Bears

The bears worldwide consist of eight species of the family Ursidae, of which three species are found in Arctic areas. The first bearlike ancestor was the small doglike carnivore Ursavus that was common in the Pliocene (5 to 2 million years ago). Similar in size to a racoon, this ancestral bear eventually gave rise to the modern bears and another branch that resulted in the now extinct giant short-faced bear (Arctodus simus). During the Pleistocene epoch or Ice Age (2 million to 10,000 years ago), the giant short-faced bear ranged over much of North America and into the Arctic. This bear was likely the most powerful predator in North America during the Ice Age and the largest individuals would have reached 700 kg (1500 pounds). In the late Pliocene, the first member of the genus Ursus evolved: Ursus minimus. In this period, bears grew in size in response to a sudden change in climate that included greater seasonality. Larger body size would have aided regulation of body temperature. The next bear to evolve was Ursus etruscus (about 2.5 million years ago), and this bear gave rise to the modern bears of the Northern Hemisphere.

The most northern bear is the polar bear (Ursus maritimus) that is restricted to the Arctic and Subarctic areas. The brown or grizzly bear (Ursus arctos) has the widest distribution of any bear: from the northern edges of North America, Europe, and Russia south to Italy, Spain, Turkey, and historically as far south as Mexico. The third and smallest species is the black bear (Ursus americanus), whose range is restricted to North America and reaches the Arctic in Alaska, Yukon, Northwest Territories, Nunavut, northern Qu├ębec, and Labrador but is also found as far south as Mexico. Where brown and black bears overlap, the brown bear is dominant and black bears can become prey. Black bears are only found in tundra areas in northern Labrador. Exclusion of black bears from other tundra areas is thought to be due to the presence of brown bears. Black bears are the only Arctic bear species capable of climbing trees as adults. In most areas, they rarely wander above the treeline where they cannot escape brown bears. Polar bears and brown bears overlap along the southern coast of the Beaufort Sea, but they do not interbreed in the wild. However, in zoos, brown and polar bears can interbreed and produce fertile young that are intermediate in traits. Brown bears will sometimes wander out onto the sea ice to utilize seal carcasses left by polar bears.

In evolutionary terms, polar bears and brown bears are very closely related with black bears branching off earlier. All bears share some common traits: stocky build, massive shoulders, strong limbs, short fur, plantigrade (walk with the sole on the ground), five toes, strong nonretractable claws, large canine teeth, a small tail, males have a baculum (penis bone), small ears, a heavy skull, a well-developed sense of smell, and good vision. Other mammals seldom prey on bears, so a heavy body and slow gait are not detriments. However, bears are capable of attaining speeds of 30-35 km h-1 for short periods. Over longer distances, bears rapidly overheat and can suffer hyper-thermia. All bears are good swimmers and can swim long distances if required.

In body size, polar bears are the largest with weights in the 200-600 kg range. Brown bears are the next largest with weights in the Arctic about 150-400 kg. Black bears weigh about 60-150 kg in the Arctic part of their range. Both brown and black bears can be substantially heavier in the southern parts of their range. All species are sexually dimorphic, with males typically 30-100% heavier than females. The large size and strength of bears likely evolved to avoid predation, digging for food, making shelter, and preying on other animals.

Bears live a solitary life for the most part. Bears are only found in groups in families, during the breeding season, or at sites with abundant food. Brown and black bears maintain defined home ranges or territories that overlap and may be defended against intruders. In contrast, polar bears use the same areas year after year but do not defend these areas from other bears. The areas used by males are typically 2-5 times larger in area than that used by females. The exception may be polar bears where the size of areas used may be similar in both sexes.

The reproductive ecology of bears is similar across all species. There can be intense competition between males for access to breeding females. Broken bones, teeth, and substantial wounding can occur during fights. It is believed that larger males are more dominant and thus sexual dimorphism is favored. The mating system can be considered polygynous or serially monogamous. Males do not contribute to the rearing of young. Females mature at 3-6 years of age and males at about 4-9 years. The breeding season is in the spring and, following fertilization, the egg develops up to a multicelled state called a blastocyst. At this stage of development, the blastocysts stop developing and enter a state of suspended growth until the autumn when implantation in the uterus occurs. Pregnant females of all species stop feeding and enter dens at this point. The embryos develop while the female is fasting and the mother gives birth to young in the den during winter. The young are born very altricial (poorly developed) with little hair, eyes closed, and weighing less than 600 g. The litter sizes are largest in black bears (up to six cubs) followed by brown bears with up to four cubs, and then polar bears with a maximum of three. The mother nurses the cubs with fat-rich milk (up to 50% fat). By the time cubs emerge from the den at 3-5 months of age, their weights have increased many fold, reaching up to 10 kg. Cubs remain with their mothers for 1-3 years depending on species.

Upon emergence from the den, the mother has undergone an extended fast of upwards of 7 months in some areas. All age and sex classes of bears can enter dens to avoid inclement weather and lack of food over winter. However, nonpregnant polar bears only den for shorter periods. Specialized fasting physiology conserves muscle mass by recycling nitrogenous body wastes. Bone strength is also maintained so that the bear can emerge from the den ready for renewed foraging. Whether the winter sleep of bears is hibernation or a state of torpor is debated and is largely dependent upon the definition used. When bears are in dens, the body temperature drops, the heart rate slows, and metabolism is reduced. The net result is conservation of energy. During this state, bears do not feed, drink, urinate, or defecate. The energy used while in dens is largely derived from fat stores and bears can lose up to 50% of their body mass over winter. However, in contrast to smaller hibernating mammals, the body temperature of bears does not drop more than a few degrees. This allows bears to arouse much faster than a deeply hibernating animal. In addition, if a bear allowed its body temperature to drop to very low levels, storing enough energy to rewarm would be difficult. The small size of bear cubs at birth may in part relate to the fasting physiology of their mothers that must undergo pregnancy and early lactation without any dietary intake.

The polar bear is the most carnivorous of all the bears and most of the diet consists of ringed and bearded seals. However, similar to all bears, polar bears will even prey on reindeer and feed on berries. The diet of brown and black bears varies greatly by region, but they are omnivores (eat a mix of plant and animal material). Berries, roots, grasses, insects, small mammals, fish, and bird eggs can form the bulk of caloric intake in black and brown bears in some areas. However, some brown and black bears are active predators and prey on moose, caribou, muskox, and other mammals. Often, newborn ungulates are taken. All bears will scavenge carrion and will take prey from other predators. If a large kill or carcass is available, bears will often spend several days feeding on it. Over 50 polar bears have been observed feeding on a dead bowhead whale frozen into the ice. A seasonal pattern of feeding means that bears typically reach their peak condition in late summer or early autumn. This means that they will have stored sufficient fat to assist with food shortages or denning over winter. Reflecting their carnivorous ancestry, bears do not have the elaborate digestive system of ungulates nor an elongated intestinal tract to assist with digesting plant material. The dentition of bears is that of a generalist (not specialized to meat or plants), and because they are unable to digest plant materials very well, they rely on the flattened molars to crush vegetation that helps release the contents to aid digestion. Therefore, bears must be more selective than a moose or reindeer when feeding on plants and restrict themselves to the most energy-rich parts.

The population dynamics of all bear species is hallmarked by low reproductive rates due to the prolonged mother-offspring bond and small litter sizes. Adult survival rates are high (80-98%), but juvenile survival varies widely between years. These factors result in low population growth rates but stable population sizes that do not fluctuate widely. In comparison to other mammals, the density of bears is typically low. The main sources of mortality are food shortages, disease, intraspecific aggression (cannibalism and infanticide are known in all three species), and harvest by humans. For many areas of the Arctic, humans regulate population numbers through harvest. Habitat loss in the Arctic is not as threatening to bears as it is for southern populations.

Bears are a prominent element of biodiversity wherever they are found. Through prehistoric and historic times, bears have affected human art, folklore, mythology, and culture. To a large degree, the influence of bears on humans is tied to occupation of a similar niche: both bears and humans are omnivores, often feed on the same vegetation or prey, and occupy the same habitats. That bears can prey on humans and humans can prey on bears created a close relationship that predates history. Bears are formidable prey, and in many locations bears were hunted by groups of people often with dogs aiding in the control of the bear. Dogs are well able to locate a bear, harass a bear, and keep it occupied while hunters shoot arrows or use spears to kill the animal. Today, polar bears rarely kill humans, with one or two people killed every few years. In Arctic areas, black and brown bears are responsible for one or two human deaths every year. Tourism to see polar bears and brown bears is an important element of the developing tourist trade in Arctic regions. The economic returns of tourism to see bears are rapidly increasing. In addition, the economic returns of sports hunting can be substantial in small communities and provide employment for guides and those associated with hunting. Selling of hides is an additional source of income in some areas.

The main threat to bears comes from loss of habitat due to encroachment of human activities such as mining, oil development, agriculture, forestry, and urbansuburban expansion. Human encroachment results in refuse dumps that bears feed in and can habituate bears to humans. Habituation can result in bears becoming less wary. Often, habituated bears are killed when they approach humans in search of food. Climate change is also a threat to all three Arctic species, but perhaps more so for polar bears. Pollution represents another threat to polar bears. The distribution of polar bears is largely intact. Globally, the distribution of brown bears is vastly reduced but the Arctic maintains large populations. The range of brown bears in the European and Russian Arctic is reduced, particularly in Europe. Black bears still occupy most of their range in the Arctic. Bears are often drawn into human settlements. Dozens of bears are destroyed every year in defense of life and property in the Arctic. Overharvesting in some populations may result in local declines for all three species. Poaching of bears (particularly gall bladders, paws, and bacu-lum) for the Asian medicinal market is a threat to all bear species. International trade in bears and their parts is covered by the Convention on International Trade in Endangered Species (CITES).

Andrew E. Derocher

See also Convention on International Trade in Endangered Species (CITES); Polar Bear

Further Reading

Lynch, W., Bears: Monarchs of the Northern Wilderness,

Toronto: Douglas and McIntyre, 1993 Murie, A., The Grizzlies of Mount McKinley, Washington, District of Columbia: US Department of the Interior, National Park Service, 1981 Servheen, C., S. Herrero & B. Peyton (compilers), Bears. Status Survey and Conservation Action Plan, IUCN/SSC Bear and Polar Bear Specialist Groups, Gland, Switzerland and Cambridge, UK: IUCN, 1999 Stirling, I. (editor), Bears: Majestic Creatures of the Wild,

Sydney, Australia: Weldon Owen Publishing, 1993 Stirling, I. & A.E. Derocher, "Factors affecting the evolution and behavioral ecology of the modern bears." International Conference on Bear Biology and Management, 8 (1990): 189-204

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