Animal Info - Polar Bear

(Other Names: 北极熊, БЕЛЫЙ МЕДВЕДЬ, Eisbär, Ice Bear, Isbjørn, Nanook, Nanuq, Nanuk, Oso Polar, Ours Blanc, Ours Polaire, Sea Bear, White Bear)

Ursus maritimus (Thalarctos m.)

Status: Vulnerable


Contents

1. Profile (Picture)
2. Tidbits
3. Status and Trends (IUCN Status, Countries Where Currently Found, Taxonomy, Population Estimates, History of Distribution, Threats (Major Threat, Climate Change, Pollution, Development Activities, Hunting))
4. Data on Biology and Ecology (Size and Weight, Habitat, Age to Maturity, Gestation Period, Birth Season, Birth Rate, Early Development, Dispersal, Maximum Reproductive Age, Maximum Age, Diet, Physical and Behavioral Characteristics (Feet, Senses, Temperature Control, Dens (Location, Construction), Movement (General, Navigation), Activity Patterns, Swimming, Hunting), Social Organization, Age and Gender Distribution, Mortality and Survival, Density and Range)
5. References


Profile

Pictures: Polar Bear #1 (Mom & Cub) (16 Kb JPEG) (Guangxi TV)Polar Bear #2 (Big Bear Coming) (14 Kb JPEG) (WWF/Polar Bear Tracker); Polar Bear #3 (Mom & 2 Cubs) (19 Kb JPEG) (Czech Web Site); Polar Bear #4 (Swimming Underwater) (35 Kb JPEG) (World Wildl. Fund); Polar Bear #5 (Three Bears Hanging Out) (46 Kb JPEG) (Vargarda Fotoklubb); Polar Bear #6 (2 Bears Sparring) (68 Kb JPEG) (Natl. Geog.); Polar Bear #7 (Mom & Cub) (110 Kb) (Duke Univ. - OBIS - SEAMAP) 

The polar bear has an elongated neck and a stocky body. It can be up to 285 cm (9.3') long, stand 160 cm (5.2') high at the shoulder, and weigh up to 800 kg (1800 lb). A polar bear is completely furred except for the tip of its nose and its foot pads. Polar bear fur appears white when it is clean and in normal daytime sunlight. However, because the fur actually has no pigment, a bear may take on the yellow-orange hues of the setting and rising sun. In spring and late winter, many polar bears are "off-white" or yellowish because of oils from their prey and other impurities that have attached to their fur.

For its habitat, the polar bear favors the perennial ice that lies adjacent to the continental arctic coastlines or island archipelagoes. Seals, which are hunted on the ice, constitute the majority of a polar bear's prey.  A female polar bear constructs a maternal den, usually consisting of a single chamber. A location is selected for the den where a pregnant female can create a snow cave early in winter and subsequently be covered over by snow. Although newborn polar bears are among the most undeveloped of mammal young, by the time a mother bear breaks out of her den in the spring, her cubs are ready to go to the sea ice with her to hunt seals. 

Most maternal denning appears to occur on the coastlines of mainlands and islands, although more than half of the dens in the Beaufort Sea region of Alaska and southwestern Canada are on offshore sea ice. A polar bear can swim for many hours while going from one piece of ice to another. Individual bears have been recorded swimming as far as 100 km (60 mi).  Polar bears are solitary animals throughout most of the year, with the exception of breeding pairs and mothers with their cubs. An adult bear occupies a multi-annual activity area. Annual activity areas of female polar bears in the Beaufort Sea region average 150,000 sq km (60,000 sq mi). 

The polar bear occurs throughout the Arctic. It is still found in most of its original range. Significant depletion of polar bear populations, due to over-hunting, occurred in the early and middle 1900's. As a result, an international agreement was reached in which the five nations with polar bears (Canada, Norway, USA, the former USSR and Denmark/Greenland) agreed to prohibit unregulated hunting and to outlaw the hunting of the bears from aircraft and icebreakers. This agreement and the resulting actions by the signatory nations were responsible for a significant recovery of the polar bear by the 1980's. 

The greatest threat to polar bears may be large scale ecological change resulting from climatic warming.  In some areas, contaminants may have an additive negative effect.  Widespread impacts from localized human activities or developments seem unlikely.  Although hunting can still pose a threat to the welfare of polar bears, it is maintained at sustainable levels in most jurisdictions by a combination of regulations and users’ agreements. 


Tidbits

*** The polar bear is the largest living land carnivore (Arkive 2004).

*** A polar bear is so well insulated that its body heat is virtually invisible to a heat sensor (Matthews 1999).

*** Inuit hunters report that the polar bear covers its black nose when stalking a seal or waiting for a seal to surface at a breathing hole. Scientists have yet to observe this behavior. (Stirling 1998)

*** Scientists determine the age of a polar bear by removing a small premolar tooth from behind the canine tooth while the bear is sedated. In the laboratory, the tooth is softened in acid, sliced into very thin sections, stained, and then the annual growth lines are counted (similar to counting the growth rings on a tree). (Stirling 1998) 

*** Scavenging on the remains of seals killed by polar bears is probably of great importance to the survival of arctic foxes  through the winter. These foxes have learned to follow the bears out on the sea ice during the winter. (DeMaster & Stirling 1981, Stirling 1998) 

*** Many of the characteristics of polar bears vary significantly from region to region.

*** Etymology of the scientific name of the polar bear: the genus name, "Ursus," is Latin for "bear"; the species name "maritimus" is Latin for "of the sea." 


Status and Trends

IUCN Status:

Countries Where the Polar Bear Is Currently Found:

2006: Occurs in Canada (Labrador; Manitoba; Newfoundland; Northwest Territories; Nunavut; Ontario; Quebec; Yukon), Greenland/Denmark, Norway (Svalbard & Jan Mayen Islands), Russia ((Krasnoyarsk; North European Russia; West Siberia; Yakutiya), and USA (Alaska). In addition, vagrants occasionally reach Iceland. (IUCN 2006)

Taxonomy:

Genetic analysis of polar bears from all over the Arctic has not shown any significant variations between populations from different areas (Stirling 1998).

Population Estimates:

[Note: Figures given are for wild populations only.]

History of Distribution:

The polar bear is found throughout the Arctic on ice-covered waters. There are no reports of polar bears in the vicinity of the North Pole itself. The northernmost location where they have been seen is about 160 km (100 mi) south of the North Pole, at 88°N latitude. The farthest south that polar bears live on a year-round basis is in James Bay (which is at about the same latitude as London) in Canada, where bears den at about 53°N latitude on Akimiski Island. They have also been recorded as far south as St. Mathew Island and the Pribilof Islands in the Bering Sea and Newfoundland in Canada, and they have occasionally been seen in the Gulf of St. Lawrence, Canada in years when heavy pack ice has been drifting farther to the south than normal (50°N latitude). (DeMaster & Stirling 1981, Polar Bear Spec. Gr. 2002, Arkive 2004. Polar Bear Spec. Gr. 2005) 

Significant depletion of polar bear populations, attributed to over-hunting, occurred in Greenland and the Soviet Arctic beginning in the 1930's. Polar bear populations in Alaska, USA declined in the late 1960s and early 1970s in response to excessive hunting with the use of aircraft. As a result of the population losses of polar bears due to over-hunting, an international agreement was reached between the five nations with polar bears (Canada, Norway, USA, the former USSR and Denmark, which governed Greenland at that time). These nations signed the "International Agreement on the Conservation of Polar Bears" in 1973, which prohibits unregulated hunting and outlaws the hunting of the bears from aircraft and icebreakers. The agreement also obligates each nation to protect polar bear denning sites and migration routes, as well as to undertake, and share information on, polar bear research. This agreement and the resulting actions by the signatory nations were responsible for the recovery of the polar bear. The total polar bear population is generally considered to have remained stable since the 1980's [but see "Threats - Climate Change" below]. (IUCN 1966, USFWS 1995Arkive 2004) 

Most of the original habitat of polar bears is still intact (although not protected) and uninhabited by man. The polar bear still occurs throughout most of its original range. (Lunn et al. 2002)

Studies using radio-tracking have demonstrated that polar bears are divided into relatively discrete groups ("stocks") in 20 different regions. Although there is considerable overlap in the regions occupied by members of these groups, the regions are thought to be ecologically meaningful, and the groups that occupy them are  managed as populations. The regions are (See Distribution Map #2): 1, Western Hudson Bay - WH; 2, Southern Hudson Bay - SH; 3, Foxe Basin - FB; 4, Lancaster Sound - LS; 5, Baffin Bay - BB; 6, Norwegian Bay - NW; 7, Kane Basin - KB; 8, Queen Elizabeth Islands - QE; 9, Davis Strait - DS; 10, Gulf of Boothia - GB; 11, McClintock Channel - MC; 12, Viscount Melville Sound - VM; 13, Northern Beaufort Sea - NB; 14, Southern Beaufort Sea - SB; 15, Chukchi Sea; 16, Laptev Sea; 17, Kara Sea or Novaya Zemlya; 18, Barents Sea or Svalbard; 19, East Greenland; 20, Arctic basin. (Amstrup 2004)  

Distribution Map #1 (27 Kb JPEG) (Light gray - approximate worldwide winter distribution of polar bears; hatched areas - known coastal regions preferred for maternal dens.) (Polar Bears Intl.)
Distribution Map #2 (Polar Bear Regions (and Population Estimates - place the cursor over a region)) (Polar Bear Spec. Grp 2006) 
Distribution Map #3 (78 Kb GIF) (San Diego Zoo Library) 

Threats 

Major Threat: Research in several geographic areas indicates that the greatest threat to polar bears may be large scale ecological change resulting from climatic warming, if the trend documented in recent years continues as projected. A recent review of the status of polar bears by the IUCN’s Polar Bear Specialist Group concluded that there is the likelihood of an overall decline in the size of the total polar bear population of more than 30% within the next 35 to 50 years. The principal cause of this projected decline is climatic warming and its consequent negative affects on the sea ice habitat of polar bears. On the basis of this review, the Polar Bear Specialist Group recommended that the IUCN Red List classification of the polar bear should be upgraded from "Least Concern" to "Vulnerable." This change was carried out in the 2006 IUCN Red List. (Polar Bear Spec. Gr. 2005, IUCN 2006) 

Climate Change: Temperatures have increased sharply in recent decades over most of the Arctic region. The average extent of sea ice cover in summer has declined by 15 - 20% over the past 30 years. A new analysis of the long-term polar bear subpopulation data base in Western Hudson Bay, Canada, confirms that the size of that subpopulation has declined from 1200 to less than 1000. The decline was apparently caused by reductions in condition and survival, especially of young bears, because climatic warming has caused the sea ice to break up about three weeks earlier now than it did 30 years ago. Thus, polar bears have less time to feed and to store the fat needed to survive on shore for four months before the ice re-freezes. Climate change is also likely to increase bear deaths directly. For example, increased frequency and intensity of spring rains is already causing some dens to collapse, resulting in the death of females and cubs. (Arctic Clim. Imp. Asst. 2004, Polar Bear Spec. Gr. 2005) 

Pollution: The Polar Bear Specialist Group also concluded that in some areas, contaminants may have an additive negative influence to the effects of climatic warming. Throughout the 1900s, numerous organic compounds were released into the global environment. Many of these compounds are resistant to physical as well as biological degradation and persist in the environment for extended periods. Their persistence allows these compounds to be spread by atmospheric and oceanic circulation, and many have concentrated in the Arctic. Polar bears, at the top of the marine food chain, can accumulate contaminants in their fat by eating ringed seals and other marine mammals which have absorbed the chemicals by eating contaminated species lower in the food chain. Many of the chlorinated compounds bond tightly to fat molecules. Polar bears are particularly vulnerable to these chemicals because they eat a fat rich diet. Seals comprise the main food of polar bears and the seal's blubber layer is preferentially eaten by the bears; consequently, the intake of pollutants is high. High levels of chlorinated compounds and heavy metals have been found in polar bears. Recent studies have explicitly linked contaminants to polar bear health. (Amstrup 2004, Arctic Clim. Imp. Asst. 2004, Polar Bear Spec. Gr. 2005, IUCN 2006) 

Development Activities: Polar bears show considerable tolerance of human activities near dens in the Beaufort Sea region. In this area, there is relatively intense industrial activity while polar bear populations have flourished. Polar bears seem secure in their dens, and appear to be tolerant of aerial and ground traffic very near maternal dens in winter and spring. There is evidence that management of human activities currently can eliminate most conflicts between those activities and maternal denning. (Truett & Johnson 2002, Amstrup 2004) 

On the other hand, oil development in the Arctic poses threats to polar bears, such as oil spills. It is probable that an oil spill in sea ice habitat would result in oil being concentrated in leads and between ice floes, resulting in both polar bears and their seal prey being directly exposed to oil. (IUCN 2006)

Hunting: Currently, although hunting can still pose a threat to the welfare of polar bears, it is maintained at sustainable levels in most jurisdictions by a combination of regulations and users’ agreements. This situation is mainly the result of the Agreement on the Conservation of Polar Bears and their Habitat, which was signed in 1973 by the 5 polar bear range countries (Canada, Norway, USA, the former USSR and Denmark/Greenland). The Agreement called for the restriction of hunting, the protection of habitat, and the carrying out of cooperative research on polar bears. In the jurisdictions where formal agreements or rules are lacking, the need for limits on the take are increasingly obvious and gaining acceptance. (Lunn et al. 2002, Amstrup 2004) 


Data on Biology and Ecology

Size and Weight:

Male adult polar bears weigh 300 - 800 kg (660 - 1800 lb) and have a total length of 200 - 285 cm (6.6 - 9.3') from tip of nose to tip of tail. Adult females weigh 150 - 300 kg (330 - 660 lb) and their total body length is 180 - 250 cm (5.9 - 8.2'). It is not meaningful to give average weights because there is a cline in size from Spitzbergen, where the bears are smallest, to the Bering Strait where they are largest. (DeMaster & Stirling 1981, USFWS 1995Amstrup 2004)

The height of adult polar bears at the shoulder varies from 130 - 160 cm (4.3 - 5.2'). (DeMaster & Stirling 1981)

Habitat:

The favored habitat of the polar bear is on the annual ice that lies adjacent to the continental arctic coastlines or island archipelagoes. Data collected from radio-collared polar bears have confirmed their close ties to the ice. One study in the Chukchi and Beaufort Seas showed that only 7% of weekly locations were on land. Most of those were bears occupying maternal dens for the winter. (Amstrup 2004) Polar bears generally avoid areas where the ice is very rough. Nor do they like very smooth ice with little drifted snow on the surface to hide behind near the breathing holes of seals. They also tend to avoid areas of multiyear ice such as that which characterizes much of the northerly portion of the polar basin, probably because the density of seals is very low there. On the ice, the bears are found near the edges of leads in the ice or in areas where the ice regularly cracks open because of the wind and currents and then refreezes. Seals are more abundant in these areas, and they are accessible to the bears when the seals surface to breathe in narrow cracks or at breathing holes in patches of thin ice that have just frozen over. (Stirling 1998) 

The polar bear is found in the Alaskan North Slope Coastal Tundra, Chukotsky Coastal Tundra, Low Arctic Tundra, Svalbard/Franz Joseph Land Marine Ecosystems, and Taimyr Coastal Tundra Global 200 Ecoregions. (Olson & Dinerstein 1998, Olson & Dinerstein 1999)

Age to Maturity:

Age of first reproduction is normally 5 - 6 years for females (IUCN 2006). Female polar bears in the Beaufort Sea usually give birth for the first time at age 6. In contrast, across many areas of Canada, females produce their first young at age 5, except for an average age of first breeding in the Hudson Bay area of 4.1 years.  (Amstrup 2004)  

Male polar bears become sexually mature around 4 - 5 years of age, but they usually do not mate successfully at that age. Several more years must pass for a bear to grow big, strong, and skillful enough to dominate his competitors. Most of the mating is probably done by males 8 - 10 years old and older. (Stirling 1998, Polar Bear Spec. Gr. 2005) 

Gestation Period:

Because the polar bear experiences delayed implantation, gestation (from conception to birth) is relatively long (195 - 265 days) (DeMaster & Stirling 1981)

Birth Season:

Breeding occurs in March to May. However, the polar bear experiences delayed implantation, and births do not occur until late November to mid-January. (IUCN 2006)

In one study of female bears in the Beaufort Sea area, most bears entered their dens in mid-November. Average dates were November 11 and 22 for land dens and pack ice dens, respectively. Female bears appeared to continue foraging right up to the time when they entered the den. Then they denned near where they happened to be foraging. On average, bears emerged from their dens with new cubs on March 26 if they were on the pack ice and on April 5 if they were on land. (Truett & Johnson 2002)

Birth Rate:

The average litter size is less than two (IUCN 2006). Twins are most common in polar bears and account for about 2/3 of the litters in all areas. Single cubs are the next most common, accounting for 20 - 30 % of litters. The occurrence of triplets varies, being more common in lower latitudes. For example, of 119 litters observed in the spring on the Manitoba coast of Hudson Bay, 20 % were single cubs, 67 % were twins, 12 % were triplets, and 1 litter had 4 cubs. In comparison, only 1 - 3 % of the litters observed in the spring on the southeastern coast of Baffin Island, Svalbard, the Canadian High Arctic, or the Beaufort Sea were triplets. (Stirling 1998)  Younger and older females often have only one cub, while 2 or 3 cubs may be born to females between the ages of about 8 and 20. (Polar Bear Spec. Gr. 2005) 

Female polar bears are available to breed again after weaning. Therefore, in most areas, the minimum successful reproductive interval for polar bears is 3 years. The time could be shorter if the cubs died before weaning. (Amstrup 2004) The average time between births, calculated from eight females who were captured and recaptured with different litters of cubs, was 3.1 years and ranged from 2 - 4 years. (DeMaster & Stirling 1981) It should be noted that the fact that most females only breed once every 3 years also implies that only about 1/3 of the females are available for reproduction in any particular year.

One study derived an estimate of the annual rate of production of female cubs by assuming that females between 6 and 18 years of age have an average litter size of 1.7 cubs, an equal number of male and female cubs on the average, and an average reproductive interval of 3.1 years. This resulted in an average annual rate of reproduction of 0.274 female cubs per year per adult female. (DeMaster & Stirling 1981) In another study, the average annual rate of production of cubs in the central Canadian Arctic Islands was estimated to be 0.47 cubs per year per adult female (Furnell & Schweinsburg 1984)

A female polar bear can usually produce only five litters in its lifetime. This is one of the slowest reproductive rates of any mammal. (USFWS 1995) 

Early Development:

A newborn polar bear is among the most undeveloped of mammal young. It weighs only 600 ­700 g (1.3 - 1.5 lb). It is blind, only lightly furred, and totally helpless. A cub grows very fast after birth. In Alaska, cubs average 13 kg (29 lb) on emergence from the den in late March or early April, with a maximum weight of 22 kg (48 lb) having been observed. Cubs continue to grow rapidly through their first summer on the sea ice, and some weigh over 100 kg (220 lb) as they approach 1 year of age. Cubs can double their weight between their first and second birthdays. (Amstrup 2004)  

A polar bear cub takes solid food at about 5 months of age. Usually it is weaned at about 24 - 28 months, but geographic differences in the age of weaning exist. In Western Hudson Bay, cubs may be weaned as yearlings. In other areas, some cubs are reported not to have been weaned until after they were 3 years old. (DeMaster & Stirling 1981, Burnie & Wilson 2001, Polar Bear Spec. Gr. 2005)  

By the time a mother bear breaks out of her den, the cubs are ready to go to the sea ice with her to hunt seals. Yearling and 2 year old polar bear cubs hunt about 4% and 7 % of the time respectively, while their mothers spend from 35 - 50 % of the time hunting. In one study, yearlings caught an average of one seal every 22 days of hunting, while their mothers caught one every 4 - 5 days. 2 year olds caught an average of one seal every 5 - 6 days of hunting. (Stirling 1998) 

Dispersal:

Polar bear cubs remain with their mothers for 1.5 - 2.5 years, depending on the region, and sometimes even longer. In some areas where the marine ecosystem is less productive they may remain with their mothers for 3.5 - 4.5 years. (Stirling 1998, Polar Bear Spec. Gr. 2002)

Maximum Reproductive Age:

The productivity of female polar bears, as measured by estimated pregnancy rates, remains high between 5 and 20 years of age and generally declines thereafter, although some females apparently are able to breed successfully throughout life. (Amstrup 2004) The maximum age of breeding in males has been estimated to be 19 years (DeMaster & Stirling 1981).

Maximum Age:

Polar bears have a normal life span of about 25 years for males and 30 years for females, although a small number of individuals may live longer (32 year old females have been aged in the wild). In captivity, there have been a number of individuals that have survived to be older than 40 years of age. (Stirling 1998, Polar Bear Spec. Gr. 2002)

Diet:

Polar bears feed primarily on ringed seals. Bearded seals are taken less often than ringed seals, but are important prey items. Polar bears also eat harp seals and hooded seals, and they scavenge on carcasses of caribou, musk-oxen, whale, walrus (usually pups) and seal. They occasionally eat mammals such as Svalbard reindeer and lemmings, as well as birds, eggs, lichens, moss, berries, grass and kelp. (DeMaster & Stirling 1981, Burnie & Wilson 2001, Polar Bear Spec. Gr. 2002) 

After a female polar bear mates in the spring, she has only a few short months in which to store away the large deposits of fat she will need to live on and support her new cubs after she enters her maternity den in the coming fall. A typical female, weighing 150 - 175 kg (330 - 385 lb), needs to gain at least 200 kg (440 lb) of fat to have a successful pregnancy. This time of the year is also when the seals the polar bear eats are most abundant and vulnerable to predation, especially the baby seals that have fattened up from their mother’s milk and have not learned to avoid predators. (Stirling 1998)  

When food is available, the polar bear has a remarkable ability to eat large amounts of food rapidly. On the other hand, one of the most significant adaptations of the polar bear to the uncertainties of food availability in the Arctic is its ability to slow down its metabolism (in order to conserve energy) after 7 - 10 days of not being able to feed, for whatever reason and at any time of year, until food becomes available again. (Polar Bear Spec. Gr. 2002, Arkive 2004) 

Physical and Behavioral Characteristics:

Feet: The polar bear has large forepaws which are used as paddles for swimming. Its feet are partially webbed and have long, non-retractable claws which dig into snow and ice. This helps to prevent the bear from slipping on snow and ice. The soles of its feet also have small projections and indentations which act like suction cups and help it to walk on ice without slipping. (Polar Bear Spec. Gr. 2002, Arkive 2004)   

Senses: The polar bear’s eyesight is about as good as that of a man, but its sense of smell is superb. It is thought that a polar bear can detect prey that is almost 1 km (0.6 mi) away and up to 1 m (3') under the compacted snow using its heightened sense of smell. (Stirling 1998, Arkive 2004) 

Temperature Control: The polar bear is well adapted to its arctic surroundings. Its thick winter coat, with glossy guard hairs and dense under-fur, and its thick layer of subcutaneous fat (5 - 10 cm (2 - 4") thick) protect it against the cold. Its guard hairs also shed water easily, so that after a swim a polar bear can shake itself off like a dog to decrease chilling and to dry itself fairly quickly. In some areas, such as the Hudson Bay region, polar bears go inland during the warmer weather and stay cool by digging caves or pits down into the permafrost. (DeMaster & Stirling 1981, Polar Bear Spec. Gr. 2002) 

Dens:

Location: Over much of their range, female polar bears den in a pattern where individual dens are scattered over broad reaches of habitat at low density. All denning areas (e.g. mountains, and coastal and river banks) have topographic characteristics that predictably catch snow in the autumn and early winter, where a pregnant female can create a snow cave early in winter and subsequently be covered over. Maternity dens are often dug on slopes of 20 - 40 deg where snow has accumulated to depths of 1 - 3 m (3.3 - 9.8'). Across the range of the polar bear, most denning, whether in concentrated areas or dispersed, occurs relatively near the coast (within 10 - 20 km (6 - 12 mi) of the coast). (DeMaster & Stirling 1981, Amstrup 2004) 

Although most maternal denning appears to occur on the coastlines of mainlands and islands, 53% of the dens in the Beaufort Sea region of Alaska and southwestern Canada are on offshore sea ice. The polar bears were captured in the Beaufort Sea and adjacent areas and followed to dens by radio-telemetry during 1981 - 1999. The sea ice is a less stable platform for denning than land. Bears that denned at sea drifted up to nearly 1000 km (620 mi) during the winter. Therefore, even if the den remained intact through the winter, the predictability of resources upon emergence of the female and her new cubs from their den was limited. The production of cubs from dens at sea, however, was not significantly different than that from dens on land. Polar bears in Alaska and adjacent Canada do not den in the same place each time they are pregnant. They usually used the same substrate (either land- or sea-ice) for denning from year to year but not the same geographic location. (Truett & Johnson 2002, Amstrup 2004) 

Construction: Female polar bears everywhere construct dens that are similar. Most common is a single chamber, which is slightly elevated from the entrance tunnel. This prevents the warmer air in the den from flowing out through the den entrance. Den measurements include single-room dens with average dimensions of about 2 x 1.5 m (6.6 x 5') and 1 m (3') high. The entrance tunnels are usually about 2 m (6.6') long but very narrow, about 0.65 m (2') in diameter. Some females dig 2- or 3-room dens. The cubs sometimes also dig small alcoves off to the side. (Stirling 1998) 

Measurements of the difference in air temperature between the inside and outside of two dens showed the air inside was 21 deg C (37 deg F) warmer in one den and 7.8 deg C (18 deg F) warmer in the other (Stirling 1998).

Movement:

General: Throughout the polar region, polar bears spend their summers concentrated along the edge of the persistent pack ice. Significant northerly and southerly movements appear to be dependent on seasonal melting and refreezing of ice near shore. In areas such as Hudson Bay, James Bay, and portions of the Canadian High Arctic, when the sea ice melts, polar bears are forced onto land for up to several months while they wait for winter and new ice. In winter, sea ice extends as much as 400 km (250 mi) south of the Bering Strait, which separates Asia from North America, and polar bears extend their range to the southernmost extreme of the ice. Sea ice disappears from most of the Bering and Chukchi Seas in summer, and polar bears occupying these areas may migrate as much as 1000 km (620 mi) to stay with the southern edge of the pack ice. (Amstrup 2004)  

The steady gait of a polar bear averages about 5.5 kph (3.5 mph), and it can continue for hours at a time (Stirling 1998). Polar bears do not normally like to run for long periods, but on a good surface a polar bear can reach a speed of 30 kph (20 mph) (Polar Bear Spec. Gr. 2002).

The total distance a polar bear travels during a year can be very large compared to that of most terrestrial mammals. This distance varies in different regions, presumably because of variation in patterns of prey availability and other sea ice characteristics. In the Beaufort Sea region, where polar bears have been followed by radio-telemetry for 20 years, total annual movements averaged 3415 km (2117 mi) and ranged up to 6200 km (3800 mi). The average annual distance moved by bears in the Chukchi Sea region was 5542 km (3436 mi). Movements of more than 50 km/day (30 mi/day) were observed. (Amstrup 2004)  Even females with cubs a few months old are capable of traveling more than 30 km/day (20 mi/day) for several days in a row (Stirling 1998).

Navigation: The occurrence of sea ice denning raises questions about the navigational capabilities of polar bears. Bears that den on sea ice may drift up to 1000 km (620 mi) during the winter while they are buried beneath the snow. No other animal is passively transported this far "in the blind." Thus, not only do polar bears range far and wide, but they can determine where they are and return to previously used areas after long distances of passive transport. How they accomplish this is unknown. (Truett & Johnson 2002, Amstrup 2004) 

Activity Patterns: Polar bears are most active in the first 1/3 of the day and least active in the last 1/3 of the day (DeMaster & Stirling 1981). A female polar bear with cubs spends 35 - 50 % of its time hunting (Stirling 1998).

Swimming: A polar bear can swim for many hours, at speeds of up to 10 kph (6 mph), while going from one piece of ice to another. It has a water-repellent coat and partially webbed feet, both of which are beneficial for swimming. The polar bear paddles with its large forepaws, the rear legs trailing as rudders. Its body fat and its coat’s hollow, air-filled guard hairs aid buoyancy. Individual bears have been recorded swimming as far as 100 km (62 mi), and polar bears are probably capable of swimming much farther if necessary. However, this uses up a significant amount of energy, so swimming such long distances is likely not done frequently. (Stirling 1998, Polar Bear Spec. Gr. 2002)

The longest a polar bear in the wild has been timed holding its breath while diving is 72 seconds (Polar Bear Spec. Gr. 2002). Polar bears are capable of diving under the ice and surfacing in the haul-out holes of seals (DeMaster & Stirling 1981). When diving, a polar bear's eyes remain open but its nostrils close as the bear holds its breath (Burnie & Wilson 2001).

When getting out of ice that is too thin to walk on, a polar bear spreads its feet far apart to reduce the pressure on the ice and keep from breaking through (Stirling 1998).

Hunting: A polar bear will remain on the sea ice all year, if possible, because it needs a platform from which to hunt. A polar bear catches very few seals in the open water. Therefore, when the edge of the ice retreats to the north during the summer, a bear will travel many miles to remain with the ice floes, where it has access to the seals. If a bear is stranded on land, it must remain there until the sea ice freezes again in the fall. When the new ice forms again in the fall, the polar bear quickly moves out onto it to resume hunting. (Stirling 1998) 

The polar bear’s chief prey - seals - are mainly caught by two hunting methods: In the "stalk", the bear moves slowly nearer to its prey, relying on the camouflage of its white coat, making effective use of available cover, and "freezing" if the seal looks up. It charges the last 15 - 30 m (50 - 100') at up to 55 kph (34 mph). In the "still-hunt", the bear waits motionless next to a seal’s breathing hole and grabs the prey as it surfaces. During the summer, a polar bear hunts seals by still-hunting 77 % of the time and by stalking 23 % of the time. During the winter and early spring, almost all hunting is done by still-hunting. (DeMaster & Stirling 1981, Burnie & Wilson 2001)

Another hunting method is based on the seal's use of its breathing holes during the winter and spring. As the winter progresses, drifting snow can accumulate over a breathing hole, removing all trace of it on the surface. Sometimes a seal will excavate a small haul-out lair, like a small snow cave, in the snow above one of its breathing holes. In the spring, pregnant females use these as birth lairs. Although there is no visible evidence on the surface of the lair's presence, the polar bear’s acute sense of smell enables it to detect a seal in one of these lairs beneath the snow, and the bear can smash through the snow to catch the seal or its pup. (Stirling 1998) 

Social Organization:

Polar bears are solitary animals throughout most of the year, with the exception of breeding pairs and mothers with their cubs. They have overlapping home ranges which are not defended (i.e. polar bears are not territorial). (Arkive 2004)  

Adult females with cubs of the year are known to avoid interaction with adult males, presumably because of potential predation by the males upon the cubs (DeMaster & Stirling 1981).

Age and Gender Distribution:

The age distribution of polar bears varies geographically. Adult males (age 6 and above) comprise 12 % of the Alaskan Arctic population, 18 % of the western Canadian Arctic population, and 17 % of the Hudson Bay population. Adult females comprise 26 %, 19 %, and 17 % of these three populations, respectively. Cubs of the year, yearlings, and 2 year olds constitute 32 % and 26% of the Alaskan Arctic and western Canadian Arctic populations, respectively. (DeMaster & Stirling 1981) 

The ratio of females to males at birth as well as among adults appears to be approximately 1:1. (DeMaster & Stirling 1981, Stirling 1998)  

Mortality and Survival:

The mortality rate of cubs in their first year of life varies between 20 - 40 % in different populations. The annual mortality of cubs prior to weaning is 10 - 30%. Subadult mortality has been estimated to be 3 - 16%. Mortality of adult males and females has been estimated to be 8 - 16%. Typically, an annual mortality rate of 8 - 12% has been assumed. Adult males have a higher annual mortality rate than females. (DeMaster & Stirling 1981, Stirling 1998)

In the Hudson Bay region during the 1980s, the survival rate of more than 200 cubs from spring through the ice-free period of autumn was 44%. Survival of Hudson Bay cubs (n > 400) from their first to their second autumn was 35%. (Amstrup 2004) 

Annual survival of Hudson Bay yearlings ranged from 43% to 53%. (Amstrup 2004) Data from Alaska, western Canadian Arctic, central Canadian Arctic, and Baffin Island produced survival estimates for yearlings, based on the difference in litter size between yearlings and 2-year-olds, that ranged between 70% and 75%.  An average survival rate for Alaskan Beaufort Sea bears one year and older is estimated at 88%. This is close to estimates of the survival rate for bears in the western Canadian Arctic and central Canadian Arctic. The annual survival rate of radio-collared adult female polar bears tracked via satellite has been estimated at 97%. (USFWS 1995) 

Density and Range:

Density

In Alaska, polar bear densities have been estimated to be between 1 bear/38 sq km (1 bear/15 sq mi) and 1 bear/139 sq km (1 bear/54 sq mi). In the western Canadian Arctic, where numbers experienced large fluctuations due to natural causes, the density of polar bears was estimated to be 1 bear/37 - 52 sq km (1 bear/14 - 20 sq mi). (DeMaster & Stirling 1981) 

Range

Telemetry data have proven that polar bears do not wander aimlessly on the ice, nor are they carried passively with the ocean currents as previously thought. Rather, they occupy multi-annual activity areas outside of which they seldom travel. Annual activity areas of female polar bears monitored by radio-tracking for multiyear periods varied among years. Radio-collared animals, however, seemed to use seasonally preferred or "core" regions every year, despite variation in annual activity area boundaries. This suggests that activity areas of polar bears, when viewed over multiyear periods, might be called home ranges. All areas of the home range, however, will not be used each year. In areas where ice coverage is subject to frequent change, a large multi-annual home range of which only a portion is used in any one season or year is an important part of the polar bear life history strategy. Annual activity areas/home ranges of 75 radio-collared female polar bears in the Beaufort Sea region averaged 149,000 sq km (58,000 sq mi). The average activity area/home range size for six bears in the Chukchi Sea was 244,000 sq km (94,000 sq mi). (Amstrup 2004) 


References

Amstrup 2004, Arctic Clim. Imp. Asst. 2004, Arkive 2004Burnie & Wilson 2001, Czech Web Site, DeMaster & Stirling 1981, Duke Univ. - OBIS - SEAMAP, Furnell & Schweinsburg 1984, Garner 1997, Guangxi TV, IUCN 1966, IUCN 1994, IUCN 1996, IUCN 2004, IUCN 2006, Lunn et al. 2002, Matthews 1999, Natl. Geog., Nowak & Paradiso 1983, Olson & Dinerstein 1998, Olson & Dinerstein 1999Polar Bear Spec. Gr. 2002, Polar Bear Spec. Gr. 2005, Polar Bear Spec. Gr. 2006, Polar Bears Intl., San Diego Zoo Library, Schliebe 2001, Schuhmacher 1967, Stirling 1998, Truett & Johnson 2002, USFWS 1995Vargarda Fotoklubb, Watson 1996, World Wildl. Fund, WWF/Polar Bear Tracker


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