Reproduction: Bears' Mating Behaviors
Female black bears become sexually mature when they are large enough about 100 pounds (45 kg) and strong enough to withstand pregnancy. They reach that size faster if they eat more nutritious food.
A bear's food in Idaho is usually a healthy mix of berries and other plants, but it's not as nutritious as the abundant nuts available to bears in the eastern United States. That's why black bears in Idaho begin mating at a later age and have smaller litters than eastern bears.
Beginning when they are four to six years old, female bears will usually mate every other year and produce one to three cubs. Mating season occurs in May and June (and may extend into August), but not all bears participate. Females with new cubs won't be part of this event because they must devote all their energy and time to finding enough food for their family. Yearlings and 2-year-old bears also will be out of the action. And not all of the mature males will find a mate.
Generally, the younger males lose out to the bigger and more aggressive older males. If a female becomes pregnant, the egg won't begin rapid development until late fall or early winter when she enters her den. Even then, the egg will develop only if the bear has enough fat reserves to ensure her survival through the winter.
As the female bear settles into her deep winter sleep, her own energy needs decline and her stored resources go toward nurturing the embryo. She gives birth to cubs in late January or early February, waking long enough to clean them and guide them to her nipples. The cubs are helpless when born, their eyes are closed, and their thin fur isn't enough to keep them warm. But if they snuggle close against their mother’s belly, they will find warmth and plenty of her milk to drink.
For the next few months, they grow in this dark, cozy place. When a cub greets the outside world in late April or early May, it weighs 4-10 pounds (1.84.5 kg) and may be strong enough to climb trees. It still can't outrun predators, and so stays close to its mother for the first few weeks.
As the cub explores this bright new world, it begins to sample the same plants and other solid food that its mother eats. But the cub will depend on the sow’s milk throughout the summer to help it grow strong and ready for winter hibernation.
During the summer, female bears will visit their dens so that cubs learn the den locations. Usually the mother and cubs will sleep together that winter. If for some reason a cub loses its mother, though, the youngster can usually still find a den.
Male and female bears tend to have the same size dens. They might dig a den in a hillside or under a tree, or they might find a spot inside a hollow tree, log, or rock cavity. They might even expand an old coyote or badger den. Wherever a bear makes its winter home, it lines the den with soft brush, grass, and the thickly-needled branches of evergreen trees.