Polar Bear
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Bear Conservation status: Lower risk
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| Ursus
maritimus Phipps, 1774 |
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The Polar Bear (Ursus maritimus), aka white bear aka northern bear aka sea bear, is a large bear native to the Arctic. It is the largest land carnivore species and the apex predator within its range. It is well-adapted to its habitat: its thick blubber and fur insulate it against the cold, its white colour camouflages it from its prey, and it hunts well both on land and in the water.
Natural range
The Polar Bear is a circumpolar species found in and around the Arctic Ocean whose southern range limits are determined 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 estimates are generally just over 20 000, with about 60% in Canada [1].
Their main population centers are:
- Wrangel Island and western Alaska
- Northern Alaska
- Canadian Arctic archipelago
- Greenland
- Svalbard- Franz Josef Land
- North-Central Siberia
Their range is limited by the availability of sea ice that they use as a platform to hunt seal, the mainstay of their diet. The destruction of its habitat on the Arctic ice by global warming threatens the bear's survival as a species; it may become extinct within the century. Signs of this have already been observed at the southern edges of its range [2]. There have been no proposals to date to transplant the species to some other environment, such as Antarctica.
Size and weight
It is the largest extant species of land predator (the Kodiak subspecies of the Brown Bear is generally larger), twice the weight of a tiger or lion. Adult males weigh from 400 to 600 kg (900 to 1300 lb) and occasionally exceed 800 kg (1750 lb). Females are about half the size of males and normally weigh 200–300 kg (450 - 650 lb). Adult males measure 2.4 to 2.6 m (7' 10" to 8' 6"); females, 1.9 to 2.1 m (6' 3" to 6' 11"). At birth, cubs weigh 600 to 700 g.
Subspecies
It is generally believed that there are no living Polar Bear subspecies [3]. In fact because "[p]olar bears bred with brown bears have produced fertile hybrids" [4] it can be argued that Polar Bears are a subspecies of Brown Bear.
The number of distinct populations depends on who is counting. The IUCN/SSC Polar Bear Specialist Group ("PBSG"), the preminent international scientific body for research and management relating to Polar Bears, currently recognizes twenty populations, or stocks, worldwide [5]. Other scientists [6] recognize six distinct populations, but no (living) subspecies:
- Chukchi Sea population on Wrangel Island and western Alaska
- Northern and northwestern Alaska and northwestern Canada (the Beaufort Sea population)
- Canadian Arctic archipelago
- Greenland
- Spitzbergen-Franz Josef Land
- Central Siberia
Other sources list these subspecies: Ursus maritimus maritimus [7] Ursus maritimus marinus [8].
Fur and skin
The nose and skin are black and the fur is white. The fur is good camouflage as well as insulation. Stiff hairs grow on the soles of its paws; these insulate and provide traction on ice.
Unlike other arctic mammals, Polar Bears never shed their coat for a darker shade in the summer. The individual hairs are transparent and unpigmented (like a single water drop in a cloud). The fur appears white rather than transparent for the same reasons as clouds, water foam, and cotton ( reflection and dispersion of visible light). The fur absorbs ultraviolet light. The hair does not have fiber-optic properties nor does it transmit light or heat to the skin (an urban legend). The coat does however insulate the bears to the point where they overheat at temperatures above 10°C (50°F) and are nearly invisible in the infrared; only their breath and muzzles can be seen [9].
In July 2005, several Polar Bears in the Brookfield Zoo turned green as a result of algae growing in their hollow hair tubes. Chicago experienced an extremely hot, humid and dry summer that year. However, the zoo took no action since it has been shown the algae does not negatively affect the bears in any way. The staff believed that exposing the bears to chlorine or bleach would be more harmful than letting the algae run its course. Previously, in February 2004, two Polar Bears in the Singapore Zoo turned green due to algae growth. A zoo spokesman said that the algae had formed as a result of Singapore's hot and humid conditions. The bears were washed in a peroxide blonde solution to restore their expected colour. A similar algae grew in the hair of three Polar Bears at San Diego Zoo in the summer of 1979. They were cured by washing the algae away in a salt solution.
Hunting, diet and feeding
It is the most completely carnivorous member of the bear family, and the one that is most likely to prey on humans as food. It feeds mainly on seals, especially Ringed Seals that poke holes in the ice to breathe (their success rate at catching the seals rarely goes beyond 5% of attempts, though), but will eat anything it can kill: birds, rodents, shellfish, crabs, Beluga Whales, walruses, occasionally Musk Oxen, and very occasionally other Polar Bears.
Polar Bears are excellent swimmers and can often be seen in open waters miles from land. This may be a sign that they have begun aquatic adaptations to better catch their prey, although recent evidence indicates that they may be drowning in the open sea because global warming is melting the ice shelves that previously made up their homes [10]. They also hunt very efficiently on land due to their prodigious speed; they are more than capable of outrunning a human.
As a carnivore feeding largely upon fish-eating carnivores, the Polar Bear ingests large amounts of Vitamin A, which is stored in its liver; in the past, humans have been poisoned by eating Polar Bear liver. Although largely carnivorous, they will also feed on carrion (e.g. beached whales) and also eat some vegetable matter, mainly berries and roots in the late summer. Perhaps somewhat predictably, they have also developed a liking for garbage; the dump in Churchill, Manitoba is frequently scavenged by Polar Bears, who have been observed eating (among other things) grease and motor oil.
Breeding
Polar Bears mate in the spring; pairing is temporary, only lasting for the mating with no permanent bond between males and females. The gestation period is 240 days (8 months), with the cubs born in early winter in a cave dug in October by the mother in deep snow. Usually, two cubs are born, less often one or three; litters of four cubs have been recorded. As with other Ursus bears, the cubs are very small at birth, typically 30 cm long and weighing 700 g. The cubs are born nearly helpless and blind, opening their eyes at about one month old, able to walk at 1.5 months, and weaned at 4-5 months. They remain with their mother, learning how to hunt and protect themselves against adult males (which have been known to cannibalize cubs), until 10 months old. Sexual maturity is reached at 3-4 years. Adult Polar Bears are known to live to over 30 years. Polar Bears do not hibernate, though the lactating females will not emerge from their cave while the cubs are very young. The mother will not have eaten for nine months, thus relying on stored body fat also known as blubber for both her own nutrition and that of the cubs.
Phylogeny
The Raccoon and bear families are believed to have diverged about 30 million years ago and around 13 million years ago the Spectacled Bear split from the other ursine bears. The 6 distinct ursine species originated some 6 million years ago. The Brown Bear and Polar Bear diverged from a common ancestor around 2 million years ago and crosses between the two species produce fertile hybrids. They only lost their Brown Bear molars between 10 and 20 thousand years ago.
Entertainment and commerce
Polar Bears have been made both controversial and famous for their distinctive white fur and their habitat. Companies like Coca-Cola, Nelvana, Bundaberg Rum and Good Humor-Breyers have used images of this bear in logos. Also, Polar Bears have appeared on the island featured in the TV drama Lost. Polar bears are known to be very curious and interesting animals to many scientists, who have been studying them for decades.