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This article is about the animal. For other uses, see Polar bear (disambiguation).

Polar Bear
Scientific classification
Domain:
Eukarya
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
Ursus groenlandicus
Ursus jenaensis
Ursus labradorensis
Ursus marinus
Ursus polaris
Ursus spitzbergensis
Ursus ungavensis
Thalarctos maritimus

The polar bear (Ursus maritimus) is a bear native to the Arctic. Polar bears and Kodiak bears are the world's largest land carnivores, with most adult males weighing 300–600 kg (660–1320 lb); adult females are about half the size of males. Its fur is hollow and translucent, but usually appears as white or cream colored, thus providing the animal with effective camouflage. Its skin is actually black in color. Its thick blubber and fur insulate it against the cold. The bear has a short tail and small ears that help reduce heat loss, as well as a relatively small head and long, tapered body to streamline it for swimming.

A semi-aquatic marine mammal, the polar bear has adapted for life on a combination of land, sea, and ice,[2] and is the apex predator within its range.[3] It feeds mainly on seals, young walruses, and whales, although it will eat anything it can kill.

The polar bear is a vulnerable species at high risk of extinction. Zoologists and climatologists believe that the projected decreases in the polar sea ice due to global warming will reduce their population by two thirds by mid-century.[4][1][5][6] Local long-term studies show that 7 out of 19 subpopulations are declining or already severely reduced.[7][8] In the USA, the Center for Biological Diversity petitioned to list the polar bear as an endangered species under the U.S. Endangered Species Act in 2005.[9] The U.S. Fish and Wildlife Service formally proposed to list the polar bear as a threatened species on January 9, 2007. A final decision was due on January 9, 2008 but has been delayed.

Physiology

Size and weight

Polar bear compared to larger prehistoric carnivores

Polar bears rank with the Kodiak bear as among the largest living land carnivores, and male polar bears may weigh twice as much as a Siberian tiger. Males are generally 25–45% larger than females.[10] Most adult males weigh 350–650 kg (770–1500+ lb) and measure 2.5–3.0 m (8.2–9.8 ft) in length. Adult females are roughly half the size of males and normally weigh 150–250 kg (330–550 lb), measuring 2–2.5 m (6.6–8.2 ft), but double their weight during pregnancy.[11][12] The great difference in body size makes the polar bear among the most sexually dimorphic of mammals, surpassed only by the eared seals.[13] At birth, cubs weigh only 600–700 g or about a pound and a half. The largest polar bear on record was a huge male, allegedly weighing 1002 kg (2200 lb) shot at Kotzebue Sound in northwestern Alaska in 1960.[14] Their tail is 2.8–4.8 inches long.[10]

Fur and skin

A polar bear resting.

A polar bear's fur provides camouflage and insulation. Although the fur appears white, in fact the individual hairs are translucent, like the water droplets that make up a cloud; the coat may yellow with age. Stiff hairs on the pads of a bear's paws provide insulation and traction on the ice.

Polar bears gradually molt their hair from May to August;[15] however, unlike other Arctic mammals, polar bears do not shed their coat for a darker shade to camouflage themselves in the summer habitat. It was once conjectured that the hollow guard hairs of a polar bear coat acted as fiber-optic tubes to conduct light to its black skin, where it could be absorbed—a theory disproved by recent studies.[16]

An infrared image of a polar bear.

The thick undercoat does, however, insulate the bears: they overheat at temperatures above 10 °C (50 °F), and are nearly invisible under infrared photography; only their breath and muzzles can be easily seen.[11] When kept in captivity in warm, humid conditions, it is not unknown for the fur to turn a pale shade of green. This is due to algae growing inside the guard hairs — in unusually warm conditions, the hollow tubes provide an excellent home for algae. Whilst the algae is harmless to the bears, it is often a worry to the zoos housing them, and affected animals are sometimes washed in a salt solution, or mild peroxide bleach to make the fur white again.

The guard hair is 5–15 cm over most of the body of polar bears.[17] However, in the forelegs, males have significantly longer, increasing in length until 14 years of age. The ornamental foreleg hair is suggested as a form of an attractive trait for females, likened to the lion mane.[13]

Allen's rule

The polar bears ears and tail are smaller than other bears, and its legs are stocky, as expected from Allen's rule for a northerly animal. Its feet are very large, however, presumably to distribute load like snowshoes when walking on snow or thin ice.

Disease

The bears sometimes have problems with various skin diseases with dermatitis caused sometimes by mites or other parasites. The bears are especially susceptible to Trichinella, a parasitic roundworm they contract through cannibalism.[18] Sometimes excess heavy metals have been observed, as well as ethylene glycol (antifreeze) poisoning. Bears exposed to oil and petroleum products lose the insulative integrity of their coats, forcing metabolic rates to dramatically increase to maintain body heat in their challenging environment. Bacterial Leptospirosis, rabies and Morbillivirus have been recorded. Interestingly, the bears are thought by some to be more resistant than other carnivores to viral disease.[citation needed] The pollutant effect on the bears' immune systems, however, may end up decreasing their ability to cope with the naturally present immunological threats it encounters, and in such a challenging habitat even minor weaknesses can lead to serious problems and quick death.

Taxonomy and evolution

Speciation

The ursidae family is believed to have differentiated from other Carnivorans about 38 million years ago. The ursinae genus originated some 4.2 million years ago. According to both fossil and DNA evidence, the polar bear diverged from the brown bear roughly 200 thousand years ago. The oldest known polar bear fossil is less than 100 thousand years old. Fossils show that between 10 and 20 thousand years ago, the polar bear's molar teeth changed significantly from those of the brown bear.

However, more recent genetic studies have shown that some clades of Brown Bear are more closely related to polar bears than to other brown bears, meaning that the polar bear is not a true species according to some species concepts.[19] In addition, polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids,[20][21] indicating that they have only recently diverged and are not yet truly distinct species. But neither species can survive long in the other's niche, and with distinctly different morphology, metabolism, social and feeding behaviors, and other phenotypic characters, the two bears are generally classified as separate species.

A comparison of the DNA of various brown bear populations showed that the brown bears of Alaska's ABC islands shared a more recent common ancestor with polar bears than with any other brown bear population in the world.[22] Polar bears still have vestigial hibernation induction trigger in their blood, but they do not hibernate in the winter as the brown bear does. Only female polar bears enter a dormant state referred to as "denning" during pregnancy, though their body temperature does not decrease during this period as it would for a typical mammal in hibernation.[11][23]

Subspecies and Subpopulations

A polar bear in Churchill, Manitoba

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 S. Pallas in 1776.[24] This distinction has since been invalidated. The IUCN/SSC Polar Bear Specialist Group (PBSG), the pre-eminent international scientific body for research and management of polar bears, recognizes only one species distributed in nineteen discrete subpopulations across five countries.[7][8]

  1. Canadian Arctic Archipelago
  2. Greenland, Denmark
  3. Svalbard, Norway
  4. Central Siberia and Franz-Josef Land, Russia
  5. Alaska, USA

The 19 subpopulations show seasonal fidelity to geographic areas, but DNA studies show significant interbreeding among them.[25]

Habitat

Three polar bears investigate the submarine USS Honolulu 280 miles (450 km) from the North Pole.
Mother and two cubs climbing up Guillemot Island, Ukkusiksalik National Park.

Though it spends time on land and ice, the polar bear is regarded as a marine mammal due to its intimate relationship with the sea.[11] The circumpolar species is found in and around the Arctic Ocean, its southern range limited by pack ice. Their southernmost point is James Bay in Canada. While their numbers thin north of 88 degrees, there is evidence of polar bears all the way across the Arctic. Population is estimated to be between 20,000 to 25,000.[8]

The main population centers are:

Their range is limited by the availability of sea ice which they use as a platform for hunting seals, the mainstay of their diet. Seals and polar bears tend to gather around fissures in the ice called polynyas.[26] The destruction of its habitat on the Arctic ice threatens the bear's survival as a species.[5][27][28][9][29]

File:Churchill-polar-bears.jpg
Tourists watching Polar Bears from a "tundra buggy" near Churchill, Manitoba.

The most severe and topically recognized threats to the polar bear are the drastic changes taking place in their natural habitat, which is literally melting away due to global warming.[6][30] The United States Geological Survey, for example, in November 2006, stated that the Arctic shrinkage in the Alaskan portion of the Beaufort Sea has led to a higher death rate for polar bear cubs.[31]

A 1999 study by scientists from the Canadian Wildlife Service of polar bears in the Hudson Bay showed that global warming is threatening polar bears with starvation. Rising temperatures cause the sea-ice from which the bears hunt to melt earlier in the year, driving them to shore weeks before they have caught enough food to survive the period of scarce food in the late summer and early fall[32] and leading to a 22% decline in the local subpopulation.[33]

There is a photographically confirmed case from the beginning of the 20th century of a Svalbard polar bear drifting on ice as far south as the northern coast of the Norwegian mainland. It was found and killed near the village of Berlevag. More recent sightings in Berlevag, including one in the summer of 2005, remain unconfirmed.[citation needed]

Behavior

Polar bears are enormous, aggressive, curious, and potentially dangerous to humans. Wild polar bears, unlike most other bears, are barely habituated to people and will quickly size up any animal they encounter as potential prey. Males are normally solitary except for mating season, and females are usually social towards one another. Despite a recurring internet meme that all polar bears are left-handed,[34][35] there is no scientific evidence to support such a contention. Researchers studying polar bears have failed to find any evidence of left-handedness in all bears and one study 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.[36]

Hunting, diet and feeding

The polar bear is the most carnivorous member of the bear family. It feeds mainly on seals, especially ringed seals that poke holes in the ice to breathe,[37] but will eat anything it can kill: birds, eggs, rodents, shellfish, crabs, beluga whales, walrus calves, muskox, reindeer, and other polar bears. Although carnivorous, they have been observed to eat plants, including berries, roots, and kelp, however these do not form a significant part of their diet.[38] Its biology is specialized to digest fat from marine mammals and cannot derive much nutrition from terrestrial food.[39][40] Most animals can easily outrun a polar bear on the open land or in the open water, and polar bears overheat quickly: thus the polar bear subsists almost entirely on live seals and walrus calves taken at the edge of sea-ice in the winter and spring, or on the carcasses of dead adult walruses or whales. They live off of their fat reserves through the late summer and early fall when the sea-ice is at a minimum.[11] They are enormously powerful predators, but they rarely kill adult walruses, which are twice the polar bear's weight, although such an adult walrus kill has been recorded on tape.[41][38] Humans are the only regular predators of polar bears, although the bears have occasionally been found in the stomachs of orcas.[42] As a carnivore which feeds largely upon fish-eating carnivores, the polar bear ingests large amounts of vitamin A, which is stored in their livers. The resulting high concentrations make the liver poisonous to humans, causing Hypervitaminosis A.[43]

A polar bear diving in a zoo.

Polar bears are excellent swimmers and have been seen in open Arctic waters as far as 60 miles (100 km) from land. In some cases they spend half their time on ice floes. Their 12 cm (5 in) layer of fat adds buoyancy in addition to insulating them from the cold. Recently, polar bears in the Arctic have undertaken longer than usual swims to find prey, resulting in four recorded drownings in the unusually large ice pack regression of 2005.[44]

Polar bears, being both curious and scavengers[45][38] will, where they come into contact with humans, investigate and consume garbage.[38] This has been documented at the dump in Churchill, Manitoba prior to its closure.[46] 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.[38][45] To protect the bears, the Churchill dump was closed in 2006. Garbage is now recycled or transported to Thompson, Manitoba.[47]

Polar bears accumulate high levels of artificial halocarbons such as PCBs and pesticides because of their diet. Their position at the top of the food pyramid tends to concentrate pollutants, particularly halocarbons because of their lipophilicity: halocarbons are soluble in the blubber which makes up the bulk of the polar bear's diet. Halocarbons are known to be toxic to other animals because they mimic hormone chemistry, and biomarkers such as immunoglobulin G and retinol suggest similar effects on polar bears. The overall significance to population health is uncertain because of unique features of polar bear biology such as summertime fasting. PCBs have received the most study, and they have been associated with birth defects and immune system deficiency.[48] Polar bears in Svalbard have the highest concentrations of PCBs, and biologists suggest this may explain the high incidence of hermaphroditic bears in the area.[49]

The relevant chemicals have been classified as persistent organic pollutants by the UN, with the aim of discouraging their production. The most notorious of these, PCBs, DDT and other, have been banned, but their concentrations in polar bear tissues continued to rise for decades as these chemicals spread upwards on the food pyramid. The most recent data now indicates a decreasing trend.[50]

Breeding

Mother with cub at Svalbard
A mother and cubs in Churchill, Manitoba

Polar bears mate in April and May over a one-week period, which is needed to induce ovulation. The fertilized egg then remains in a suspended state until August or September. During these four months, the females eat prodigious amounts of food in preparation for pregnancy, often more than doubling their body weight. When food becomes scarce in August because of ice breakup, they dig a maternity den in a snow drift and enter a dormant state similar to hibernation. In areas where food is available year-round, they may not enter a den until October. Cubs are born in December without awakening the mother. She remains dormant while nursing her cubs until March, when the family emerges from the den. The biggest threat to cubs are strange male bears, who will kill cubs to make the female sexually receptive. Cubs are weaned at two or three years of age and are separated from their mother. Sexual maturity typically comes at the age of four, but may be delayed by up to two years.[11] In the 1990s, less than 20% cubs in the Western Hudson Bay were weaned at eighteen months, as opposed to 40% of cubs in the early 1980s.[32]

In Alaska, the United States Geological Survey reports that 42% of cubs now reach 12 months of age, down from 65 percent 15 years ago.[31] In other words, less than two of every three cubs that survived 15 years ago are now making it past their first year.

The USGS has also published research which purports that the percentage of Alaskan polar bears that den on sea ice has changed from 62% between the years 1985–1994, to 37% over the years 1998–2004. The Alaskan population thus now more resembles the world population, in that it is more likely to den on land.[51]

Conservation status

Projected change in polar bear habitat from 2001–2010 to 2041–2050. From USGS

The World Conservation Union listed polar bears as a vulnerable species, one of three sub-categories of threatened status, in May 2006.[52] Their latest estimate is that 7 out of 19 subpopulations are declining or already severely reduced.[7] The United States Geological Survey forecasts that two-thirds of the world's polar bears will disappear by 2050, based on moderate projections for the shrinking of summer sea ice caused by global warming.[4] The bears would disappear from Europe, Asia, and Alaska, and be depleted from the Arctic archipelago of Canada and areas off the northern Greenland coast. By 2080, they would disappear from Greenland entirely and from the northern Canadian coast, leaving only dwindling numbers in the interior Arctic archipelago.[4]

Global warming has already had an impact on polar bear population health and size. Recent declines in polar bear numbers can be linked to the retreat of sea ice and its formation later in the year. Ice is also breaking up earlier in the year, forcing bears ashore before they have time to build up sufficient fat stores, or forcing them to swim long distances, which may exhaust them, leading to drowning.[44] The results of these effects of global warming have been thinner, stressed bears, decreased reproduction, and lower juvenile survival rates.[53]

Polar bear

Because of the inaccessibility of the Arctic, there has never been a comprehensive global survey of polar bears, making it difficult to establish a global trend. The earliest preliminary estimates of the global population were around 5,000–10,000 in the early 1970s, but this was revised to 20,000–40,000 in the 1980s.[11] Part of this increase may indicate recovery as a result of conservation measures implemented in the early 1970s, but it is principally a revised estimate based on a growing base of data.[11] Current estimates bound the global population between 20,000-25,000.[7] Long-term studies of local populations of polar bears show they have been shrinking in the Western Hudson Bay and Baffin Bay areas, and are under stress in the Southern Beaufort Sea area.[8][9] In the Western Hudson Bay in Canada, for example, there were an estimated 1194 polar bears in 1987, and 935 in 2004[33] – though there were an estimated 500 polar bears there in 1981.[54]

The need for species' protection has been disputed by H. Sterling Burnett and Mitchell K. Taylor. Burnett, a Senior Fellow of the conservative advocacy group National Center for Policy Analysis, has claimed that the total global population of polar bears increased from 5,000 to 25,000 between the 1970s and 2007.[55] Mitchell Taylor, the Nunavut Government Manager of Wildlife Research, wrote a letter to the U.S. Fish and Wildlife Service arguing that local studies are insufficient evidence for global protection at this time.[56] Even critics of these positions stipulate that the population of polar bears "might appear secure" but argue that such appearances are superficial and more study is required.[57][58]

First polar bear shot in the S. A. Andrée's Arctic balloon expedition of 1897.

Hunters from around the Arctic have harvested hundreds of polar bears annually since at least the 18th century.[11] The harvest grew rapidly in the 1960s, peaking around 1968 with a global total of 1,250 bears that year.[59] Although the polar bear was not deemed endangered at the time, the growing threat encouraged countries to regulate polar bear hunting around that time. Norway passed a series of increasingly strict regulations from 1965 to 1973. Canada began imposing hunting quotas in 1968. The U.S. began regulating in 1971 and adopted the Marine Mammal Protection Act in 1972. In 1973, the International Agreement on the Conservation of Polar Bears (known as the Oslo Agreement among experts) was signed by the five nations whose Arctic territory is inhabited by polar bears: U.S., Canada, Norway, Denmark (via its territory Greenland) and Russia (then the Soviet Union). Although the agreement is not enforceable in itself, member countries agreed to place restrictions on recreational and commercial hunting, completely ban hunting from aircraft and icebreakers), and conduct further research.[60][61] The treaty allows hunting "by local people using traditional methods," although this has been liberally interpreted by member nations. All nations except Norway allow hunting by the Inuit, and Canada and Denmark allow trophy hunting by tourists.[citation needed]

Canada

Play fight

About 60% of the world's polar bears live in Canada,[8] where conservation laws are a provincial jurisdiction. Hunting quotas and restrictions relating to Indian status are in effect, but vary by province. About 500 bears are killed per year by humans across Canada,[62] a rate believed by scientists to be unsustainable in some areas, notably Baffin Bay.[8] Canada has allowed recreational hunters accompanied by local guides and dog-sled teams since 1970,[63] but the practice was not common until the 1980s.[64] Conservation initiatives conflict with northern resident's income from fur trade and recreational hunting, which can bring in $20,000 to $35,000 Canadian dollars per bear, mostly from American hunters.[65] Inuit are skeptical of conservation concerns because of increases in bear sightings near settlement in recent years.[5]

The territory of Nunavut accounts for 80% of Canadian kills.[62] Their government has condemned the American initiative to grant threatened status to polar bears,[66] and northern residents are strongly concerned about it.[67] In 2005, the government of Nunavut increased the quota from 400 to 518 bears,[68] despite protests from some scientific groups.[69] While most of that quota is hunted by the indigenous Inuit people, a growing share is sold to recreational hunters. (0.8% in the 1970s, 7.1% in the 1980s, and 14.6% in the 1990s)[64] Nunavut polar bear biologist, M.K. Taylor, who is responsible for polar bear conservation in the territory, insists that bear numbers are being sustained under current hunting limits.[56]

The Government of the Northwest Territories maintain their own quota of 72–103 bears within the Inuvialuit communities of which some are set aside for sports hunters.

United States

Polar bears at the Detroit Zoo

Because many marine mammal populations had plummeted due to over-hunting, the United States passed the federal Marine Mammal Protection Act in 1972, which prohibited the harassment, injuring or killing of all marine mammal species, including polar bears. This prohibited the importation of polar bear trophies into the U.S. by sport hunters.[70]

In 1994, the United States modified the Marine Mammal Protection Act, allowing the importation of sport-hunted polar bear trophies into the country and clearing the way for an increase in polar bear hunting. Since 1994, more than 800 sport-hunted polar bear trophies have been imported into the U.S.[71] In May 2007, legislation was introduced in both houses of the United States Congress (H.R. 2327, called the Polar Bear Protection Act) to reverse the 1994 legislation and ban the importation of dead polar bears.[72] On June 27, this legislation was defeated in congress and not passed.[73]

In February 2005, the Center for Biological Diversity petitioned the United States Fish and Wildlife Service to list the bears as an endangered species under the Endangered Species Act.[9][74] The agency did not respond to the petition, despite being required to do so within 90 days.[74] On 14 December 2006 the Center for Biological Diversity, Greenpeace and the Natural Resources Defense Council filed a successful lawsuit to compel a decision.[75]

On January 9, 2007, the Fish and Wildlife Service formally proposed to list the polar bear as a threatened species. A final decision was due on January 9, 2008, but has been delayed. It is expected to be completed in early 2008. The proposed listing did not include a proposal to designate critical habitat areas for the polar bear, thus the final rule will necessarily forgo critical habitat as well. The Center for Biological Diversity has announced it will continue litigation to obtain critical habitat and to seek endangered status which provides more protection than threatened status.

If listed, the polar bear would be only the third species protected under the Endangered Species Act due to global warming. The elkhorn coral and staghorn coral were listed as endangered species on May 9, 2006 due to separate petition from the Center for Biological Diversity.

Russia

Russia declared a complete protection in 1955,[11] but allows hunting by the indigenous people on the basis that it is part of their culture. It signed the Agreement between the Government of the United States of America.

Denmark

Until 2005, Greenland placed no limit on hunting by indigenous people. In 2005, it imposed a limit of 150 for 2006. It also allowed recreational hunting for the first time.[76]

Norway

  • Since 1973, Norway has had a complete ban on polar bear hunting.

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See also

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References

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  2. ^ Gunderson, Aren (2007). "Ursus Maritimus". Animal Diversity Web. University of Michigan Museum of Zoology. Retrieved 2007-10-27.
  3. ^ Polar Bears International
  4. ^ a b c Amstrup, Steven C.; Marcot, Bruce G.; Douglas, David C. (2007), Forecasting the Range-wide Status of Polar Bears at Selected Times in the 21st Century (PDF), Reston, Virginia: U.S. Geological Survey, retrieved 2007-09-29
  5. ^ a b c Stirling, Ian (2006). "Possible Effects of Climate Warming on Selected Populations of Polar Bears (Ursus maritimus) in the Canadian Arctic" (PDF). Arctic. 59 (3): pp. 261-275. ISSN 0004-0843. Retrieved 2007-09-15. {{cite journal}}: |pages= has extra text (help); Cite has empty unknown parameters: |laysource=, |laydate=, and |laysummary= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
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  7. ^ a b c d Compiled and edited by Jon Aars, ed. (2005). "Status of the Polar Bear" (PDF). Proceedings of the 14th Working Meeting of the IUCN/SSC Polar Bear Specialist Group. Polar Bears. Vol. 32. Nicholas J. Lunn and Andrew E. Derocher. Gland, Switzerland: IUCN. pp. pp. 33-55. ISBN 2-8317-0959-8. Retrieved 2007-09-15. {{cite conference}}: |editor= has generic name (help); |pages= has extra text (help); External link in |conferenceurl= (help); Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |conferenceurl= ignored (|conference-url= suggested) (help); Unknown parameter |month= ignored (help) See also HTML excerpts: population status reviews and Table 1 summarizing polar bear population status per 2005.
  8. ^ a b c d e f Polar Bears and Conservation and "Polar Bear FAQ". Polar Bears International. Retrieved 2007-09-15.
  9. ^ a b c d Siegel, Kassie; Cummings, Brendan (2005), Petition to List the Polar Bear (Ursus maritimus) as a Threatened Species Under the Endangered Species Act (PDF), Before the Secretary of the Interior: Center for Biological Diversity, retrieved 2007-09-08 {{citation}}: More than one of |location= and |place= specified (help) See also the Center's website on the issue.
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  12. ^ Stirling makes no mention of length, these are from SeaWorld
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  15. ^ Kolenosky G. B. 1987. Polar bear. pp. 475–485 in Wild furbearer management and conservation in North America (M. Novak, J. A. Baker, M. E. Obbard, and B. Malloch, eds.). Ontario Fur Trappers Association, North Bay, Ontario, Canada.
  16. ^ Koon, Daniel W. (1998), "Is Polar Bear Hair Fiber Optic?", Applied Optics, vol. 37, no. 15, Optical Society of America, pp. pp. 3198-3200, ISSN 0003-6935 {{citation}}: |pages= has extra text (help).
  17. ^ Uspenskii, S. M. (1977). The Polar Bear. Moscow: Nauka.
  18. ^ Larsen, Thor; Kjos-Hanssen, Bjørn (October 1983), Goldman, Helle V. (ed.), "Trichinella sp. in polar bears from Svalbard, in relation to hide length and age", Polar Research, vol. 1, no. 1, Oslo: Norwegian Polar Institute, pp. pp. 89-96, doi:10.1111/j.1751-8369.1983.tb00734.x, ISSN 0800-0395 {{citation}}: |pages= has extra text (help).
  19. ^ Marris, E. 2007. Nature 446, 250-253. Linnaeus at 300: The species and the specious
  20. ^ DeMaster, Douglas P.; Stirling, Ian (May 8, 1981), "Ursus Maritimus", Mammalian Species, vol. 145, American Society of Mammalogists, pp. pp. 1-7, doi:10.2307/3503828, ISSN 0076-3519, OCLC 46381503, retrieved 2008-01-21 {{citation}}: |pages= has extra text (help)CS1 maint: date and year (link).
  21. ^ Schliebe, Scott; Evans, Thomas; Johnson, Kurt; Roy, Michael; Miller, Susanne; Hamilton, Charles; Meehan, Rosa; Jahrsdoerfer, Sonja (December 21, 2006), Range-wide Status Review of the Polar Bear (Ursus maritimus) (PDF), Anchorage, Alaska: U.S. Fish and Wildlife Service, retrieved 2007-10-31{{citation}}: CS1 maint: date and year (link).
  22. ^ Lisette P. Waits, Sandra L. Talbot, R.H. Ward and G. F. Shields (1998). "Mitochondrial DNA Phylogeography of the North American Brown Bear and Implications for Conservation". Conservation Biology. pp. pp. 408-417. {{cite web}}: |pages= has extra text (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  23. ^ Bruce, D. S.; Darling, N. K.; Seeland, K. J.; Oeltgen, P. R.; Nilekani, S. P.; Amstrup, S. C. (March 1990), "Is the polar bear (Ursus maritimus) a hibernator?: Continued studies on opioids and hibernation", Pharmacology Biochemistry and Behavior, vol. 35, no. 3, pp. pp. 705-711, ISSN 0091-3057 {{citation}}: |pages= has extra text (help).
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