Female black bears typically reach sexual maturity at three years of age, depending upon their physical condition. Biologically, male bears may reach maturity at three to four years of age, but their ability to breed is often limited due to the presence of older, more mature males. The breeding season occurs from June to mid-July and males will breed with as many females as possible. Births occur in January while the female is denning. Females give birth to one to five cubs, with an average litter size of two. Litters are produced every other year and cubs generally weigh only 8–16 ounces at birth.
While pregnancy lasts approximately 220 days, embryonic development does not continue throughout the gestation period. In fact, bears possess a unique reproductive adaptation called “delayed implantation” that allows births to occur when females are generally in prime physical condition. After females are bred in June, the fertilized eggs remain in a state of arrested development and are not implanted in the uterus until the fall. Once implanted, embryonic development resumes and growth only occurs during the remaining 10 weeks of pregnancy. Consequently, cubs are born in late January while still in the den and weigh only ounces at birth.
The advantage of this reproductive strategy is that females are able to devote all energy reserves towards milk production at a time when natural foods are practically nonexistent. In years of poor acorn production, however, females may not be able to store adequate fat reserves and reproduction can be significantly decreased. Generally, however, cubs that weighed only ounces at birth will emerge from their dens weighing 5–10 pounds and will be physically prepared to meet their new world.
Offspring remain with their mother for the entire year after birth and generally throughout the second denning season. After spring emergence, the mother will run off the yearling bears as she comes into estrus and prepares for the new breeding season. This is actually for the benefit for her young as adult males will often kill smaller bears during the breeding season. Yearlings can weigh anywhere from 20–100 pounds, depending upon food availability, as they strike out to establish their own new territory.
Watch a video taken inside the den of a radiocollared female black bear - listen to hear the sounds of cubs nursing.