Bears are large carnivores of the family Ursidae. They are members of the order Carnivora, which also includes dogs, cats, and seals, although these animals are in different families than bears. All of these carnivores have a pair of modified teeth in the upper and lower jaw, called carnassials, which are used to tear meat into smaller chunks during feeding. Bears are not strictly meat-eaters, however, and their molars are well adapted for grinding plant food. In fact, bears are opportunistic, omnivorous feeders.
Bears first appear in the fossil record of about 27 million years ago, as a fox-sized animal known as the dawn bear. About 6 million years ago, there were numerous species of bears, some of them huge, but all now extinct. Seven species of bear survive today. The most recent one is the polar bear (Thalarctos maritimus ), which evolved from the brown bear (Ursus arctos ) only about 70,000 years ago. The most recent extinction was the cave bear (Ursus spelaeus ),
a huge animal that co-existed with humans as recently as 20,000 years ago. The giant panda (Ailuropoda melanoleuca ) is sometimes classified in the bear family, based on morphological and genetic similarity.
Bear species are quite variable in size. The smallest is the Malayan sun bear (Helarctos malayanus ), males of which are about 4 ft (1.2 m) long from head to tail, stand about 28 in (70 cm) tall at the shoulder, and weigh up to 440 lb (200 kg). The largest species is the polar bear, which may be up to almost 10 ft (3 m) long, and weigh a ton (about 900 kg). Most male bears, called boars, are considerably larger than the females, or sows. This is especially important when boars compete for females. Sun bears and sloth bears (Melursus ursinus ), however, take only one mate and so the sexes are nearly the same size.
Bears from cold regions do not truly hibernate. Instead, during the coldest part of the winter when food is not readily available, they enter a long period of lethargy. However, their body temperature and heart rate do not drop much, as would occur during true hibernation. During this time bears sleep a great deal and do not eat; instead, they live off energy stored in their body fat. The exception to this pattern is the polar bear, which continues to hunt for seals during winter.
Bears have an amazing ability to adjust physiologically to seasonal ecological changes. Non-tropical bears mate during the spring or summer, but the fertilized eggs float free in the uterus, rather than immediately implanting and developing. Later, in early winter, the sow finds or creates a den in which to sleep away the winter. At that time, the eggs implant and their gestation starts. The cubs are poorly developed when born, being blind, nearly hairless, almost helpless, and extremely small. For example, a female brown bear weighing 450 lb (205 kg) produces cubs weighing less than a pound (about 450 g). The cubs are born during the winter, and are so small that they are incapable of regulating their body temperature. The warm den in which the mother winters provides a snug place for them to nurse and grow until they can maintain their body temperature. The sow’s milk, which is extremely rich in fat, sustains the rapid growth of the cubs during the winter denning. By the time the sow is ready to leave her den, the cubs have grown enough to follow her.
All bears are thickly furred, often with a coat of a single color. With the exception of the sun bear, all species have fur on the bottom of their feet, around the pads of the soles. This is especially important for the polar bear, which spends most of its life walking on snow and ice. The sun bear, on the other hand, has furless feet as an aid in climbing trees. The feet of bears are well-armed with heavy claws. Bears walk in plantigrade fashion, meaning they walk on the heel and sole of the foot. Most other mammals walk on their toes. Although they may look rather lumpish and clumsy, bears can run for short distances at speeds up to 40 mph (64 km/h).
Bears have a reputation for having poor eyesight, but their sight is actually quite good. Their sense of smell, however, is extremely acute. Bears can identify the odor of animals that passed by as much as several days before. Unlike other carnivores, bears do not have their lips attached to their gums. This means that they can make facial expressions and use their lips to suck in food, such as insects or honey.
Brown or grizzly bears (Ursus arctos ) live in forest, tundra, and grassland across the top of the Northern Hemisphere, including both North America and Eurasia. Some biologists separate them into three subspecies: the Eurasian brown bear (U. a. arctos ) of much of temperate and subarctic Eurasia; the grizzly bear (U. a. horribilis ) of Canada, Russia, and the United States; and the Kodiak bear (U. a. middendorffi ) of Kodiak Island and two smaller islands in the Bering Sea off Alaska. These brown bears may vary in color from white to cinnamon to black, but their fur is most commonly brown. Some brown bears have white tips on the end of the hairs, a coloring called grizzled. A grizzly bear can be distinguished from other bears in North America by the profile of its body. The outline of its head as seen from the side is concave, or scooped inward, and its shoulders are high due to a thick layer of muscle and fat, which makes the back appear to slope downward.
Grizzlies that live near rivers accessible to the ocean feed on migrating salmon as much as they can. The bears that feed well can be huge in size. Even with their great size, they like to frolic in the water and are adept at catching fish, which they carry to land to strip the flesh from the bones. They may also hunt rodents, and will opportunistically predate large prey such as moose, caribou, and even black bears. When meat is not available, grizzlies feed primarily on roots, sedge leaves, and berries.
Grizzly bears mate in the late spring. In the autumn, the sow finds a den in a cave or hollow tree, and settles in for her winter lethargy. Two or three cubs are born, usually in February. The cubs stay close to their mother for at least two years, continuing to nurse during most of that time. The mother is exceedingly protective, and teaches the cubs to climb trees to escape danger. Most attacks on humans are made by sow grizzlies protecting their cubs. One of the major enemies from which she must defend her cubs is male grizzlies, which will kill and eat them.
After a young female leaves its mother, it may continue to share the same feeding range. A male, however, will go off on its own, traveling up to 100 mi (160 km) before finding a place to settle down. It may have difficulty finding a suitable habitat because older males will fight to keep new ones out of their territory.
The range of the grizzly bear originally extended from Alaska to Mexico, as far east as Hudson Bay, through the prairie region, and even extending into desert habitat. Grizzlies have now disappeared from most of the western United States, with only small numbers surviving in Wyoming, Montana, Idaho, and Washington. There are larger numbers in Alberta, British Columbia, Yukon, Alaska, and the Northwest Territories. The populations of grizzlies in the United States (outside of Alaska) have declined mostly because of excessive hunting and habitat loss. These are still important problems for the species.
Eurasian brown bears vary greatly in size. The few remaining in Spain rarely weigh more than 250 lb (114 kg). Those in Siberia may rival the huge Kodiak bear in size. Brown bears are still found throughout much of Europe and Asia, but in rapidly decreasing numbers because of excessive hunting and habitat loss. Those that survive, live primarily in hardwood forest in mountainous regions. About 120,000 brown bears are thought to live in Eurasia, the vast majority of these in Russia.
Polar bears are huge, whitish or yellowish, marine bears that are well adapted to life in and around the icy Arctic Ocean. They are relatively slender bears, with a longer neck and head than brown bears. The individual hairs of their thick fur are transparent; it is reflected sunlight that makes their coat appear white. Oddly, their skin is black. Any sunlight that gets through its fur is absorbed by the black skin, helping to keep the animal warm. Polar bears swim by paddling their furry, slightly webbed front feet and steering with the back feet.
Polar bears hunt by wandering extensively over the sea ice, often moving from ice floe to floe, and sometimes swimming for hours in the cold water. They do this to find good places for hunting their favorite food, the ringed seal (Phoca hispida ). Their claws are longer than those of other bears, and are used to grasp their seal prey as it rises out of the water at a breathing hole or along the edge of an ice floe. During the spring, polar bears break into the snowy dens where female seals have given birth to their young.
Polar bears may congregate in areas where ice floes move freely in the wind, because seals are more easily obtained in that habitat. Polar bears will tolerate each other if they find a stranded whale carcass, or if a walrus has been killed. During the summer, when the ice is gone from the mainland coast, polar bears may move onto the land and feed on berries, or they may fast. They may also be attracted to garbage dumps near towns, where there can be dangerous encounters with people.
Polar bears are usually solitary animals, coming together only to mate in late spring (March-June). One to three cubs are born in December or January in a snow den constructed by the mother. The cubs average about 23 oz (650 g) at birth, but weigh 20 lb (9 kg) or more when they emerge from the den in April. This rapid weight gain is possible because the milk of the sow contains more than 30% fat. The cubs stay with their mother for at least two years, learning to hunt and defend themselves. At that time the mother will mate again. Young females become sexually mature at five years of age. Polar bears live to be 20 or more years old.
Five countries have populations of polar bears: the United States (Alaska), Canada, Denmark (in Greenland), Norway, and Russia. These nations are cooperating in the management of their polar bears, allowing only a tightly controlled hunt. The hunt is mostly carried out by aboriginal people, who eat the bear meat and sell the valuable hides.
Global warming is a more recent threat to polar bear populations due to its impact on sea ice. Sea ice reduction due to global warming influences not only the availability of hunting and denning habitat, but may impact the overall distribution and abundance of polar bears. Scientists have found evidence of polar bear drownings because some bears must swim longer distances (up to 60 mi/96 km) over open ocean in their search for food. The bears are forced to swim these longer distances because of contractions in the sea ice. Although polar bears are strong swimmers, they are best adapted to swimming near shore and these longdistance swims leave them vulnerable to exhaustion and swamping by waves.
The American black bear (Ursus americanus ) occurs largely in forested habitat, but also in grassy meadows and tundra. It ranges across most of North America, from Alaska to Newfoundland, and south into Mexico. Although its populations are depleted in many parts of its range, it is still a widespread species. The black bear’s fur is usually black, but may also be brown, light tan, or even white. Different colors may occur within a litter of cubs.
Black bears have larger ears than other bears. Size varies among populations; the overall range of adult weight is about 125–600 lb (57–272 kg). Males typically weigh about one-third more than females. The weight of individuals varies considerably during the year, depending on the amount of nourishing food available. Because they are much smaller than grizzlies, black bears try to stay out of sight when territories of the two species overlap.
Each black bear has a territory where it forages for berries, nuts, honey, insect grubs, fish, rodents, and carrion. A male’s territory typically overlaps those of several sows. Black bears mate during the summer, with each female being visited several times by nearby males. The fertilized egg does not implant until the autumn. However, if the female bear is poorly nourished, the fertilized eggs do not implant at all. One to four cubs are born in January, after a gestation of 8–10 weeks.
The Asiatic black bear (Selenarctos thibetanus )is a black-colored animal with a white crescent on its chest; its alternate common name is moon bear. The hair around its neck is considerably longer than elsewhere on the body. A male may weigh up to 350 lb (159 kg), while females usually weigh less than 200 lb (91 kg). This bear inhabits mountain forests from Afghanistan, across China to Japan, and south to Southeast Asia.
The Asiatic black bear is generally nocturnal, but will sometimes venture out in the daytime to feed on sun-warmed fruit. It is also an opportunistic predator, and can kill fairly large animals by breaking the neck. Northern populations of Asiatic black bears sleep away the winter, but in warmer parts of their range they remain active year-round. They mate in the autumn, and the cubs are born 3–4 months later. The cubs are weaned by four months of age, much earlier than in other species of bears.
The sun bear (Helarctos malayanus ) is the smallest species of bear, with some adults weighing less than 100 lb (45 kg). It has black fur, a whitish snout, and often a whitish or orange, U-shaped mark on the chest. It has a much longer tongue than other bears, which is used to lap honey, bees, or termites from their nests. They also eat a wide variety of tropical fruits. The sun bear is a tropical species, found in rainforest from southern China to Borneo and Sumatra. Sun bears climb trees well, and often build nests by breaking and bending branches together. Cubs are born at any time of the year, after a gestation of 14 weeks.
The sloth bear (Melursus ursinus ) of India and other countries in South Asia has the longest hair in the bear family, although its belly is almost hairless. Otherwise, it looks much like the Asiatic black bear. However, it has several distinctive behavioral traits: it carries its young on its back, and the male remains with the female to help raise the cubs. The sloth bear has long, strong claws, which are used to break open termite nests during feeding.
The spectacled or Andean bear (Tremarctos ornatus ) is the only bear species in South America. It occurs in the Andean region, even living above 14,000 ft (4,300 m). It has a whitish, eyeglasses-shaped pattern around its eyes and a band of white on its neck and chest. It climbs trees well and may sleep in them. Male spectacled bears may reach a head-body length of almost 6 ft (1.8 m) and a weight of 400 lb (182 kg). In profile, they have a shorter nose than other bears.
The spectacled bear is primarily nocturnal, sleeping during the day in an excavated cavity or under tree roots. At night, it often climbs high into trees to feed on fruit. If it finds a good supply and decides to stay a while, it will build a platform of branches as a nest.
All bears except polar bears are regarded as therapeutic in traditional medicine in Asia. In particular, fluid from the gall bladder is thought to have many health-enhancing qualities. Many Asiatic black bears are kept in captivity, where they have tubes implanted
Carnassial teeth— Specialized teeth of mammals in the order Carnivora, which are longer and sharper than other teeth and useful in tearing meat.
Plantigrade— Walking on the heel and sole of the foot instead of on the toes.
into their gall bladder from which bile fluid is continuously withdrawn without killing the animal. It is also thought that an aphrodisiac, or love potion, can be made from the gall bladders of bears. The flesh of bear paws is regarded as a gourmet food in eastern Asia. Because of these uses, the Asiatic black bear has long been intensively hunted, and is now an endangered species. Many American black bears and Eurasian brown bears are also killed so that their gall bladder can be harvested and exported to eastern Asia.
Even the extremely rare spectacled bear of South America is hunted by indigenous people, who believe their fat is useful in the treatment of arthritis.
In addition to the problem of excessive hunting, all bears are being affected by habitat destruction, mostly to develop agricultural land. When humans move into natural habitat, bears are often the first animals to be eliminated. All species of bears have declining populations, and four species (Polar bear, Asiatic black bear, spectacled bear, and sloth bear) are classified as vulnerable by the World Conservation Union (IUCN). If their populations are not better conserved, it is possible that some species of bears will become extinct.
See also Pandas.
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Jean F. Blashfield