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A bear is a large mammal of the order Carnivora, family Ursidae. Common characteristics of bears include a short tail, excellent senses of smell and hearing, five un-retractable claws per paw, and long, dense, shaggy fur.

Bears have a large body with powerful limbs. They are capable of standing up on their hind legs. They have broad paws, long snouts, and round ears. Their teeth are used for defense and tools and depend on the diet of the bear. Their claws are used for ripping, digging, and catching. A bear's eyesight is probably similar in acuity (sharpness) to the human eye. Black bears, and likely other bears, have color vision to help them identify fruits and nuts.

Depending on the species, bears can have 32 to 42 teeth. Bear teeth are not specialized for killing their prey like those of cats. Normal canine teeth in a carnivore are generally large and pointed used for killing prey, while bears' canine teeth are relatively small and typically used in defense or as tools. Bears' molar teeth are broad, flat and are used to shred and grind plant food into small digestable pieces.

Bears have four limbs that end in paws. Each paw has five long, sharp claws that are unretractible, unlike cats. These claws can be used to climb trees, rip open termite nests and beehives, dig up roots, or catch prey, depending on the species. While most carnivores tend to walk on their toes in a way that is adapted for speed, bears have a plantigrade stance. They walk with their weight on the soles of their hindfeet, with the heel touching the ground, while the toes of the forefeet are used more for balance. Although slower than most carnivores, a running bear can reach speeds of up to 50 km/h (30 mph). They are also stronger than most carnivores and their limbs are more flexible and agile.

A bear's fur is long and shaggy. Fur color varies among species, ranging from white, blonde or cream, black and white, to all black or all brown. Colors of a bear's fur can also vary within species. For example, American black bears may be black, brown, reddish-brown, or bluish-black. Several species, such as the sun bear and spectacled bear have a light-colored chest with facial markings.

In all bear species, males are larger than females, but the difference between sexes varies and is greatest in the largest species. Large male polar bears may weigh twice as much as females, while smaller male and female bears are much more similar in weight. A bear's life span seems to last about 25 to 40 years. Bears living in the wild tend to die younger than their zoo-counterparts.

Bears live in a variety of habitats from the tropics to the Arctic and from forests to snowfields. They are mainly omnivorous, although some have a more specialised diet, such as polar bears. They eat lichens, roots, nuts, and berries. They can also go to a river or other body of water to capture fish. Bears will commonly travel far for food. Hunting times are usually in the dusk or the dawn except when humans are nearby.

Bears mostly live alone, except for mothers and their cubs, or males and females during mating season. Bears form temporary groups only when food is plentiful in a small area. Alaskan brown bears group in the same area to feed on salmon during the annual salmon runs, when the fish swim upriver to reach their spawning grounds. Other bears may live alone but exist in a social network. A male and female may live in an overlapping home range, each defending their range from other bears of the same sex. Male young usually leave their mothers to live in other areas, but females often live in an area that overlaps that of their mother.

Bears travel over large territories in search of food, remembering the details of the landscape they cover. They use their excellent memories to return to locations where food was plentiful in past years or seasons. Most bears are able to climb trees to chase prey or gain access to additional vegetation. The only exceptions are polar bears and large adult brown bears, whose heavy weight makes it difficult to climb trees.

Some of the large species, such as the polar bear and the grizzly bear, are dangerous to humans, especially in areas where they have become used to people. For the most part, bears are shy and are easily frightened of humans. They will, however, defend their cubs ferociously.

The bear's courtship period is very brief. Bears reproduce seasonally, usually after a period of inactivity similar to hibernation. Cubs come out toothless, blind, and bald. The cubs, usually born in litters of 1–3, will stay with the mother for six months. They will be fed by milk at first and will start hunting with the mother in three months. Then, they are weaned. However, they will still remain nearby for three years. The cubs reach sexual maturity at seven years. Normally, bears are very solitary and will not remain close together for long periods of time.

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