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Ursus maritimuspolar bear

By Aren Gunderson

Geographic Range

Polar bears have a circumpolar distribution. They range throughout the arctic region surrounding the North Pole. The limits of their range are determined by the ice pack of the Arctic Ocean and the landfast ice of surrounding coastal areas. Bears have been reported as far south as the southern tips of Greenland and Iceland. During the winter, polar bears will range along the southern edge of the ice pack or northern edge of ice formed off the coasts of the continents. Pregnant females will overwinter on the coastlines where denning habitat is available for bearing young. During the summer, bears will remain at the edge of the receding ice pack or on islands and coastal regions that retain landfast ice. Six different populations are recognized as: Wrangel Island and western Alaska, northern Alaska, the Canadian Arctic archipelago, Greenland, Svalbard-Franz Josef Land, and Central Siberia. (DeMaster and Stirling, 1981; Nowak, 1999)

Habitat

Polar bears are considered by many to be marine mammals. The name Ursus maritimus means maritime bear. Their preferred habitat is the pack ice of the Arctic Ocean. The ice edge and pressure ridges where fractures and refreezing occur provide the best hunting ground. Bears will travel as much as 1,000 km north and south, as the ice melts and freezes. During summer bears may remain on islands or coastlines with landfast ice, drift on ice flows, or get stranded on land where they are forced to endure warm weather. (DeMaster and Stirling, 1981; Nowak, 1999; Stirling and McEwan, 1975)

  • Terrestrial Biomes
  • tundra
  • icecap
  • Range elevation
    0 (low) m
    0.00 (low) ft
  • Average elevation
    0 m
    0.00 ft

Physical Description

The body of a polar bear is large and stocky, similar to that of a brown bear, except it lacks the shoulder hump. The head is relatively smaller than the heads of other bears and the neck is elongated. At the shoulder a polar bear can measure 1.6 m in height. Adult males weigh between 300-800 kg (660-1760 lbs) and can reach 2.5 m in length from tip of nose to tip of tail. Females are smaller, weighing 150 to 300 kg (330 to 660 lbs) and measuring 1.8 to 2 m in length. The pelage generally has a white appearance, but it can be yellowish in the summer due to oxidation or may even appear brown or gray, depending on the season and light conditions. Polar bear skin is black and the fur is actually clear, lacking in pigment. The white appearance is the result of light being refracted from the clear hair strands. The forepaws are broad and make excellent paddles while swimming. The soles of both hind and fore feet are furred for insulation and traction while walking on ice and snow. Polar bears have a plantigrade gait. Females have four functional mammae. (DeMaster and Stirling, 1981; Nowak, 1999)

  • Sexual Dimorphism
  • male larger
  • Range mass
    150 to 800 kg
    330.40 to 1762.11 lb
  • Range length
    180 to 250 cm
    70.87 to 98.43 in

Reproduction

Polar bears have a sequential polygynous mating system. Male and female breeding pairs remain together for a short time while females are in estrus (3 days). (DeMaster and Stirling, 1981; Nowak, 1999; Ramsey and Stirling, 1988)

Mating occurs in late winter and early spring, from March to June. Delayed implantation extends gestation to 195 to 265 days. Pregnant females establish a winter den on land dug into the snow usually within 8 km of the coast in October or November. An average of 2 cubs are born in the mother's den between November and January, litter sizes can range from 1 to 4. She remains in hibernation, nursing her cubs until April. The mortality rate for cubs is estimated to be 10-30%. The average annual rate of reproduction calculated by DeMaster and Stirling (1981) was 0.274 females per adult female. (DeMaster and Stirling, 1981; Nowak, 1999; Ramsey and Stirling, 1988; Stirling and McEwan, 1975)

  • Breeding interval
    Females breed yearly.
  • Breeding season
    Breeding occurs from March to June.
  • Range number of offspring
    1 to 4
  • Average number of offspring
    2
  • Average number of offspring
    2
    AnAge
  • Range gestation period
    195 to 265 days
  • Range weaning age
    24 to 36 months
  • Range age at sexual or reproductive maturity (female)
    3.5 (low) years
  • Average age at sexual or reproductive maturity (female)
    5-6 years
  • Range age at sexual or reproductive maturity (male)
    3.5 (low) years
  • Average age at sexual or reproductive maturity (male)
    5-6 years

Cubs are born with their eyes closed; they have a good coat of fur and weigh about 600 grams. They will emerge from the den in spring weighing 10 to 15 kg. Mothers provide all parental care of their offspring. The cubs remain with their mother for 2 to 3 years. They will not reach sexual maturity until 5 to 6 years old. (DeMaster and Stirling, 1981; Nowak, 1999; Ramsey and Stirling, 1988)

  • Parental Investment
  • altricial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female
  • extended period of juvenile learning

Lifespan/Longevity

In the wild polar bears are estimated to live 25 to 30 years. Annual adult mortality is estimated to be 8 to 16%. In captivity the oldest recorded lifespan was a female that died at the Detroit Zoo in 1991 at 43 years and 10 months old. (DeMaster and Stirling, 1981; Nowak, 1999)

Behavior

Polar bears are solitary. The exceptions to this are when a mother is caring for her cubs and when males and females are paired during mating. Bears may also come into competition with one another when a seal kill attracts other bears looking to scavenge. In instances where bears encounter each other, the smaller bear will tend to run away. A female with cubs, however, will charge males that are much larger to protect her young or a kill that they are feeding on. Polar bears are inactive most of the time (66.6%), either sleeping, lying, or waiting (still hunting). The rest of their time is spent traveling (walking and swimming; 29.1%), stalking prey (1.2%), or feeding (2.3%). Polar bears are excellent swimmers, they may range widely in search of food and sightings as far south as Maine, in the United States, have been documented. (DeMaster and Stirling, 1981; Stirling and McEwan, 1975; Stirling, 1974)

Communication and Perception

Like other bear species, polar bears have a keen sense of smell and use their sensitive lips and whiskers to explore objects. They vision and hearing are not exceptionally well developed. Polar bears use a "chuffing" sound as a form of greeting. (DeMaster and Stirling, 1981)

Food Habits

Polar bears are carnivores. In the summer, they may consume some vegetation but gain little nutrition from it. Their primary prey are ringed seals (Pusa hispida). They also hunt bearded seals (Erignathus barbatus), harp seals (Pagophilus groenlandicus), hooded seals (Cystophora cristata), walruses (Odobenus rosmarus), sea birds and their eggs, small mammals, fish and scavenge on carrion of seals, walruses, or whales. Bears often leave a kill after consuming only the blubber. The high caloric value of blubber relative to meat is important to bears for maintaining an insulating fat layer and storing energy for times when food is scarce. Polar bears do not store or cache unconsumed meat as other bears do. (DeMaster and Stirling, 1981; Stirling and McEwan, 1975)

Polar bears have two hunting strategies. Still-hunting is used predominately. This involves finding a seal's breathing hole in the ice and waiting for the seal to surface to make the kill. When a bear sees a seal basking out of the water it will use a stalking technique to get close, then make an attempt at catching it. One stalking technique is crouching and staying out of sight while creeping up on the seal. Another technique is to swim through any channels or cracks in the ice until it is close enough to catch the seal. Using this technique a bear may actually dive under the ice and surface through the breathing hole in order the surprise the seal and eliminate its escape route. Feeding usually occurs immediately after the kill has been dragged away from the water. Polar bears consume the skin and blubber first and the rest is often abandoned. Other polar bears or arctic foxes then scavenge these leftovers. After feeding, polar bears will wash themselves by licking and rinsing their fur. (DeMaster and Stirling, 1981; Nowak, 1999; Ramsey and Stirling, 1988; Stirling, 1974)

  • Primary Diet
  • carnivore
    • eats terrestrial vertebrates
  • Animal Foods
  • birds
  • mammals
  • fish
  • eggs
  • carrion
  • Plant Foods
  • leaves

Predation

Humans and other polar bears are the only predators of Ursus maritimus. Male polar bears may prey on cubs if they come into contact. Females with cubs tend to avoid other bears for this reason. Historically, polar bears have been hunted by the native peoples of the arctic for fur and meat. Commercial and sport hunting of polar bears increased in the 1900s as the price of pelts reached as much as $3000. (DeMaster and Stirling, 1981; Nowak, 1999)

  • Anti-predator Adaptations
  • cryptic
  • Known Predators

Ecosystem Roles

Polar bears are a top carnivore of the arctic. The remains of seal kills left unconsumed by bears are likely an important source of food for younger, less-experienced polar bears and for Artic foxes. (DeMaster and Stirling, 1981; Stirling and McEwan, 1975; Stirling, 1974)

Economic Importance for Humans: Positive

Polar bear materials have historically been used by native people of the arctic for fur, meat, and medicines. Hunting by those groups is still allowed in the United States, Canada, and Greenland (Denmark). Trophy and commercial hunters have taken bears for pelts that sold for $3000 in the past. (Nowak, 1999)

Economic Importance for Humans: Negative

Polar bears are viewed as potentially dangerous to humans. Contact between humans and bears is rare due to the large home range of individual bears and the sparse human population throughout their distribution. Two deaths resulting from polar bear encounters have been reported. (Nowak, 1999)

  • Negative Impacts
  • injures humans
    • bites or stings

Conservation Status

Polar bear populations were recently considered to be stable or growing in some areas. In 1993, the estimated world population was 21,470 to 28,370 bears. In 1972, the United States Marine Mammal Protection Act prohibited all hunting, except for subsistence, of polar bears in the U.S. In 1973 the United States, Russia, Norway, Canada, and Denmark came to an agreement to protect polar bear habitat, limit hunting, and cooperate on research. Polar bear populations are currently threatened by trends in global warming, which continues to decrease the extent of their habitat (pack ice) and their prey base. In 2008 the U.S. Fish and Wildlife Service listed polar bears as threatened. The IUCN lists Ursus maritimus as vulnerable. (DeMaster and Stirling, 1981; Hilton-Taylor, 2000; Nowak, 1999)

Other Comments

Polar bears bred with brown bears have produced fertile hybrids (DeMaster and Stirling 1981). In fact, polar bears have been shown to be genetically more closely related to certain brown bear populations than are some brown bear populations to others. This suggests that polar bears have evolved fairly recently from a brown bear ancestor and that brown bear genetic structure is more complicated than previously thought. (DeMaster and Stirling, 1981; Talbot and Shields, 1996)

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Aren Gunderson (author), University of Northern Iowa, Jim Demastes (editor), University of Northern Iowa.

Glossary

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

carrion

flesh of dead animals.

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

delayed implantation

in mammals, a condition in which a fertilized egg reaches the uterus but delays its implantation in the uterine lining, sometimes for several months.

diurnal
  1. active during the day, 2. lasting for one day.
drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

food

A substance that provides both nutrients and energy to a living thing.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

keystone species

a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nomadic

generally wanders from place to place, usually within a well-defined range.

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynous

having more than one female as a mate at one time

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

terrestrial

Living on the ground.

threatened

The term is used in the 1994 IUCN Red List of Threatened Animals to refer collectively to species categorized as Endangered (E), Vulnerable (V), Rare (R), Indeterminate (I), or Insufficiently Known (K) and in the 1996 IUCN Red List of Threatened Animals to refer collectively to species categorized as Critically Endangered (CR), Endangered (EN), or Vulnerable (VU).

tundra

A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

"Division of Endangered Species, Species Information" (On-line). Accessed Dec 5, 2001 at http://endangered.fws.gov/wildlife.html.

DeMaster, D., I. Stirling. 1981. *Ursus maritimus*. Mammalian Species, 145: 1-7.

Hilton-Taylor, C. 2000. "The 2000 IUCN Red List of Threatened Species" (On-line). Accessed Dec 5, 2001 at http://www.redlist.org/search/details.php?species=22823.

Nowak, R. 1999. Walker's Mammals of the World. Baltimore and London: The Johns Hopkins University Press.

Ramsey, M., I. Stirling. 1988. Reproductive biology and ecology of female polar bears (*Ursus maritimus*). Journal of Zoology, 214: 601-634.

Stirling, I. 1974. Midsummer observations on the behavior of wild polar bears (*Ursus maritimus*). Canadian Journal of Zoology, 52: 1191-1198.

Stirling, I., E. McEwan. 1975. The caloric value of whole ringed seals (*Phoca hispida*) in relation to polar bear (*Ursus maritimus*) ecology and hunting behavior. Canadian Journal of Zoology, 53: 1021-1027.

Talbot, S., G. Shields. 1996. Phylogeography of Brown Bears (Ursus arctos) of Alaska and Paraphyly within the Ursidae. Molecular Phylogenetics and Evolution, 5: 477-494.

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