Bear

G

Brown Bear

The brown bear is distinguished from other bear species by the presence of a prominent shoulder hump and an upturned snout. The brown bear’s vast range includes parts of Europe, Asia, and North America, where the bears inhabit a variety of habitats, including tundra, dense forests, and deserts. Within these regions, populations are mostly small, isolated, and restricted to remote areas. Coat color in brown bears ranges from a creamy white through various shades of brown to almost jet-black. In certain subspecies of brown bears, such as the grizzly bear found in inland areas, the hairs on the back may be white-tipped, giving the outer coat a grayish, or grizzled, appearance.

Brown bears, along with polar bears, are the largest of the bear species. In general, brown bears range in weight from less than 90 kg (200 lb) for small females to more than 800 kg (1,760 lb) for the largest males. Brown bear populations differ significantly in size, depending largely on available diet. Coastal populations that feed on abundant, fatty salmon in Alaska, for instance, may be three times larger than inland bears, such as grizzlies, which subsist on plant parts, insects, and some meat they steal from wolves or catch themselves. As a result of their fatty fish diet, the Alaskan brown bear, a subspecies that ranges throughout coastal Alaska and western Canada, and the Kodiak bear, a subspecies found on three Alaskan islands that make up the Kodiak archipelago, are the largest brown bears. The Alaskan brown bear may weigh up to 800 kg (1,760 lb) and reach 2.3 m (7.5 ft) in length. The Kodiak bear can grow to 770 kg (1,700 lb) and reach the towering height of 3 m (10 ft).

The brown bear population in Alaska is estimated at 32,000. Elsewhere in North America, brown bears may number fewer than 1,200 individuals, and these animals are listed as threatened by the U.S. Fish and Wildlife Service. In 2007 the population of grizzly bears in Yellowstone National Park was removed from the list of threatened and endangered species, although grizzlies in other areas are still listed as threatened.

The IUCN Red List of Threatened Animals does not list the brown bear, although populations in Western Europe have been greatly reduced by habitat destruction and hunting. Fewer than 10 bears remain in the Pyrenees Mountains between Spain and France, and a similar number survive in the Italian Alps. About 5,000 to 6,000 brown bears live in Eastern Europe, and more than 120,000 live in Russia.

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H

Polar Bear

The polar bear inhabits the polar regions of the Northern Hemisphere, where its white fur blends perfectly with its ice- and snow-covered Arctic habitat. Although primarily found along coasts and ice floes, polar bears may winter in dens up to 48 km (30 mi) inland. Polar bears are one of the largest bear species and can grow to a length of 2 m (7 ft) and weigh up to 800 kg (1,760 lb). Excellent long-distance swimmers, polar bears use their large front feet as paddles and trailing back feet as rudders.

With little vegetation in their frigid environment, polar bears are the most carnivorous of bears, eating primarily seals and sometimes young walruses. They prefer to hunt seals on land or ice but may stalk them from under water. During the summer months when ice floes melt, polar bears are unable to hunt seals and may wander far inland seeking berries, bird eggs, and other foods.

Polar bears are listed as vulnerable on the IUCN Red List of Threatened Species. The United States is considering adding the polar bear to its list of threatened and endangered species under the Endangered Species Act. From 22,000 to 25,000 polar bears make up 19 population groups that live in five Arctic areas of Greenland, Norway, Russia, Canada, and the United States.

Loss of sea ice as a result of global warming may pose the greatest threat to polar bear populations—the bears use the ice for hunting seals and for resting. As much as two-thirds of the world’s population of polar bears may disappear within 50 years as sea ice dwindles. Concentration of pollutants in the Arctic food chain can also affect health and reproduction in polar bears. In addition, polar bear populations remain at risk from ongoing oil and gas exploration and development in the Arctic region. Hunting policies that target adult male polar bears may reduce reproduction success in some regions of Canada.

IV

Bear Behavior

Bears mostly live alone, except for mothers and their cubs, and males and females during mating season. Bears form temporary groups only in exceptional circumstances, when food is plentiful in a small area. Alaskan brown bears may collect in the same area to feed on salmon during the annual salmon runs, when salmon swim up river to reach their spawning grounds. Recent evidence also suggests that giant pandas may form small social groups, perhaps because bamboo is more concentrated than the patchy food resources of other bear species. Other bears may live alone but exist in a social network. A male and female may live in an overlapping home range—although they tolerate each other, each defends its range from other bears of the same sex. Male young usually leave their mothers to live in other areas, but female young often live in a range that overlaps with that of their mother.

The key to a bear’s survival is finding enough food to satisfy the energy demands of its large size. Bears travel over huge territories in search of food, and they remember the details of the landscape they cover. They use their excellent memories to return to locations where they have had success finding food in past years or seasons. Most bears are able to climb trees to chase prey or gain access to additional plant vegetation. The exceptions are polar bears and large adult brown bears—their heavy weight makes it difficult for them to climb trees.

Little is known about communication among bears. Vocalizations and movements of the mouth and ears appear to be communication methods used by most species. Scent marking for communication is done by urine or by rubbing trees. When they need to act threatening or fight, bears tend to stand up on their hind legs, perhaps to appear larger to their rivals. They use their clawed forepaws to slash at their opponents.

Bears that live in regions with cold winters spend the coldest part of the year asleep in sheltered dens, including brown bears, American and Asiatic black bears, and female polar bears. Pregnant females give birth in the winter in the protected surroundings of these dens. After fattening up during the summer and fall when food is abundant, the bears go into this winter dormancy to conserve energy during the part of the year when food is scarce. Winter sleep differs from hibernation in that a bear is easily aroused from sleep. In addition, a bear’s body temperature drops only a few degrees—an American black bear with an active body temperature of around 38°C (100°F) will undergo a moderate drop in body temperature to about 34°C (93°F). In contrast, a true hibernator undergoes more extensive changes in bodily functions. For instance, the body temperature of the Arctic ground squirrel drops from 38°C (100°F) to as low as -3°C (27°F).

V

Reproduction

Reproduction is seasonal for most bears. For bears living in temperate zones with four seasons, mating occurs in the spring after bears emerge from winter sleep. Bears that live in tropical regions, such as sloth bears, mate during the dry season. A male and female typically stay together for one to two weeks during mating season, mating many times before going their separate ways.

After mating, the gestation, or pregnancy, period generally ranges from seven to nine months, depending on the species. The actual development period of the unborn young is not as long as the gestation period suggests because the embryo passes through a period of delayed implantation. In this process, the newly forming embryo halts development and becomes inactive for as long as six months. The embryo then resumes its activity and becomes implanted in the wall of the female’s uterus, where its development proceeds rapidly. In the American black bear an embryo formed from a June mating may not become implanted until sometime in November. This delayed implantation enables birth to coincide with a bear’s winter sleep. The vulnerable newborns, born within the safety of dens, receive additional warmth and protection as they share close quarters with their mother.

Bears usually produce litters of one to three cubs. Like most carnivores, bear babies are smaller at birth than the young of most other mammals, sometimes weighing 1 percent or less of their mother’s body weight. A newborn giant panda cub weighs around 113 g (4 oz). All bear young are born blind, toothless, mostly hairless, and completely helpless. They grow quickly, nourished by milk from their mother that is high in fat. By about three months of age, cubs leave the den to forage for food with their mother.

Cubs are weaned at about six months of age but stay with their mother for two to three or more years while they learn from her what to eat and where to find food. Mothers also try to protect their cubs from adult males, which sometimes kill cubs. This behavior on the part of males may be a strategy to increase breeding opportunities, as females without cubs will be more readily available to mate.

Females become sexually mature at four to seven years of age and may then produce young every two to five years. In general, the age at which a bear first reproduces comes earlier when food is abundant and later when food is limited.

VI

Evolution

Bears are the youngest of the carnivore families, having arisen from doglike ancestors during the Eocene Epoch, around 55 million to 38 million years ago. The earliest bears had the characteristics of both dogs and bears, with heavy-set features and blunter teeth than those of true dogs. Modern bears appeared in Eurasia around 5 million years ago during the late Miocene Epoch. These bears were relatively small animals, but some types eventually grew quite large. They diversified and spread through Europe, Asia, and North America, eventually reaching South America. Fossils indicate that bears once lived in Africa, with one large primitive species found as far south as present-day South Africa.

Among the extinct bears was the largest land-living mammalian carnivore. The giant short-faced bear was almost twice as big as today’s brown bear. Unlike modern bears, it was lightly built, with long legs and feet that enabled it to run fast. It was likely a powerful hunter capable of killing large prey. However, recent research suggests it may also have used its size and strength to chase other predators away from their kills. The giant short-faced bear lived in North America during the Pleistocene Epoch, from 1.6 million to 10,000 years ago. The earliest people to reach North America, perhaps as early as 15,000 years ago, must have found this animal truly frightening. The short-faced bear spread into South America. Scientists believe the spectacled bear is a smaller relative of this extinct bear.

Perhaps the most famous extinct species is the great cave bear, which lived in Europe around the Middle and Late Pleistocene and became extinct around 10,000 years ago. Its bones have been found in caves from Spain to the Caucasus. The largest deposit of remains is that of the Drachenhohle caves in Austria, which contain the remains of about 30,000 cave bears. Many of the remains are those of animals that over the centuries died in the caves during their winter dormancy, probably as a result of illness, lack of food, or old age. Distinguished by a massive skull and a domed forehead, this bear was as large as the modern Alaskan brown bear. Its teeth indicate that it ate an almost exclusively vegetarian diet. Cave drawings created by Ice Age humans show that they occasionally hunted cave bears. For several thousand years the modern brown bear and the cave bear co-existed, but the cave bear became extinct around 10,000 years ago.

The modern brown bear first appeared in Asia during the Middle Pleistocene Epoch and then spread to North America and Europe, even reaching northern Africa. A population of brown bears that lived along the coast became specialized for hunting seals, eventually evolving into the polar bear around 700,000 years ago. The American black bear dates to before the Late Pleistocene, and unlike today’s black bears, these prehistoric black bears grew as large as modern grizzlies. The first giant pandas appeared in Europe in the late Miocene and were found in eastern Asia during the Pleistocene Epoch.

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