Diversity
Bears are a small group of mostly large mammals, with 8 species in 5 genera (
Ursus
,
Tremarctos
,
Melursus
,
Helarctos
, and
Ailuropoda
). Although
Ursidae
is not diverse, species in this family are widespread and culturally significant
to human populations throughout their range.
Geographic Range
Bears are found on all continents except Antarctica and Australia, but are primarily
found throughout the northern hemisphere, historically occurring as far south as the
Atlas Mountains of northwestern Africa, the Andes of South America, and the Sunda
shelf region. This range has been reduced in historical times as a result of human
persecution and habitat destruction. For example, brown bear (
Ursus arctos
) populations in the Atlas Mountains are thought to be extinct and their range has
been significantly altered in North America and Europe.
- Biogeographic Regions
- nearctic
- palearctic
- oriental
- neotropical
Habitat
Bears occur in nearly all terrestrial habitats throughout their range, from Arctic
tundra and polar ice floes to tropical and temperate forests, mountains, grasslands,
and deserts. Although some bear species occur in arid areas, proximity to water is
important. Bears are most abundant and diverse in temperate and boreal regions.
Most species are habitat generalists, changing preferred foods, activity patterns,
and denning quarters with local conditions.
Ailuropoda melanoleuca
, however, is found primarily in the montane bamboo forests of southern China.
- Habitat Regions
- temperate
- tropical
- polar
- terrestrial
- Terrestrial Biomes
- tundra
- taiga
- desert or dune
- savanna or grassland
- chaparral
- forest
- rainforest
- scrub forest
- mountains
Physical Description
Bears are large, robustly built animals. The smallest species,
Helarctos malayanus
ranges in size from 25 to 65 kg, the largest individuals can weigh up to 800 kg (
Ursus maritimus
). Males are larger than females, sometimes more than twice their size.
Bears have small, rounded ears, small eyes, and very short tails. Most species have
long, rough fur, and the hairs that make it up are generally unicolored (rather than
being
agouti
, the common pattern among mammals).
Sun bears
have a smooth coat. Most bears are brown, black, or white; some have striking white
marks on the chest or face.
Giant pandas
are well-known for their distinctive bands of black and white fur.
Bear skulls are massive, with unspecialized incisors, elongate canines, reduced premolars,
and
bunodont
cheek teeth. All bear species possess robust, recurved, non-retractile claws that
they use for digging and ripping. The feet of bears are
plantigrade
, and most have hairy soles, although tree climbing bears, such as
Helarctos
, have naked soles. There are five digits on each foot. Giant pandas (
Ailuropoda melanoleuca
) have an additional, opposable feature of the forepaws, sometimes called a panda's
"thumb". It is not a true digit but a pad-covered enlargement of the radial sesamoid
bone. Pandas use this opposable structure to manipulate bamboo.
The skulls of bears are elongated. They have an alisphenoid canal , and the paroccipital processes are large and not fused to the bullae , which are not enlarged. Curiously, the lacrimal bone of bears is vestigial. Their cheekteeth are bunodont , and the carnassials are flattened and specialized for crushing, not secodont . The incisors are unspecialized; the canines are long and slightly hooked; and the first three premolars are small and weakly developed if present at all. The dental formula is 3/3, 1/1, 3-4/4, 2/3 = 40-42.
- Other Physical Features
- endothermic
- heterothermic
- bilateral symmetry
- Sexual Dimorphism
- male larger
Reproduction
Male and female bears generally associate only briefly for mating. Males monitor
the estrus condition of females in their home range and will remain close for a few
days when females are receptive. Multiple mating is practiced by both sexes.
- Mating System
- polygynandrous (promiscuous)
Bears give birth to 1 to 4 young, usually 2, at intervals of 1 to 4 years. There
is evidence of delayed implantation in all species. Gestation lengths ranging from
95 to 266 days, with implantation being delayed from 45 to 120 days. Actual gestation
lengths may be closer to 60 to 70 days. Births in temperate species occur during
the winter when the female is dormant. The cubs nurse during the dormant period and
the entire metabolic demands of the female must be met by her fat reserves. Births
in
Helarctos malayanus
may occur at any time of the year. Sexual maturity occurs at from to 3 to 6.5 years
old, usually occurring later in males. Growth continues after sexual maturity. Males
may not reach their adult size until 10-11 years old. Females reach adult sizes usually
around 5 years old.
- Key Reproductive Features
- iteroparous
- seasonal breeding
- year-round breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- viviparous
- delayed implantation
Females give birth to their young in protected areas, often a den of some kind, until
they are capable of getting around well, at several months of age. Bears are very
small when born, from 90 (
Ailuropoda melanoleuca
) to 680 (
Ursus arctos
) grams at birth. They are born with their eyes and ears closed and are either naked
or with only a fine layer of fur. Cubs grow rapidly, polar bears go from 600 grams
at birth to 10 to 15 kg within 4 months. Weaning occurs from 3.5 (
Ursus thibetanus
) to 9 (
Ailuropoda melanoleuca
) months. Young stay with their mother for up to 3 years, but young of most species
disperse after 18 to 24 months. Females are very protective of their young and it
is likely that cubs learn about obtaining food and shelter during their extended juvenile
time with their mother.
- 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
Bears are long-lived if they survive their first few years of life. Most mortality
occurs in young cubs or dispersing juveniles as a result of food stress. Pre-weaning
cub mortality was estimated at 10-30% in
polar bears
and sub-adult mortality at between 3 and 16%. In
American black bears
in Alaska, sub-adult mortality was estimated at 52 to 86%. Estimates of longevity
in the wild are as high as 25 years. Captive animals have been known to live to 50
years or more (
Ursus arctos
).
Behavior
Bears are generally solitary, with the exception of mothers with their young. Bears
are most often nocturnal or crepuscular, but may be active during the day as well.
Polar bears (
Ursus maritimus
) are primarily diurnal. Bears generally take advantage of shelters, such as caves,
hollow logs, and cavities in tree roots, as dens.
Helarctos malayanus
individuals spend much of their time in trees and build platforms for resting.
Bears tend to move relatively slowly, with a shuffling, plantigrade gait, but are
capable of running quickly when necessary, standing and walking on the rear two feet,
and climbing. Polar bears (
Ursus maritimus
) are excellent swimmers and sun bears (
Helarctos malayanus
) are quite arboreal. Most bears move throughout a large range in order to meet their
metabolic needs. Polar bear females migrate off of pack ice in late fall to give
birth to their young in dens.
Some temperate bear species undergo extended periods of torpor during winters, retreating
to underground burrows or caves to escape from temperature fluctuations. They become
lethargic and metabolize fat reserves accumulated during the summer and fall. Some
physiologists do not consider this a true state of hibernation, as body temperatures
do not drop substantially and bears can be readily roused from this state. Others
consider this distinction a semantic one and some researchers have proposed that this
is a true hibernation because heart rates drop to almost half the normal rate. Bear
species that undergo this form of hibernation often give birth during their winter
sleep.
- Key Behaviors
- arboreal
- terricolous
- diurnal
- nocturnal
- crepuscular
- motile
- nomadic
- migratory
- sedentary
- hibernation
- solitary
Communication and Perception
Vision and hearing in bears is not well-developed, but they have a keen sense of smell
and use their sensitive lips to locate and maneuver food.
Ursus americanus
has color vision and has been demonstrated using vision to distinguish food items
at close range. Little is known about communication in bears, but grunts, moans, and
roars are known from most species. Cubs may be especially vocal, uttering "woofs"
and shrill howls when distressed. "Chuffing" is used as a greeting in
Ursus arctos
. Chemical cues may be used by males in locating receptive females. Home range boundaries,
individual identity, and sexual condition may be advertised, both visually and chemically,
by tree-scratching and by urinating and defecating on boundary trails.
Food Habits
Bears are omnivorous and opportunistic. Specific food types may vary by habitat or
season. For example, North American brown bears (
Ursus arctos
) may rely extensively on fruits and insect larvae throughout the year, or may prey
extensively on calves during ungulate breeding seasons and on migrating fish. Most
species eat primarily fruits and insect larvae but will include vertebrates when they
can, carrion, honey, forbs and grasses, seeds, nuts, tubers, fish, and eggs. Bears
use their formidable strength, massive forelimbs, and robust claws to tear apart logs
and capture prey. Giant pandas (
Ailuropoda melanoleuca
) are dietary specialists, eating primarily bamboo stems and shoots, but will also
include small vertebrates, insects, and carrion in their diet. Polar bears (
Ursus maritimus
) are the most carnivorous species, preying mainly on
seal
species, but including fish, small mammals, birds and their eggs, and will scavenge
carcasses of
whales
,
walruses
, and
seals
.
Predation
Once bears reach their adult size it is unlikely that they will be subject to predation.
Cubs are at risk of predation from conspecific bears, sympatric bear species, and
other large predators, such as large
cats
and
canids
. Female bears are aggressive in defense of their young.
Ecosystem Roles
All bear species, because of their omnivorous diet and large size, impact the populations
of prey animals and plant communities in the ecosystems in which they live.
Polar bear
populations and
brown bear
populations that rely on large prey, exert significant pressure on prey populations,
including breeding
seals
and
elk
. Bear species may help to disperse seeds from the fruits they eat. Arctic foxes
(
Alopex lagopus
) rely heavily on scavenging polar bear kills.
Bears are infected by a wide variety of endo and ectoparasites, including: protozoans
(
Eimeria
and
Toxoplasma
),
trematodes
(
Nannophyetus salminicola
,
Neoricketsia helminthoeca
),
cestodes
(
Anacanthotaenia olseni
,
Mesocestoides krulli
,
Multiceps serialis
,
Taenia
species, and
Diphyllobothrium
species),
nematodes
(
Baylisascaris transfuga
,
B. multipapillata
,
Uncinaria yukonensis
,
U. rauschi
,
Crenosoma
,
Thelazia californiensis
,
Dirofilaria ursi
,
Trichinella spiralis
, and
Gongylonema pulchrum
),
lice
(
Trichodectes pinguis
), fleas (
Chaetopsylla setosa
,
C. tuberculaticeps
,
Pulex irritans
, and
Arctopsylla
species), and
ticks
(
Dermacentor
and
Ixodes
species). Infection by
Trichinella spiralis
is especially common, affecting up to 60% of
Ursus maritimus
and
U. arctos
.
- Ecosystem Impact
- disperses seeds
- Trichinella spiralis
- Eimeria
- Toxoplasma
- Nannophyetus salminicola
- Neoricketsia helminthoeca
- Anacanthotaenia olseni
- Mesocestoides krulli
- Multiceps serialis
- Taenia
- Diphyllobothrium
- Baylisascaris transfuga
- B. multipapillata
- Uncinaria yukonensis
- U. rauschi
- Crenosoma
- Thelazia californiensis
- Dirofilaria ursi
- Gongylonema pulchrum
- Trichodectes pinguis
- Chaetopsylla setosa
- C. tuberculaticeps
- Pulex irritans
- Arctopsylla
- Dermacentor
- Ixodes
Economic Importance for Humans: Positive
Bears are important members of healthy ecosystems and are sometimes used as indicator
species of habitat health and wildness. Bears have also been hunted by humans throughout
history for their meat, fat, and fur. Other body parts are used in traditional Chinese
pharmacopias, although their usefulness in curing ailments has never been demonstrated.
Research on the metabolic pathways
black bears
use during their winter torpor may help in the development of treatments for kidney
failure, gallstones, severe burns, and other illnesses.
- Positive Impacts
- body parts are source of valuable material
- research and education
Economic Importance for Humans: Negative
Bears are often implicated in predation on livestock, although their impact on livestock
populations is most often vastly over-stated. This is particularly true of
Tremarctos ornatus
, which is persecuted for livestock predation despite its primarily frugivorous lifestyle.
Bears regularly attack and kill humans when they feel threatened. Females accompanied
by their young may be especially aggresssive and unpredictable. Bear attacks that
seem at first to be unprovoked, often prove to be inadvertently provoked when investigated.
Bears that live near humans, or have become habituated to humans, cause damage by
breaking into homes, food stores, and garbage. Some bear species damage crops, such
as manioc and corn.
- Negative Impacts
-
injures humans
- bites or stings
- crop pest
Conservation Status
Bears have been hunted and persecuted throughout human history. Most bear populations continue to face hunting pressure and have become fragmented as a result of human habitat destruction and hunting.
The IUCN ranks Malayan sun bears ( Helarctos malayanus ) as data deficient, polar bears ( Ursus maritimus ) as lower risk, Asiatic black bears ( Ursus thibetanus ), sloth bears ( Melursus ursinus ), and spectacled bears ( Tremarctos ornatus ) as vulnerable, and giant pandas ( Ailuropoda melanoleuca as endangered.
Several brown bear subspecies are listed as endangered under the United States Endangered Species Act: Mexican grizzly bears, Ursus arctos nelsoni , European brown bears, U. arctos arctos , and Tibetan brown bears or horse bears, U. arctos pruinosus . Baluchistan bears, Ursus thibetanus gedrosianus , are also considered endangered.
The following species are on Appendix I of CITES:
Ailuropoda melanoleuca
,
Helarctos malayanus
,
Melursus ursinus
,
Tremarctos ornatus
,
Ursus thibetanus
, and populations of
Ursus arctos
in Bhutan, China, Mexico and Mongolia. All other populations of
U. arctos
are included in Appendix II.
Other Comments
The fossil record of bears in North America and Eurasia extends to the earlyl Miocene.
It is thought that bears reached Africa in the late Miocene and South America in the
early Pleistocene.
Additional Links
Contributors
Tanya Dewey (author), Animal Diversity Web, Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.
- 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.
- oriental
-
found in the oriental region of the world. In other words, India and southeast Asia.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Neotropical
-
living in the southern part of the New World. In other words, Central and South America.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- temperate
-
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
- tropical
-
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
- 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.
- 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.
- taiga
-
Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.
- desert or dunes
-
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
- tropical savanna and grassland
-
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
- savanna
-
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
- temperate grassland
-
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
- chaparral
-
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
- forest
-
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
- rainforest
-
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
- scrub forest
-
scrub forests develop in areas that experience dry seasons.
- mountains
-
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
- 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.
- polygynandrous
-
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
- 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
- year-round breeding
-
breeding takes place throughout the year
- 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.
- arboreal
-
Referring to an animal that lives in trees; tree-climbing.
- diurnal
-
- active during the day, 2. lasting for one day.
- nocturnal
-
active during the night
- crepuscular
-
active at dawn and dusk
- motile
-
having the capacity to move from one place to another.
- nomadic
-
generally wanders from place to place, usually within a well-defined range.
- migratory
-
makes seasonal movements between breeding and wintering grounds
- sedentary
-
remains in the same area
- 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
- 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
- carnivore
-
an animal that mainly eats meat
- herbivore
-
An animal that eats mainly plants or parts of plants.
- folivore
-
an animal that mainly eats leaves.
- omnivore
-
an animal that mainly eats all kinds of things, including plants and animals
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