Bears are large mammals, and comprise eight species in Europe, Asia, North and South America, and the circumpolar Arctic. The earliest bears lived in North America and Europe during the late Eocene epoch, approximately thirty-seven million years ago. Bears are classified as carnivores, having evolved from small generalized predators. Functionally, however, bears range from the almost completely carnivorous polar bear to the almost completely herbivorous giant panda. The remaining six bear species are omnivorous, opportunistically feeding on a wide range of plants as well as mammals, fish, insects, and mollusks. A brown bear in Alaska, for example, emerging fromits winter den in April orMay,may feed on carrion (winter-killed moose or caribou), switch to grasses, sedges, and roots as spring progresses, and then eat mostly salmon as the summer salmon runs begin.
The Classification Controversy
While most taxonomists today recognize eight species of living bears, there has long been disagreement over whether to consider the giant panda (Ailuropoda melanoleuca) a true bear. The issue is complicated by the existence of the red (or lesser) panda (Ailurus fulgens). The red panda is raccoonlike in appearance, while the giant panda looks like a bear, yet the two pandas share some anatomical and behavioral features. The fossil record on pandas is scant. Recent molecular studies have shed new light on the issue but are not conclusive. Anatomical, biochemical, paleontological, behavioral, and reproductive evidence is all relevant to this issue but is subject to differing interpretations by different authorities. Some place both pandas in the bear family (Ursidae); some put both in the raccoon family (Procyonidae); some consider the giant panda a bear and the red panda a raccoon; some put the two pandas in their own family, naming it either Ailuridae or Ailuropodidae; and some put the red panda in Ailuridae and the giant panda in Ailuropodidae. Clearly there is no simple answer to the question of how to classify the pandas. However, in general usage among both biologists and lay persons at the beginning of the twenty-first century, the giant panda is considered a bear, and the red panda is not.
Physical Characteristics of Bears
Bears are heavy-bodied, stout-legged, short- tailed mammals with long skulls and short snouts. Most bears have thick fur. All have five toes with nonretractile claws on each foot. Some have claws modified for climbing and some for digging. Bears are plantigrade, walking on the entire sole of the foot. Even lacking some of the modifications for speed possessed by canids and ungulates, some bears can run forty miles per hour. Bears have good, but not exceptional, eyesight and hearing. Their sense of smell, however, is excellent and greatly aids them in finding food. Bears range in size from the sun bear, which weighs 60 to 145 pounds as an adult, to the male polar bear, which weighs up to 1,770 pounds. Large Kodiak bears (a type of brown bear) are almost as large as the largest polar bears. The extinct bear Arctodus, which lived during the Pleistocene epoch, was the largest bear ever, andmayhave reached fourteen feet tall when standing on its hind legs. All living ursids have forty-two teeth. They have large canine and incisor teeth, the premolars are often reduced, and the molars in all except the polar bear form crushing platforms for grinding plant material.
During winter, in areas where food supplies are scarce, many bears den up for a winter sleep. This hibernation differs from that of such deep hibernators as ground squirrels. A ground squirrel's body temperature may drop to the freezing point (32 degrees Fahrenheit) or slightly below.Abear's body temperature drops only 5 to 9 degrees Fahrenheit from its normal level of 99 degrees Fahrenheit. Its heart rate drops from forty to seventy beats per minute to eight to twelve, and its metabolism drops by about half. This slowing of the body's processes combined with the heatconserving properties of the den allow a bear to go up to eight months without eating or drinking. Stored body fat is the bear's only source of energy. Hibernating bears do not defecate or urinate. Brown bears and American and Asiatic black bears hibernate, as do pregnant female polar bears.
Abrief mating season of one to two weeks occurs in spring or summer. Females in heat are followed and courted by males. There is often fighting among males, with the strongest and most aggressive winning the right to breed. In bears there is a delay in the implantation of the fertilized egg in the wall of the uterus. In the polar bear, for example, mating may take place in April, but the embryo will not implant and begin to grow until October. Because of this delay, the cubs, when born in December, are small and relatively undeveloped, weighing 1 to 1.5 pounds. The female is in her winter den at this time and is living off her fat, and the needs of her cubs do not overwhelm her capacity to produce milk. Bear cubs generally leave their mothers at 1.5 or 2.5 years, at which time the female is ready to breed again. Therefore, most female bears give birth to cubs every second or third year.
Bears are intelligent, single-trial learners. They can remember where and when they have found food in the past-a trait that is very useful to them in the wild, but is a problem if they gain access to human food or garbage. Bears conditioned in this way can quickly lose their fear of humans and are potentially dangerous. Cubs learn how to find food and other life lessons by observing their mothers. Other than family groups of a mother and her cubs, bears are usually solitary. However, a rich, concentrated food source, such as runs of spawning salmon in a river, may cause bears to gather in one area. In this case, a dominance hierarchy is established and maintained. Bears posture and threaten one another, communicating with body language and, to a limited degree, facial expressions. Fighting occurs and sometimes results in injury but is rarely fatal. Through this dominance hierarchy, prime fishing spots are claimed by dominant bears, as are breeding rights. Conflict is minimized as the bears concentrate on what is most important: eating.
In many areas of the world, humans have converted wildlands into farms, tree plantations, oil fields, mines, suburbs, and cities. Many species of bears have been greatly reduced as a result. The giant panda, an endangered species, numbers only about one thousand in the wild. The other Asian bears-the sun, sloth, and Asiatic black bears, as well as the spectacled bear from South America-are also severely threatened. The American black bear, however, is thriving, with an estimated population of 450,000. Polar bear populations have been stabilized by a treaty limiting hunting to native peoples, but may now be threatened by global climate change. Efforts are being made to restore the grizzly (a type of brown bear) in some western areas of the United States. The conservation of bears worldwide requires the protection of wildlands such as national parks, forests, and wildlife refuges, and the animals that inhabit them. In multiuse areas, careful management of bear populations can assure their survival.
Family: Ursidae (bears)
Subfamilies: Ursinae, Tremarctinae, Ailuropodinae
Genus and species: Ursus arctos (grizzly or brown bear), U. maritimus (polar bear), U. americanus (American black bear); Selenarctos thibetanus (Asian black bear); Helarctos malayanus (sun bear); Melursus ursinus (sloth bear); Tremarctos ornatus (spectacled bear); Ailuropoda melanoleuca (giant panda)
Geographical location: Europe, Asia, North and South America, and marine areas of the Arctic
Habitat: Varies by species; temperate forests, subalpine mountains, tundra, grasslands, tropical rain forests, deserts, and sea ice
Gestational period: Seven to eight months for the American black, spectacled, brown, and polar bears; four to seven months for the Asiatic black bear, giant panda, and sun bear; unknown for the sloth bear
Life span: Wild bears typically live fifteen to twenty-five years; rarely, wild bears live past thirty, and captive bears have exceeded forty years
Special anatomy: All living bears except the polar bear have flat molars, which are adapted for crushing plant material; some bears have claws adapted for climbing and some for digging; although most bears eat a considerable amount of plant material, they have retained the short, simple gut of their meateating ancestors
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