Polar Bear

The polar bear is a carnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. A boar (adult male) weighs around 350–700 kg, while a sow (adult female) is about half that size.

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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.

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.

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Photo: Polar bears play-fighting

 

Habitat

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The polar bear is often regarded as a marine mammal because it spends many months of the year at sea. 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.  Its preferred habitat is the annual sea ice covering the waters over the continental shelf and the Arctic inter-island archipelagos.

Biology and behavior

Physical characteristics

Polar_bear_swimming_in_zooPhoto:  Polar bear swimming

Polar bears are superbly insulated by up to 10 cm of blubber, their hide and their fur; they overheat at temperatures above 10 °C (50 °F). 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. 

The guard hair is 5–15 cm over most of the body.  Polar bears gradually moult from May to August, but, unlike other Arctic mammals, they do not shed their coat for a darker shade to camouflage themselves in the summer conditions.

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. Males have significantly longer hairs on their forelegs, that 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. 

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. 

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.  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). When sprinting, they can reach up to 40 km/h (25 mph). 

 

Hunting and diet

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Photo: The long muzzle and neck of the polar bear help it to search in deep holes for seals, while powerful hindquarters enable it to drag massive prey

The polar bear is the most carnivorous member of the bear family, and throughout most of its range, its diet primarily consists of seals. 

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. Polar bears hunt primarily at the interface between ice, water, and air; they only rarely catch seals on land or in open water. 

The polar bear’s most common hunting method is called still-hunting:  The bear uses its excellent sense of smell to locate a seal breathing hole, and crouches nearby in silence for a seal to appear.

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 and then crouches. If the seal does not notice, the bear creeps to within 9 to 12 m of the seal and then suddenly rushes forth to attack. 

A third hunting method is to raid the birth lairs that female seals create in the snow. 

 Photo: Polar bear at a whale carcass

Mature bears tend to eat only the calorie-rich skin and blubber of the seal, whereas younger bears consume the protein-rich red meat. Studies have also photographed polar bears scaling near-vertical cliffs, to eat birds’ chicks and eggs. 

For sub-adult 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.

Sub-adults 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. 

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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. Ringed seals are born weighing 5.4 kg and grown to an estimated average weight of only 60 kg. 

The bearded seal, on the other hand, can be nearly the same size as the bear itself, averaging 270 kg. Adult male bearded seals, at 350 to 500 kg are believed to be too large for a female bear to overtake, and so are prey only for mature male bears. 

Enormously powerful large males also occasionally attempt to hunt and kill even larger prey items. It can kill an adult walrus, although this is rarely attempted. At up to 2,000 kg , a walrus can be more than twice the bear’s weight,  and has up to 1-metre-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 adult walruses when their diving holes have frozen over or intercept them before they can get back to the diving hole in the ice.

Since an attack on a walrus tends to be an extremely protracted and exhausting venture, bears have been known to abandon the hunt after making the initial injury. 

Polar bears have also been seen to prey on beluga whales and narwhals, 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. 

Polar bears very seldom attack full-grown adult whales. 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. 

Picture: Some characteristic postures:
1. – at rest;
2. – at an estimated reaction;
3. – when feeding

 

Behavior

Photo: Polar bear males frequently play-fight

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.  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.


Conservation status, efforts and controversies

Picture: This map from the U.S. Geological Survey shows projected changes in polar bear habitat from 2001 to 2010 and 2041 to 2050. Red areas indicate loss of optimal polar bear habitat; blue areas indicate gain.

Estimates of the status of the global population of polar bears vary widely. As of 2008, the World Conservation Union (IUCN) reports that the global population of polar bears is 20,000 to 25,000, and is declining. 

In 2006, the IUCN upgraded the polar bear from a species of least concern to a vulnerable species  It cited a “suspected population reduction of >30% within three generations (45 years)”.  Risks to the polar bear include climate change, pollution in the form of toxic contaminants, conflicts with shipping, stresses from recreational polar-bear watching, and oil and gas exploration and development. The IUCN also cited a “potential risk of over-harvest” through legal and illegal hunting. 

According to the World Wildlife Fund, the polar bear is important as an indicator of arctic ecosystem health. Polar bears are studied to gain understanding of what is happening throughout the Arctic, because at-risk polar bears are often a sign of something wrong with the arctic marine ecosystem. 

 

Climate change

The IUCN, Arctic Climate Impact Assessment, United States Geological Survey and many leading polar bear biologists have expressed grave concerns about the impact of climate change, including the belief that the current warming trend imperils the survival of the species. 

The key danger posed by climate change is malnutrition or starvation due to habitat loss. Polar bears hunt seals from a platform of sea ice. Rising temperatures cause the sea ice to melt earlier in the year, driving the bears to shore before they have built sufficient fat reserves to survive the period of scarce food in the late summer and early fall. 

Reduction in sea-ice cover also forces bears to swim longer distances, which further depletes their energy stores and occasionally leads to drowning. Thinner sea ice tends to deform more easily, which appears to make it more difficult for polar bears to access seals. 

Insufficient nourishment leads to lower reproductive rates in adult females and lower survival rates in cubs and juvenile bears, in addition to poorer body condition in bears of all ages. 

Photo: Mothers and cubs have high nutritional requirements, which are not met if the seal-hunting season is too short.

In addition to creating nutritional stress, a warming climate is expected to affect various other aspects of polar bear life: Changes in sea ice affect the ability of pregnant females to build suitable maternity dens. 

As the distance increases between the pack ice and the coast, females must swim longer distances to reach favored denning areas on land. 

Thawing of permafrost would affect the bears who traditionally den underground, and warm winters could result in den roofs collapsing or having reduced insulative value. 

For the polar bears that currently den on multi-year ice, increased ice mobility may result in longer distances for mothers and young cubs to walk when they return to seal-hunting areas in the spring. 

Problematic interactions between polar bears and humans, such as foraging by bears in garbage dumps, have historically been more prevalent in years when ice-floe breakup occurred early and local polar bears were relatively thin. 

Increased human-bear interactions, including fatal attacks on humans, are likely to increase as the sea ice shrinks and hungry bears try to find food on land. 

 Photo: A polar bear swimming

Photo:  German stamp depicting Knut and the slogan “Preserve nature worldwide”

Oil and gas development

Oil and gas development in polar bear habitat can affect the bears in a variety of ways. An oil spill in the Arctic would most likely concentrate in the areas where polar bears and their prey are also concentrated, such as sea ice leads. 

Because polar bears rely partly on their fur for insulation and soiling of the fur by oil reduces its insulative value, oil spills put bears at risk of dying from hypothermia. Polar bears exposed to oil spill conditions have been observed to lick the oil from their fur, leading to fatal kidney failure. 

Maternity dens, used by pregnant females and by females with infants, can also be disturbed by nearby oil exploration and development. Disturbance of these sensitive sites may trigger the mother to abandon her den prematurely, or abandon her litter altogether.

Predictions

The U.S. Geological Survey predicts 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 climate change.  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. 

 

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