National Park Service
Black bear and cubs at den site.
Black bears breed during summer, but due to an uncommon biological adaptation called “delayed embryo implantation,” gestation does not begin until winter hibernation. During this time they can be found in den sites, such as hollow trees, rock cavities or brush piles.
Because they do not eat during hibernation, a considerable fat reserve must be built up during summer and fall. Acorns, beechnuts and other hard mast are important food sources, especially for females about to reproduce. If mast crops are poor and the sow does not have the fat reserve necessary to support herself and young, sows may re-absorb the embryo and not give birth that year.
Cubs are born in late January while the female is denning. Normally the female will give birth to two cubs, but litters may vary from one to five cubs. At birth, each cub is extremely small, weighing between 8 to 12 ounces. Because the cubs will nurse almost continuously they will grow rapidly and usually weigh 5 to 7 pounds when they leave the ten in late march or early April.
The young will cubs stay with their mother throughout the summer and den with her again the following winter. By the time the cubs are 1 ½ years old, they leave the mother and the sow is then free to breed again. Most females reach sexual maturity between 2 ½ and 3 ½ years.
While they are primarily active at night, black bears are sometimes seen during daylight hours. Most sightings are reported between late spring and mid-summer, the peak period for young males to leave their family units and search for home territories of their own.
It is extremely rare for black bears to be aggressive toward humans. They are shy, secretive animals that do not socialize with each other, except in individual family units or during the breeding season. Normally, they will hide or run away to avoid any contact with humans. Unfortunately, providing food to bears can change their behavior and draw them close to humans.