Bears are mammals in the family Ursidae within the order Carnivora, comprising 8 extant species distributed primarily across the Northern Hemisphere. They are noted for their robust, stocky build, thick fur, and elongated snout.
Although classified as carnivorans, most bears are opportunistic omnivores that shift seasonally among plant matter, including berries, roots, nuts, and grasses, and animal resources such as insects, fish, and carrion.
Many species build dens in hillsides, natural cavities under trees or rocks, or in snow, which provide insulation and safety. In winter, these sites support prolonged dormancy (hibernation), and in several species, pregnant females give birth and nurse their cubs there.
The bear family exhibits considerable variation in size among its species. For example, polar bear males typically measure 7 ft 10 in to 9 ft 10 in (2.4 to 3 m) and weigh between 770 and 1,540 lb (350 and 700 kg).1 In contrast, males of the sun bear measure between 3.25 and 4.58 ft in (1 and 1.4 m) and weigh between 55 and 143 lb (25 and 65 kg).2
They are sexually dimorphic, with males typically being larger than females.
Bears have a robust body with massive shoulder blades and a broad pelvis. To support their bulk, they have four short yet strong limbs with five toes on each paw. Their toes are modified into non-retractable claws that help in digging burrows, climbing, or tearing prey.
They have a plantigrade stance, meaning they walk with the whole foot on the ground (like humans), not on toes. Although they appear bulky, some bears, like the grizzly bear (a subspecies of brown bear), can run quite fast, reaching speed bursts of 35 mph (56 kmph).3
Their skull is large, having powerful jaw muscles, large canines mainly for display, and flat, crushing molars suited to their omnivorous lifestyle. Their ears are small and rounded to minimize heat loss.
Although the muzzle lacks sensory whiskers, bears have an extraordinary sense of smell, which they use to detect food and communicate. In fact, a brown bear’s sense of smell is around 2,100 times better than ours. It has a massive olfactory bulb (5 to 7 times larger than ours) in the brain and millions of scent receptors lining its nasal cavities.4
Their bodies are covered with a coat of long, coarse guard hairs and short, dense underfur that traps air for insulation in winter. A thinner coat replaces this winter coat during summer.
Polar bears, in particular, have hollow, translucent guard hairs that absorb heat from the sun and keep them warm. They also have a thick layer of blubber under their skin for insulation.
The 8 extant bear species are classified into 3 subfamilies and 5 genera. These species are further classified into 45 recognized subspecies, although not every species has subspecies.
The closest relatives of bears are the canines of the family Canidae. Although some mammals, like the red panda and raccoons, are commonly called ‘bears’ in some languages, they actually belong to the superfamily Musteloidea.
Bears are found throughout most of the Northern Hemisphere, with the highest concentration in Asia, North America, and Europe. A subspecies of the brown bear, the Atlas bear, was found in North Africa, but it became extinct around the 1870s.
Depending on the species, bears occupy a wide range of habitats, including tropical rainforests, coniferous forests, grasslands, prairies, steppes, alpine slopes, and, in the case of the polar bear, Arctic sea ice.
As opportunistic omnivores, their diet ranges from plant foliage and fruits to insects, animal meat, and carrion.5 In most species, plant matter forms the bulk of the diet, while animal matter is consumed opportunistically.
Their food choices vary with species and season. For example, brown bears typically feed on young, protein-rich plant tissues in spring, while in summer they switch to berries and other fruits. During late summer to early autumn, brown bears in the coastal regions of the North Pacific particularly feed on salmon.6
During autumn, many bears consume energy-rich food, such as beechnuts and acorns, to accumulate fat before winter dormancy. A brown bear can eat up to 41 kg (90 lb) of food a day before entering its den for hibernation, gaining 2 to 3 kg.7
Exceptions
Over 99% of the giant panda’s diet consists of leaves of 30 different bamboo plants (herbivorous).8 In stark contrast, the polar bear mostly feeds on seals (carnivorous) throughout the year, particularly their high-fat blubber, to support survival in the Arctic.
The diet of sloth bears may comprise up to 90% ants and termites seasonally.9
These animals are mostly solitary and live and forage alone, except for mothers with cubs or during mating season. Depending on the species, they may be diurnal or nocturnal. For example, brown and American black bears are generally diurnal, but sloth bears are mostly nocturnal.
In winter, bears living in colder northern regions of their range hibernate in their dens, particularly to battle the scarcity of resources. During this phase, their metabolism slows down, and body temperature decreases by 7.2 to 12.6 °F (4 to 7 °C). Their heart rate slows from about the usual 55 to as low as 9 beats per minute.10 Depending on the species, bears may hibernate for 3 to 8 months.
On average, most bears can survive up to 25 years in the wild and 50 years in captivity.11
As of 2013, the oldest bear in the wild is No. 56, a female American black bear that lived in the woods of northern Minnesota. It was 39.5 years old at the time of its death.12
Breeding typically occurs annually in spring to early summer. During this period, males actively search for females in estrus and may compete with rival males. Courtship is brief in many species, though some bears show more elaborate behaviors, including muzzling, wrestling, vocalizing, and mock fighting.
After fertilization, the egg does not immediately implant in the uterus. It remains suspended for several months, and implantation occurs later, typically before winter. This delayed implantation helps synchronize the birth timing with the denning period.
Most bears give birth in winter and invest heavily in maternal care. These cubs are born blind, nearly hairless, or with a thin layer of body hair (lanugo). They are completely dependent on their mother for survival. The mother nurses her cubs with fat-rich milk high in antibodies, often for up to a year.
Because of their size and strength, bears are apex predators themselves. Adult bears have almost no natural predators, but in the Russian Far East, Siberian tigers regularly hunt adults.
Bear cubs are vulnerable to predation from wolves, cougars, coyotes, bobcats, and birds of prey.
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