Ursus Arctos
Geographic RangeUrsus arctos once ranged throughout northern and central Europe, Asia, the Atlas mountains of Morocco and Algeria, and western North America as far south as Mexico. They are now found in extremely small numbers from western Europe and Palestine to eastern Siberia and the Himalayan region, possibly the Atlas Mountains of northwest Africa, and Hokkaido. Northern North American populations in Alaska and western Canada remain fairly stable. Many populations in the United States have been extirpated, including those of the Sierra Nevada and southern Rockies. Northern Mexican populations were extirpated in the 1960's.
HabitatBrown bears occupy a variety of habitats, from desert edges to high mountain forests and ice fields. In North America they seem to prefer open areas such as tundra, alpine meadows, and coastlines. Historically, they were common on the Great Plains prior to the arrival of European settlers. In Siberia, Ursus arctos occurs primarily in forests, while European populations are restricted mainly to mountain woodlands. The main habitat requirement for Ursus arctos is some area with dense cover in which it can shelter by day.
Physical DescriptionOne of the largest of living carnivores, grizzly bears are 1 to 2.8 meters in length from head to rump and their tails are 65 to 210 mm long. They are 90 to 150 cm tall at the shoulder and can tower at an intimidating height of 8 feet when standing upright on their hind legs. They range in weight from 80 to more than 600 kg. On average, adult males are 8 to 10% larger than females. Ursus arctos is largest along the the coast of southern Alaska and on nearby islands where males average 389 kg and females average 207 kg, though some males have been weighed at as much as 780 kg. Size rapidly declines to the north and east, with individuals in southwestern Yukon weighing only 140 kg on average. Fur is usually dark brown, but varies from cream to almost black. Individuals in the Rocky Mountains have long hairs along the shoulders and back which are frosted with white, giving a grizzled appearance, hence the common name grizzly bear in that region. Brown bears are extremely strong and have good endurance; they can kill a cow with one blow, outrun a horse, outswim an Olympian, and drag a dead elk uphill
ReproductionFemale brown bears copulate with multiple males during estrus, which lasts 10 to 30 days. Males may fight over females and guard them for 1 to 3 weeks. Female receptivity is probably communicated by scent marking throughout her territory
Mating of brown bears takes place from May to July. Fertilized eggs develop to the blastocyt stage, after which implantation in the uterus is delayed. The blastocyt becomes implanted approximately 5 months after mating, usually in November when the female has entered her winter sleep. A 6 to 8 week gestation follows, with births occuring from January to March (usually while the female is still in hibernation). Total gestation time, including pre-implantation, ranges from 180 to 266 days. Females remain in estrus throughout the breeding season until mating occurs and do not ovulate again for at least 2 (usually 3 or 4) years after giving birth. Two to three offspring are generally born per litter.
Brown bears mature sexually between 4-6 years of age, but continue growing until 10-11 years old. Bears have been known to live and reproduce in Yellowstone Park at 25 years of age, and potential lifespan in captivity is as great as 50 years
Lifespan/LongevityBrown bears in the wild can live for 20 to 30 years, although most brown bears die in their first few years of life. In captivity, brown bears have been known to live up to 50 years.
BehaviorUrsus arctos individuals may be active at any time of the day, but generally forage in the morning and evening and rest in dense cover by day. Brown bears may excavate shallow depressions in which to lie. Seasonal movements of Ursus arctos have been observed, with individuals sometimes traveling hundreds of kilometers during the autumn to reach areas of favorable food supplies, such as salmon streams and areas of high berry production.
Home ranges overlap extensively and there is no evidence of territorial defense, although bears are generally solitary. Occasionally, bears may gather in large numbers at major food sources and form family foraging groups with more than one age class of young. Under these conditions, dominance hierarchies are usually formed and maintained with aggression. Highest-ranking individuals are large adult males, although the most aggressive bears are females with young. Least aggressive and lowest-ranking are adolescents. The only social bonds formed are between females and young.
Brown bears begin a period of inactivity in October to December, and resume activity in March to May, with the exact period dependent on the location, weather, and condition of the individual. In southern areas, this period of inactivity is very brief or may not occur at all. This period is marked by a deep sleep in which these bears allow their body temperature to drop by a few degrees. It is not true hibernation and bears can generally be aroused readily from their winter sleep.
Most often, brown bears dig their own dens and make a bed out of dry vegetation. Burrows are usually located on a sheltered slope, either under a large stone or among the roots of a mature tree. Dens are sometimes used repeatedly year after year.
Ursus arctos moves with a slow, lumbering walk, although it is capable of moving very quickly and can easily catch a black bear. Brown bears are mainly terrestrial, although they can often be found swimming or preying upon fish in the water. Adults are unable to climb trees.
Communication and PerceptionBrown bears communicate primarily through smells and sounds. Brown bears can be heard making moaning noises sometimes while they are foraging. They scratch and rub on trees and other landmarks to communicate territorial boundaries and reproductive status.
Brown bears have an excellent sense of smell (able to follow the scent of a rotting carcass for more than two miles), human-level hearing, but relatively poor eyesight.
Food HabitsBrown bears are omnivorous, eating almost anything nutritious. Their diet changes with seasonal availability of different food sources. They eat a wide variety of plant foods, including grasses, sedges, roots, moss, and bulbs. Fruits, nuts, berries, bulbs, and tubers are taken extensively during summer and early autumn. They consume insects, fungi, and roots at all times of the year and also dig mice, ground squirrels, marmots, and other fossorial animals out of their burrows. Moth larvae have been demonstrated to be especially important sources of protein and fat when brown bears are putting on fat in the fall. In the Canadian Rockies and other areas, grizzly bears (the subspecies of brown bear in that area) are quite carnivorous, hunting moose, elk, mountain sheep, and mountain goats. Occasionally black bears are preyed upon. In Alaska, brown bears have been observed to eat carrion and occasionally capture young calves of caribou and moose. Brown bears have also been observed to feed on vulnerable populations of breeding salmon in the summer in these areas.
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