Ursus arctos, Brown Bear, Grizzly Bear

Physical Characteristics

The largest of living carnivores, grizzly bears are 5 - 8 feet in length from head to rump. They are 3½ - 5 feet tall at the shoulder and can tower at an intimidating height of 8 - 10 feet when standing upright on their hind legs. On average, adult males are larger than females. Ursus arctos is largest along the coast of southern Alaska and on nearby islands where mature males can weigh as much as 1,400 pounds and mature females 700 pounds. Size rapidly declines to the north and east. Fur is usually dark brown, but varies from light blond to almost black. Individuals in the Rocky Mountains have long hairs along the shoulders and back, which are frosted with white, thus giving a grizzled appearance and hence the common name "grizzly bear". Ursus arctos has an excellent sense of smell (able to follow the scent of a rotting carcass for miles), human-level hearing and eyesight. Brown bears are extremely intelligent, resourceful and opportunistic, allowing the grizzly to live in a variety of habitats.

Adult males of the coast of Katmai National Park and the Alaska Peninsula are among the biggest brown bears on the planet, even as large as the famous Kodiak brown bears, considered to be the largest in the world.

Food Habits

The diet of Ursus arctos consists mainly of vegetation, and shifts to different foods as the seasons progress. In spring, grasses, sedges, roots, moss and bulbs are mainly eaten. During summer and early autumn, berries are essential, with bulbs and tubers also eaten. Ursus arctos consumes insects, fungi and roots at all times of the year and also digs mice, ground squirrels and marmots out of their burrows. Grizzly bears are also carnivorous, feeding on moose, caribou, mountain sheep and goats. In Alaska, along the coast, brown bears feed on the vulnerable populations of salmon in the summer, as well as other marine mammals which wash ashore or get trapped in tidal areas.

Reproduction

Mating of brown bears takes place from May to July, although the fertilized eggs are not implanted in the uterus until hibernation. Births occur during January and February, while the female is in hibernation. 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. Generally, two offspring, which are blind at birth, are born per litter and remain with their mother until their third summer of their life (usually until the fourth, at 3 1/2 years old) unless premature death occurs, commonly caused by other bears, leading to the belief that males may intentionally kill young cubs so females return to estrus, although this is currently a controversial issue and there is no scientific data supporting this belief. Brown bears mature sexually between 5 - 7 years of age - females first, then males - but continue growing until they are 10 - 11 years old. Bears have been known to live and reproduce at 25 years of age, with a potential life span of up to 35 years.

Behavior

Ursus arctos may be active at any time of the day, but generally forages in the morning and evening, and rests in dense cover by day. Brown bears may excavate shallow depressions in which to lay, commonly referred to as day beds. Seasonal movements of Ursus arctos have been observed, with individuals sometimes traveling many miles during the autumn to reach areas of favorable food supplies, such as salmon streams, and areas of high berry production. Home ranges can be as large as 800 square miles, but are on average between 50 and 150 square miles, with male ranges being greater than female ranges. Home ranges overlap extensively, and there is no evidence of territorial defense, although bears are generally solitary. Occasionally, bears 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. During the breeding season, males may fight over females and guard their mates for 1 - 3 weeks. On the coast of Alaska, where bears gather in large concentrations, females often mate with several males, resulting in cubs being born by different fathers.

Ursus arctos begins hibernation in November - December, and resumes activity in March - April, with the exact period dependent on the location, weather, and condition of the individual. In certain southern coastal locales, hibernation is very brief or may not occur at all. 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. In certain areas on the Alaska Peninsula lava caves formed by past volcanic eruptions are used. Dens are sometimes used repeatedly year after year.

Ursus arctos moves with a slow, lumbering, ungainly gait as both feet on a side move forward together. Bears are also plantigrade – like humans, but unlike most other animals, they walk flat-footed on both the soles and the toes of their feet. Awkward appearing or not, brown bears are able to run over 30 mph for a short distance. They are mainly terrestrial, although they can often be found swimming or preying upon fish in the water along coastal areas. It is very unusual for adults to climb trees.

Habitat

Brown bears occupy a variety of habitats, but in the New World 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, they are found primarily in forests, while European populations are restricted mainly to mountain woodlands. The main habitat requirement for Ursus arctos is an area with dense cover in which it can have shelter by day.

The brown bear has been long considered the most dangerous animal in North America, although real danger of attack is often exaggerated. In general, brown bears attempt to avoid human contact and will not attack unless startled at close quarters with young or engrossed in a search for food. However, they are very unpredictable in temperament, and often exhibit impulsive and petulant behavior.

On the coast of Katmai National Park, where bears are not hunted or harassed, the bears are tolerant and curious about the rare human visitors, but mostly, they completely ignore our presence.

Some populations are clearly endangered, others are not. Grizzly bear numbers have dropped dramatically since the turn of the century, when settlers and livestock flooded the western U.S., driving the grizzly out of much of its former range. Grizzlies now cling to a mere 2 per cent or less of their former range. Logging, mining, road construction, resorts, subdivisions, golf courses, etc., have all encroached on suitable bear habitat, resulting in a decrease in bear numbers. Grizzly bear numbers were estimated at 100,000 in the conterminous United States in the early 1900's, but there are now fewer than 1,000. In the greater Yellowstone area, there were 250 bears in 1972; the grizzly was then included in the endangered species list. Today, population estimates account for 600 individuals and the grizzly may soon be removed from the list, which might result in the species being hunted once again. Brown bears are still fairly common in the mountainous regions of western Canada and Alaska, perhaps numbering about 30,000 individuals. In Eurasia, there are an estimated 100,000 bears, with about 70,000 of those living in the former Soviet Union.

For more information on Alaska's bears, visit
 http://www.wildlife.alaska.gov/index.cfm?adfg=bears.main