Life History of the Black Bear (Ursus americanus) 

Black bears number more than 600,000 in North America. They select a variety of habitats typically in relatively remote terrain, although they also can be found in housing developments. Wetlands and streams interspersed among mixed conifer and hardwood forests, with dense understory vegetation where food resources are abundant, make up prime bear habitat in Pennsylvania. In western Maryland female bears prefer evergreen and mixed forest, and wetland habitat; areas with high stream densities are selected, and large conifers are important as escape, concealment, and thermal cover. Where no wetlands exist, bears may select residential habitats in spring through summer. Brush/extractive habitats (mines, sand and gravel pits, quarries) are avoided. Class 1 (primary highways) and heavily traveled roads are avoided and act as barriers to dispersal (and thus home-range boundaries), fragment forested habitat, and precipitate human-induced mortality from vehicle collisions. However, bears may be attracted to restricted-access roads (logging roads, lightly traveled paved roads) that can be used as travel corridors. 

Most of the diet of black bears consists of soft (berries) and hard (nuts, including acorns and beechnuts) mast and vegetation, and their social organization is tied to the abundance and distribution of this food. In most areas food is distributed in scattered patches that cannot support groups of individuals, and bears are solitary. Bears congregate and form social hierarchies where food is abundant and clumped in distribution. In spring, they eat newly sprouting plants (skunk cabbage and grasses), leaves and flowers, raid ant colonies for pupae, and search for juvenile mammals and chicks. Fish and carrion are rarely available and thus not significant components of the diet. Running speed can reach 50 km per hour (30 mph) but is more useful for escape than predation. Black bears accumulate most of their fat during summer and fall. They have color vision and forage mainly in daylight, but they may become nocturnal around humans. Summer and fall foraging may take individuals up to 200 km (126 miles) from their home ranges before they return to hibernate. 

Young males disperse at 1-3 years of age and travel an average of 61 km (37 miles) before establishing adult home ranges. Male home ranges average 81 km2 (30 mi2) and usually overlap the ranges of several to many females. Annual female ranges in Maryland average 36 km2 (13 mi2) and can reach 49 km2 (18 mi2). Male home ranges are so large that they cannot be defended, so ranges overlap and males compete. Both sexes scent mark their home ranges with urine. Adult males, especially, rub and scent mark "bear trees" before and during the mating season. 

Mating occurs in early summer. This species exhibits delayed implantation, with the embryo not implanting in the uterus until November. Females give birth to 1-6 cubs (usually 2-3) in dens in January. Cubs weigh 200-450 g (0.4-1.0 pounds) each at birth, the smallest newborns, relative to the mother's weight, of any placental mammal. The short gestation and small size of the young are adaptive responses to reproducing during hibernation. Cubs weigh 2-5 kg (4.4-11.0 pounds) when they emerge with their mother from the den in the spring. Males do not participate in rearing the cubs. Cubs remain with their mothers until they are about 17 months old, at which time the mothers approach estrus and force the young to disperse. At dispersal, yearlings weigh 7-49 kg (15-109 pounds), depending on food availability. 

Females produce their first cubs at 2-9 years of age, depending on food availability, and mate usually every other year thereafter. They reach maximum size (about 236 kg, or 520 pounds) at about 6 years of age. Males become sexually mature at 3-4 years of age and continue to grow until 10-12 years of age, when they weigh up to 409 kg (902 pounds) and are dominant over younger, smaller males. Black bears can live more than 30 years in the wild but rarely live longer than 10 years because of encounters with humans, which account for more than 90 percent of deaths of individuals older than 18 months. 

In the northern portions of their geographic range, black bears hibernate for up to 7 months (about 4 months in Maryland). During winter sleep, bears defer eating, drinking, urinating, or exercising until emergence in the spring. Weight loss during hibernation can reach up to 40 percent of body weight in lactating females. In the north, metabolic rate can be reduced 50 percent. Heart rate drops from 66-140 beats per minute in summer to 8-22 beats per minute. Body temperature drops 1-7 ( C (1.8-12.6 ( F), resulting in reduced circulation to the limbs and slowed reactions to disturbances. Still, mothers remain alert enough to tend to cubs and react to danger. Fewer than 1 percent of bears die during hibernation. Native Americans revered bears for their ability to survive for months without eating. Medical research on the metabolic adaptations black bears possess for hibernation is providing information potentially important for the treatment of kidney failure, gallstones, burns and other ailments in humans. 

Primary Source: 
Rogers, L. L. 1999. Pp. 157-160 in The Smithsonian Book of North American Mammals (D. E. Wilson and S. Ruff, eds.). Smithsonian Institution Press, Washington, DC. 

Additional Sources: 
Dateo, D. M. 1997. Use of home ranges and influences of habitat selection on crop depredation by black bears (Ursus americanus) in western Maryland. M.S. thesis, Frostburg State University, Frostburg, Maryland. 

Fecske, D. M., R. E. Barry, F. L. Precht, H. B. Quigley, S. L. Bittner, and T. Webster. 2002. Habitat use by female black bears in western Maryland. Southeastern Naturalist 1:77-92. 

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