Body length: 220 to 250 cm (males); 180 to 210 cm (females)
Average weight:300 to 800 kg (661 to 1,764 lbs) for males; 150 to 450 kg (331 to 992 lbs) for females
Lifespan: Wild: 20–30 years; Captivity: up to 42 years
Distribution: Circumpolar distribution (Arctic and subarctic) of Canada, Russia, Greenland (Denmark), Norway, and the United States of America
Status: Vulnerable (IUCN), Appendix II (CITES)
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Ursidae
Subfamily: Ursinae
Genus: Ursus
Species: Ursus maritimus Phipps, 1774
English: Polar bear
French: Ours polaire, Ours blanc
Spanish: Oso polar
Russian: Bélyj medvédj (бе́лый медве́дь)
Danish: Isbjørn
Norwegian: Isbjørn
Inuit: Nanook
Yupik: Nanuuk
Chukchi: Umka
Cree: Wabusk, Wapusk
Polar bears are the largest and most sexually dimorphic of the extant bear species. Adult males can have a mass between 300 and 800 kg, and their adult body length ranges from 1.8 to 2.5 m. Adult females are about half the size of males, with mass ranging from 150 to 450 kg and having an adult body length between 1.9 and 2.1 m. Note that an individual’s mass varies with age, by season, and, in females, with reproductive status.
Although quite different in appearance, polar bears are closely related to brown bears, and fertile wild hybrids have been recorded in the Canadian Arctic. The differences between the two species reveal how polar bears have evolved to meet the challenges of life in the Arctic. The characteristic white fur of polar bears provides camouflage while hunting seals on the sea ice. In fact, polar bear fur is actually hollow and translucent, and may appear yellowish, grey, or even brown depending on season and lighting. Polar bears are good at retaining body heat. The hollow nature of their fur aids in buoyancy and insulation [1], and the relatively smaller size of their ears and tails minimizes heat loss from the extremities.
Polar bears have longer necks than brown bears and lack the brown bear’s characteristic shoulder hump. Their skulls are narrower and longer, their eyes are situated higher on the skull, and they have a “Roman nose” that slopes down from the forehead [1, 2]. The streamlined shape of the neck and head allows polar bears to easily stick their head through a breathing hole to capture seals, which are their main prey. Because of their carnivorous diet, polar bears’ skulls are structurally weaker and less suitable for chewing tough plant material than those of brown bears [3]. Polar bears have enlarged canine teeth that enable them to grasp seals, and their cheek teeth are smaller and sharper than are those of brown bears, which allows them to shear meat.
Polar bears are marine mammals that spend most of their lives on sea ice and only move to land to have cubs or when their preferred habitat melts in summer. Consequently, they have large paws, which act as paddles when swimming and distribute the animal’s body weight when walking on the ice [2]. Hair growing between the toes and around the soles of the feet minimizes heat loss and also maximizes traction on the ice. To further increase traction, their footpads are covered in papillae (small bumps) and their claws are sharp and deeply recurved, which also allows them to grasp prey. Polar bears’ claws are usually black and—as in all bears—are non-retractable [4]. The muscles of their hind limbs are especially well developed [5], enabling them to reach speeds greater than 32 km/h over short distances [1].
The modern polar bear (Ursus maritimus) has no recognized subspecies [6, 7].
Polar bears are highly carnivorous, with a diet mainly composed of ringed seals and bearded seals. They consume their prey on the sea ice during the hunting season, which peaks from April to July. These seal species are high in fat and are necessary for polar bears to survive the late-summer/early-autumn ice-free fasting season common to many subpopulations [9]. Polar bears return to hunt on the sea ice after it re-forms, with the exception of pregnant females, who enter their maternity dens between August and October, and remain there until sometime between February and early April, depending on latitude. Only pregnant females hibernate in dens throughout the winter, although other individuals may use temporary shelters [10]. Dens are generally made in snow banks.
While in the den, females give birth to extremely altricial cubs between late November and mid-January. Average litter size at emergence from the den is about 1.5, meaning about half of females emerge with one cub and about half with two cubs. Litters of three at emergence are rare, and only a single litter of four has ever been observed. Birthing litters of three or four could be more common than estimated, however, as cubs are only observed after den emergence, and any that die in the den would go undetected.
Cubs weigh 600–700 grams at birth, lack subcutaneous fat, are only very lightly furred, and are born blind [11]. They grow rapidly because of the high fat content of polar bear milk [12]. Mother and cubs emerge from the den in the spring and usually linger in the denning area for a week or two before heading for the sea ice. In southerly subpopulations such as those around Western and Southern Hudson Bay, females that come ashore while pregnant may not eat anything from August to March—a period of eight months.
The mating season starts in the spring (March–May), during which time adult males seek out receptive females. Male–male competition is high, and when a male is successful in securing a female, he guards her for several days before mating actually occurs [13]. The male–female interactions prior to mating likely induce ovulation [14]. Implantation is delayed until the fall. Males can reach sexual maturity at 3–4 years of age, although they are more likely to reproduce several years later, when they are strong enough to secure a female [15]. Females reach sexual maturity at around 4–5 years of age [14, 15].
Polar bears are considered solitary outside of the mating season, though they may aggregate in the absence of competition for food or mates [16]. Polar bears can communicate vocally, producing a chuffing sound during aggressive encounters [1]. Predation upon polar bears is likely rare; however, wolves may eat polar bear cubs where their ranges overlap [2]. Almost half of all cubs die during their first year of life, probably as a result of starvation or hypothermia [17]. Adult polar bears generally survive up to 25 years in the wild [1].
| Gestation (months) | Fetal development (days) | Maximum litter size | Juvenile lifespan (years) | Subadult (years) | Adult lifespan (years) | Maximum age (years) |
|---|---|---|---|---|---|---|
| 6-8a | 60a | 4a | <2a | 2-3a | >3a,b | 32 (wild)c; 43 (captivity)c |
a [1], b [15], c [2]
Mating system: Polygynous
Breeding interval: Dependent on body condition; normally every 3 years but can vary from 1 to 4 years for females, males attempt to mate every year but peak in mating success in their middle teens.
Breeding season: February–June (some latitudinal variation)
Independence: ~2.5 years (exceptional weaning at 1.5 years was observed in Hudson Bay in the past but no longer)
Twenty subpopulations of polar bears have been delineated based on movement data and tag returns from hunters [18], with an estimated global total of 26,000 individuals (95% CI: 22,000–31,000) [19]. Population size estimates used to be obtained primarily using capture-mark-recapture (CMR) studies, which involve: 1) capturing a large number of bears each year (typically from a helicopter using remote injection of a tranquilizer such as Telazol®); 2) marking newly captured individuals with ear tags; and 3) counting the number of recaptured individuals (individuals that were tagged during an earlier year). A high proportion of recaptures implies a small population of bears. In many areas, scientists are now moving towards remote biopsy darting to collect DNA samples [20]. Genetic analysis of these remotely collected samples enables researchers to identify individuals via genotyping. The CMR model can be applied to these individuals without the need for tranquilization or handling to affix ear tags, as any newly genotyped individual will be considered a first “capture”, while any previously genotyped individual will be considered a “recapture”. This genetic CMR technique is useful to monitor population trends because it is less invasive and less time-consuming than methods involving direct animal handling. Aerial surveys have also been used because they are less invasive, less expensive, and less time-consuming than CMR, although they do not provide additional insights on population status such as age structure, body condition, and reproductive rates [21]. Traditional ecological knowledge (TEK) is sometimes used in combination with scientific methods to assess population size, behaviours, and current threats.
Obtaining precise population estimates is difficult, and the margins of error are large. The overall population trend is predicted to decline, although some subpopulations are stable or increasing. Poor body condition, low recruitment, low survival rates, and population declines have been linked to declining sea-ice conditions [22, 23, 24, 25]. Little is known about polar bears in the Arctic Basin; however, density is thought to be low there [18].
For the latest population estimates by region, please visit the IUCN Polar Bear Specialist Group website.
Polar bears live in the circumpolar Arctic, and their range depends on the southern extent of sea ice. They are found in five countries, known as the Polar Bear Range States: Canada (Nunavut, Northwest Territories, Yukon, Newfoundland and Labrador, Québec, Manitoba, Ontario), Greenland (Denmark), Russia (North European Russia, Siberia, Chukotka, Sakha, Krasnoyarsk), Norway (Svalbard), and the United States (Alaska). They have occasionally been observed in Iceland [27]. Approximately one half to two thirds of the world’s polar bears live in Canada.
Green = Extant. Map data courtesy of IUCN Redlist.
Polar bears prefer sea-ice habitat located over or close to the continental shelf [28, 29], as regions of annually reforming sea ice can support a more productive ecosystem than thick multiyear ice [2], where seal density is lower. Optimal habitat is about 85% ice cover, and bears move to refuge habitats (i.e., land or multiyear ice) when sea-ice concentration drops below 50% [30, 31]. Lower ice cover makes seals’ surfacing locations unpredictable and increases the time and energy polar bears spend travelling and swimming [1]. Polar bears often select sites where a higher density of prey is available [32], such as polynyas (year-round open water upwellings surrounded by sea ice) or shear zones that are close to the shore [33]. The so-called Arctic Ring of Life is a productive shore lead–polynya system spanning the edge of the polar basin, and it is excellent polar bear habitat [2]. Females in some subpopulations dig maternity dens in snowdrifts on sea ice, but in most subpopulations, pregnant females create dens on land [5]. The use of these terrestrial dens and the selection of less-dynamic sea ice lessen the energetic costs linked to movements [34]. As such, females may tend to select more stable habitats, even if they are less productive [35]. Denning on sea ice, which has only been commonly seen in Alaska, appears to be declining in response to changes in sea ice [36]
Given these habitat preferences, most polar bears are found on the ice over the continental shelf and in the archipelagos around the polar basin. The sea ice in many areas melts in summer, and elsewhere sea-ice conditions can be highly variable [37]. In some areas, loss of sea ice results in extended fasting periods for polar bears. These fasting periods last until freeze-up, at which point polar bears can begin hunting again, although the main feeding period is in spring when seals are pupping [9, 38]. In contrast, food availability is low (but constant) around the central Arctic Basin, and its permanent ice allows polar bears to hunt there throughout the year [37].
Globally, the habitat of polar bears has been broadly divided into four ecoregions based on current and projected sea-ice conditions [25]. These are: 1) the Seasonal Ice Ecoregion, from western Greenland to the southernmost extreme of the polar bear range, characterized by complete or near-complete loss of sea ice for several months each summer; 2) the Archipelago Ecoregion of the Canadian Arctic; 3) the Polar Basin Divergent Ecoregion, which stretches west from the Southern Beaufort to the Barents Sea and is characterized by ice moving away from the shore; 4) the Polar Basin Convergent Ecoregion, which stretches northeast from the Northern Beaufort to Canada’s Ellesmere Island and down the east coast of Greenland, and is characterized by ice from other regions gathering against the shore. Climate change is expected to affect polar bear populations differently in each of these ecoregions.
BB = Baffin Bay, BS = Barents Sea, CS = Chukchi Sea, DS = Davis Strait, EG = East Greenland, FB = Foxe Basin, GB = Gulf of Boothia, KB = Kane Basin, KS = Kara Sea, LS = Lancaster Sound, LVS = Laptev Sea, MC = M’Clintock Channel, NBS = Northern Beaufort Sea, NW = Norwegian Bay, SBS = Southern Beaufort Sea, SHB = Southern Hudson Bay, VM = Viscount Melville Sound, WHB = Western Hudson Bay. Polar bears in Queen Elizabeth Islands (QE) are now considered to be part of the Arctic Basin subpopulation.
Polar bears occur at low densities throughout their range. An individual’s annual home range size is highly variable, and most often extremely large (e.g., average home-range size for female polar bears is greater than 100,000 km2), depending on the ratio of land to sea ice and on seasonal variation in ice cover [39, 40]. The higher the variability in sea-ice conditions, the larger the home range size [39]. Because of the changing nature of the sea ice and interannual variability in prey distribution, polar bears are not territorial; rather, individuals’ home ranges overlap substantially [4].
Studies of home-range size generally do not include males because radio collars fall off male polar bears, since their necks are wider than their heads. Therefore, little is known about male movement. A short-term study using subcutaneously implanted GPS transmitters suggests similar home-range sizes for males and females near Alaska [41], whereas a more recent study of Greenland bears using ear-attached GPS transmitters suggests that females may maintain larger home ranges than males during the breeding season [42]. These contrasting results suggest that further research on male movement patterns is needed.
Polar bears are highly carnivorous, having a diet composed mainly of ringed seals (Pusa hispida) and—to a lesser degree—bearded seals (Erignathus barbatus). However, polar bears may consume all pinniped species in their range, and diet composition varies among subpopulations. From the eastern Canadian Arctic to Svalbard, harp seals (Pagophilus groenlandicus) are an important prey species. Polar bears rarely feed on belugas (Delphinapterus leucas), except in some High Artic populations in which they are an important food [43]. Polar bears prey extensively on naïve seal pups, and adult bears often consume only the fat and skin, leaving the rest of the carcass for scavengers [1, 2]. Polar bears detect prey visually or with their powerful sense of smell. When a polar bear locates a seal on the ice, it will approach stealthily on the ice or in the water before making a sudden final rush. Polar bears also frequently hunt by waiting for seals at the edge of the ice, or by using their immense weight to crash into a seal’s birthing lair. Hunting in open water is rare; however, polar bears commonly use aquatic stalks to approach hauled-out seals.
In regions with an extended ice-free period, polar bears may feed opportunistically while ashore. Derocher [1, Appendix B] lists more than 80 species consumed by polar bears, including Svalbard reindeer (Rangifer tarandus platyrhynchus) and thick-billed murres (Uria lomvia) as well as goose eggs, berries, and seaweed. Although some researchers have suggested that polar bears may be able to cope with loss of sea ice by switching to these terrestrial food sources [e.g., 44, 45], this is not accepted by most polar bear scientists, because evidence suggests that terrestrial foods will not be able to offset the loss of access to the calorie-rich blubber of seals caught on the ice [46, 47]. When available, polar bears may also consume human foodstuffs or garbage [48], thereby bringing them into contact with people. Due to earlier break-up and later freeze-up of sea ice, the length of hunting seasons has been shortened, and polar bears that are in poor body condition may become involved in human–bear conflicts [9].
Polar bears sometimes engage in cannibalism, typically involving an adult male feeding on younger or incapacitated individuals [e.g., 49, 50]. Though infrequent, this intraspecific aggression has likely shaped polar bear social behaviour; indeed, females with cubs generally avoid adult males [49]. Cannibalism has also been hypothesized to increase as sea ice and access to seals decrease [29].
Polar bears have been classified as Vulnerable on the IUCN Red List since 1986. Their populations are expected to decline by >30% within the next three generations (~34.5 years) due to reduced range size (i.e., area of occupancy and extent of occurrence) and habitat quality [19], as these factors influence the maximum number of individuals of a species [26]. A 2007 study headed by members of the United States Geological Survey estimated that polar bears could be reduced to one-third of their current population size by 2050 [28].
Polar bears are listed on Appendix II of CITES.
In 1973, the Polar Bear Range States signed the International Agreement on the Conservation of Polar Bears and their Habitat as a result of growing concerns regarding the consequences of overharvesting on the species’ persistence. Since then, several bilateral agreements to regulate harvest have been taken by countries that share subpopulations. However, polar bear subpopulations are not studied equally, and therefore long-term scientific data are not available throughout the species range. Because of this, a “circumpolar monitoring framework” has recently been proposed, with the goals of: 1) increasing research efforts for subpopulations for which data is currently lacking; and 2) allowing the Range States to provide coordinated responses to future changes facing polar bears [51].
Climate change is the primary threat to the long-term survival of polar bears. Climate change causes earlier break-up and later freeze-up of sea ice, which shortens the hunting season during which bears can accumulate fat and protein. This lower energy storage can result in worsened body condition [38], lower milk production, slower cub growth, and increased cub mortality [52], all of which are factors that negatively affect population growth rates and probability of persistence [9]. Owing to loss of sea ice, polar bears may be extirpated in the Seasonal Ice and Polar Basin Divergent Ecoregions by 2050 and in the Polar Basin Convergent Ecoregion by 2080 [53]. Polar bears in the Archipelago Ecoregion are likely to persist through the century, though in reduced numbers [53]. If greenhouse gas emissions continue to increase, two thirds of the world’s polar bears may disappear by 2050 as a result of habitat loss [28, 54]. Climate change may also threaten polar bears by increasing human–bear conflicts, as polar bears are more likely to seek human food owing to earlier on-shore arrival and a longer fasting season [9].
Polar bears are also threatened by pollution in the form of chlorinated, brominated, and fluorinated contaminants [55] as well as mercury [56]. As top Arctic predators, polar bears consume these substances in high concentrations as a result of biomagnification at each level of the food chain [57]. These contaminants can be detected in the fat, liver, and blood of animals, and they can have detrimental physiological consequences [58, 59], because they may affect the endocrine and immune systems [55] as well as disturb reproduction and development [60, 61].
Oil spills increase the probabilities of exposure to oil by polar bears and their prey. Oil decreases the insulative properties of polar bear fur, which can lead to hypothermia. In addition, ingestion of oil by polar bears can lead to death from kidney failure [2]. Denning activities of polar bears can also be disturbed by oil exploration and development [62]. Snow has been shown to muffle vibrations and noises linked to drilling [63], and as such, disturbances have to occur close to a den to result in den or litter abandonment [62, 63].
Polar bears have long been important to inhabitants of the Arctic such as the Inuit, Yupik, and Cree, as well as their forebears. In both behaviour and anatomy, polar bears more closely resemble humans than do any other Arctic animals. Bears can sit similarly to humans and stand on their hind legs, and polar bears are the only non-human species to regularly hunt seals on the sea ice. A dead, skinned bear resembles a human. Because of these human–bear similarities and their great size and formidableness, polar bears have been a common subject of Arctic peoples’ mythology and art [2].
In some Canadian polar bear management units, Inuit communities may allocate part of their quota for sport hunting by non-Inuit. Though subsistence hunters usually hunt with modern equipment such as snowmobiles, sport hunts involve a dogsled and are conducted by a local Inuit guide. These hunts sometimes use dead seals as bait to attract bears. Fees paid by sport hunters are a source of income for guides and outfitters in some northern communities [66]. In 2008, polar bears were listed as a threatened species under the United States Endangered Species Act, thereby banning the import of polar bear trophies into the United States and greatly reducing demand from American sport hunters.
Polar bears are among the world’s most charismatic species. As an iconic species of the Arctic and the poster species for climate change, public interest in polar bears is high. Polar bears are powerful draws for zoos, although it is challenging to create a satisfactory enclosure for an inquisitive carnivore that ranges thousands of kilometers across the sea ice every year in the wild. Polar bear tourism is well established in Canada, with thousands of visitors flocking to Churchill, Manitoba, each autumn to watch polar bears ashore before the sea ice re-forms. More recently, shipboard tours around Svalbard, Norway, have become popular in late spring and early summer, as they allow observation of polar bears in a true Arctic environment. Scientific research on polar bears is ongoing in many areas.
Agnès Pelletier, Ph.D., Trent University. Present Affiliation: WildMap Consulting.
René Malenfant, Ph.D., University of Alberta. Present Affiliation: University of New Brunswick.
Map of polar basin ecoregions courtesy of Steve Amstrup on behalf of the United States Geological Survey.
IUCN Polar Bear Specialist Group: https://www.iucn-pbsg.org/
IUCN Red List: https://www.iucnredlist.org/species/22823/14871490
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