In North America, densities of black bears range from 0.3 to 3.4 bears per square mile. Densities are highest in the Pacific Northwest due to the high diversity of habitats and long foraging season.
Black bears become sexually mature at about 3½ years of age, but some females may not breed until their fourth year or later. Black bears breed during the summer, usually in late June or early July. Males travel extensively in search of receptive females, and mating individuals do not form pair bonds. Rival males may fight one another, and unreceptive females may fight with males. Females that are dominant may suppress breeding activities of females that are subordinate.
After mating, the fertilized egg does not implant immediately, but remains unattached in the uterus until fall. Females in good condition usually produce 2 or 3 cubs that weigh 7 to 12 ounces at birth. Bears in urban areas with subsidized food have had up to 5 cubs in a single litter.
Females give birth between late December and early February while they are in their dens. After giving birth, the sow may continue torpor (winter sleep) while the cubs are awake and nursing. Females that are lactating do not come into estrus, so females generally breed every other year. Only females care for young. Males sometimes kill and eat cubs.
Cubs are weaned in late summer but usually stay close to their mother throughout their first year. After the breeding season, females and their yearlings may travel together for a few weeks. The cubs leave the mother when the female comes into her next estrus.
Sites for dens are quite variable and include piles of rocks or brush, excavations, hollow trees, and structures made by humans. Dens sometimes are lined with grass and leaves, and many dens are at ground level under fallen trees, or sometimes decks.
Black bears typically are nocturnal, although occasionally they are active during the day. In the South, black bears tend to be active year-round. In northern areas, black bears undergo a period of torpor during winter, which they spend in their dens. During torpor, individuals may remain in their dens for five to seven months, foregoing food, water, and elimination.
The home range of a black bear is dependent on the type and quality of habitat, and the sex and age of the bear. In mountainous regions, bears encounter a variety of habitats by moving up or down in elevation. Where the terrain is flat, bears typically range more widely in search of resources. Most adult females have well-defined home ranges of five to 20 square miles. Ranges of adult males are several times larger.
Black bears frequent heavily forested areas, including large swamps and mountainous regions. Black bears depend on forests for food, water, cover, and space. Mixed hardwood forests interspersed with streams and swamps are typical habitats for bears. Highest population growth rates are achieved in eastern deciduous forests, where there is a variety and abundance of foods, especially near urban areas.
Black bears are omnivorous and forage on a wide variety of plants and animals. Their diet typically is determined by the seasonal availability of food. About 80% of their diet is plant material, and typical foods include grasses, berries, nuts, tubers, inner bark, insects, small mammals, eggs, carrion, and garbage (Figure 5). Shortages of food occasionally occur in northern ranges when mast crops (berries and nuts) fail. At those times, bears travel more widely in search of food. Human encounters with bears are more frequent during such years, as are complaints of damage to crops and losses of livestock.