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.
- Biogeographic Regions
- nearctic
- palearctic
- arctic ocean
- Other Geographic Terms
- holarctic
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.
- Habitat Regions
- polar
- terrestrial
- saltwater or marine
- Terrestrial Biomes
- tundra
- icecap
- Aquatic Biomes
- coastal
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.
- Other Physical Features
- endothermic
- heterothermic
- homoiothermic
- bilateral symmetry
- Sexual Dimorphism
- male larger
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).
- Mating System
- polygynous
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.
- Key Reproductive Features
- iteroparous
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- viviparous
- delayed implantation
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.
- Parental Investment
- altricial
-
pre-fertilization
- provisioning
-
protecting
- female
-
pre-hatching/birth
-
provisioning
- female
-
protecting
- female
-
provisioning
-
pre-weaning/fledging
-
provisioning
- female
-
protecting
- female
-
provisioning
-
pre-independence
-
provisioning
- female
-
protecting
- female
-
provisioning
- 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.
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.
- Key Behaviors
- terricolous
- natatorial
- diurnal
- motile
- nomadic
- hibernation
- solitary
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.
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.
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.
- 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.
- Anti-predator Adaptations
- cryptic
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
.
- Ecosystem Impact
- keystone species
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.
- Positive Impacts
- food
- ecotourism
- source of medicine or drug
- research and education
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.
- 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.
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.
Additional Links
Contributors
Tanya Dewey (editor), Animal Diversity Web.
Aren Gunderson (author), University of Northern Iowa, Jim Demastes (editor), University of Northern Iowa.
- 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.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Palearctic
-
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Arctic Ocean
-
the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- holarctic
-
a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.
Found in northern North America and northern Europe or Asia.
- 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.
- terrestrial
-
Living on the ground.
- saltwater or marine
-
mainly lives in oceans, seas, or other bodies of salt water.
- 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.
- coastal
-
the nearshore aquatic habitats near a coast, or shoreline.
- 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.
- 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.
- 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.
- polygynous
-
having more than one female as a mate at one time
- 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).
- 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
- viviparous
-
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
- 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.
- 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.
- natatorial
-
specialized for swimming
- diurnal
-
- active during the day, 2. lasting for one day.
- motile
-
having the capacity to move from one place to another.
- nomadic
-
generally wanders from place to place, usually within a well-defined range.
- 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.
- solitary
-
lives alone
- tactile
-
uses touch to communicate
- acoustic
-
uses sound to communicate
- chemical
-
uses smells or other chemicals to communicate
- visual
-
uses sight to communicate
- tactile
-
uses touch to communicate
- acoustic
-
uses sound to communicate
- chemical
-
uses smells or other chemicals to communicate
- carrion
-
flesh of dead animals.
- 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.
- 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).
- food
-
A substance that provides both nutrients and energy to a living thing.
- 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.
- drug
-
a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease
- 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).
- carnivore
-
an animal that mainly eats meat
References
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., 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.
Stirling, I. 1974. Midsummer observations on the behavior of wild polar bears (*Ursus maritimus*). Canadian Journal of Zoology , 52: 1191-1198.
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.
"Division of Endangered Species, Species Information" (On-line). Accessed Dec 5, 2001 at http://endangered.fws.gov/wildlife.html .