossils of polar bears are uncommon.[12][15] The oldest known fossil is a 130,000- to 110,000-year-old jaw bone, found on Prince Charles Foreland, Norway, in 2004.[20][1] Scientists in the 20th century surmised that polar bears directly descended from a population of brown bears, possibly in eastern Siberia or Alaska.[12][15] Mitochondrial DNA studies in the 1990s and 2000s supported the status of the polar bear as a derivative of the brown bear, finding that some brown bear populations were more closely related to polar bears than to other brown bears, particularly the ABC Islands bears of Southeast Alaska.[20][21][22] A 2010 study estimated that the polar bear lineage split from other brown bears around 150,000 years ago.[20]
More extensive genetic studies have refuted the idea that polar bears are directly descended from brown bears and found that the two species are separate sister lineages. The genetic similarities between polar bears and some brown bears were found to be the result of interbreeding.[23][24] A 2012 study estimated the split between polar and brown bears as occurring around 600,000 years ago.[23] A 2022 study estimated the divergence as occurring even earlier at over one million years ago.[24] Glaciation events over hundreds of thousands of years led to both the origin of polar bears and their subsequent interactions and hybridizations with brown bears.[25]
Studies in 2011 and 2012 concluded that gene flow went from brown bears to polar bears during hybridization.[23][26] In particular, a 2011 study concluded that living polar bear populations derived their maternal lines from now-extinct Irish brown bears.[26] Later studies have clarified that gene flow went from polar to brown bears rather than the reverse.[25][27][28] Up to 9 percent of the genome of ABC bears was transferred from polar bears,[29] while Irish bears had up to 21.5 percent polar bear origin.[27] Mass hybridization between the two species appears to have stopped around 200,000 years ago. Modern hybrids are relatively rare in the wild.[24]
Analysis of the number of variations of gene copies in polar bears compared with brown bears and American black bears shows distinct adaptions. Polar bears have a less diverse array of olfactory receptor genes, a result of there being fewer odours in their Arctic habitat. With its carnivorous, high-fat diet the species has fewer copies of the gene involved in making amylase, an enzyme that breaks down starch, and more selection for genes for fatty acid breakdown and a more efficient circulatory system. The polar bear's thicker coat is the result of more copies of genes involved in keratin-creating proteins.[30]
The polar bear is the largest living species of bear and land carnivore, though some brown bear subspecies like the Kodiak bear can rival it in size.[31][32] Males are generally 200–250 cm (6.6–8.2 ft) long with a weight of 300–800 kg (660–1,760 lb). Females are smaller at 180–200 cm (5.9–6.6 ft) with a weight of 150–300 kg (330–660 lb).[10] Sexual dimorphism in the species is particularly high compared with most other mammals.[33] Male polar bears also have proportionally larger heads than females.[34] The weight of polar bears fluctuates during the year, as they can bulk up on fat and increase their mass by 50 percent.[31] A fattened, pregnant female can weigh as much as 500 kg (1,100 lb).[35] Adults may stand 130–160 cm (4.3–5.2 ft) tall at the shoulder. The tail is 76–126 mm (3.0–5.0 in) long.[10] 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.[36]
Compared with the brown bear, this species has a more slender build, with a narrower, flatter and smaller skull, a longer neck, and a lower shoulder hump.[31][37] The snout profile is curved, resembling a "Roman nose".[31] They have 34–42 teeth including 12 incisors, 4 canines, 8–16 premolars and 10 molars. The teeth are adapted for a more carnivorous diet than that of the brown bear, having longer, sharper and more spaced out canines, and smaller, more pointed cheek teeth (premolars and molars).[33][38][37] The species has a large space or diastema between the canines and cheek teeth, which may allow it to better bite into prey.[38][39] Since it normally preys on animals much smaller than it, the polar bear does not have a particularly strong bite.[39] Polar bears have large paws, with the front paws being broader than the back. The feet are hairier than in other bear species, providing warmth and friction when stepping on snow and sea ice.[40] The claws are small but sharp and hooked and are used both to snatch prey and climb onto ice.[41][42]
The coat consists of dense underfur around 5 cm (2.0 in) long and guard hairs around 15 cm (5.9 in) long.[10] Males have long hairs on their forelegs, which is thought to signal their fitness to females.[43] The outer surface of the hairs has a scaly appearance, and the guard hairs are hollow, which allows the animals to trap heat and float in the water.[44] The transparent guard hairs forward scatter ultraviolet light between the underfur and the skin, leading to a cycle of absorption and re-emission, keeping them warm.[45] The fur appears white due to the backscatter of incident light and the absence of pigment.[45][46] Polar bears gain a yellowish colouration as they are exposed more to the sun. This is reversed after they moult. It can also be grayish or brownish.[10] Their light fur provides camouflage in their snowy environment. After emerging from the water, the bear can easily shake itself dry before freezing since the hairs are resistant to tangling when wet.[47] The skin, including the nose and lips, is black and absorbs heat.[10][45] Polar bears have a 5–10 cm (2.0–3.9 in) thick layer of fat underneath the skin,[10] which provides both warmth and energy.[48] Polar bears maintain their core body temperature at about 36.9 °C (98 °F).[49] Overheating is countered by a layer of highly vascularized striated muscle tissue and finely controlled blood vessels. Bears also cool off by entering the water.[45][50]
The eyes of a polar bear are close to the top of the head, which may allow them to stay out of the water when the animal is swimming at the surface. They are relatively small, which may be an adaption against blowing snow and snow blindness. Polar bears are dichromats, and lack the cone cells for seeing medium, mainly green, wavelengths. They have many rod cells which allow them to see at night. The ears are small, allowing them to retain heat and not get frostbitten.[51] They can hear best at frequencies of 11.2–22.5 kHz, a wider frequency range than expected given that their prey mostly makes low-frequency sounds.[52] The nasal concha creates a large surface area, so more warm air can move through the nasal passages.[53] Their olfactory system is also large and adapted for smelling prey over vast distances.[54] The animal has reniculate kidneys which filter out the salt in their food.[55]
Polar bears inhabit the Arctic and adjacent areas. Their range includes Greenland, Canada, Alaska, Russia and the Svalbard Archipelago of Norway.[10][57][58] Polar bears have been recorded 25 km (16 mi) from the North Pole.[59] The southern limits of their range include James Bay and Newfoundland and Labrador in Canada and St. Matthew Island and the Pribilof Islands of Alaska.[10] They are not permanent residents of Iceland but have been recorded visiting there if they can reach it via sea ice.[60] Due to minimal human encroachment on the bears' remote habitat, they can still be found in much of their original range, more so than any other large land carnivore.[61]
Polar bears have been divided into at least 18 subpopulations labelled East Greenland (ES), Barents Sea (BS), Kara Sea (KS), Laptev Sea (LVS), Chukchi Sea (CS), northern and southern Beaufort Sea (SBS and NBS), Viscount Melville (VM), M'Clintock Channel (MC), Gulf of Boothia (GB), Lancaster Sound (LS), Norwegian Bay (NB), Kane Basin (KB), Baffin Bay (BB), Davis Strait (DS), Foxe Basin (FB) and the western and southern Hudson Bay (WHB and SHB) populations.[62][56] Bears in and around the Queen Elizabeth Islands have been proposed as a subpopulation but this is not universally accepted.[56] A 2022 study has suggested that the bears in southeast Greenland should be considered a different subpopulation based on their geographic isolation and genetics.[63] Polar bear populations can also be divided into four gene clusters: Southern Canadian, Canadian Archipelago, Western Basin (northwestern Canada west to the Russian Far East) and Eastern Basin (Greenland east to Siberia).[62]
The polar bear is dependent enough on the ocean to be considered a marine mammal.[14][64] It is pagophilic and mainly inhabits annual sea ice covering continental shelves and between islands of archipelagos. These areas, known as the "Arctic Ring of Life", have high biological productivity.[61][65] The species tends to frequent areas where sea ice meets water, such as polynyas and leads, to hunt the seals that make up most of its diet.[66] Polar bears travel in response to changes in ice cover throughout the year. They are forced onto land in summer when the sea ice disappears.[67] Terrestrial habitats used by polar bears include forests, mountains, rocky areas, lakeshores and creeks.[68] In the Chukchi and Beaufort seas, where the sea ice breaks off and floats north during the summer, polar bears generally stay on the ice, though a large portion of the population (15–40%) has been observed spending all summer on land since the 1980s.[69] Some areas have thick multiyear ice that does not completely melt and the bears can stay on all year,[70][71] though this type of ice has fewer seals and allows for less productivity in the water.[71]