Black bears (Ursus americanus) typically breed in June and July, although breeding can occur at any time from June through September. Females will start breeding from ages 2 to 8, depending on environmental conditions, bear density and food availability. Food availability in the fall is especially important, and in places with lots of acorns and other mast bears mature much faster. In the rich eastern forests of the mid-Atlantic bears will breed every 2 years, while western black bears typically breed only every 3 or 4 years.
Bears have an adaptation called delayed implantation that helps them solve a problem with the timing of reproduction. Bears typically mate in the late spring, but they also need to fatten themselves up as much as possible for their winter torpor period (bears technically do not hibernate since they do not suppress their metabolism as much a true hibernators and they frequently “wake up” and come out in the winter). Gaining weight for the winter and being pregnant at the same time would be difficult for females. Bears also have short gestation periods, and giving birth to a litter of cubs shortly before entering the winter torpor period would also be hard for both the new cubs and the mother. Because of these problems natural selection pressure has been strong enough to give rise to the adaptation of delayed implantation.
Delayed implantation describes a reproductive strategy where fertilized eggs remain dormant and do not implant in the uterine wall, and subsequent pregnancy and development of the young are delayed. Black bears typically delay implantation until November or December, and birth occurs in January or early February. Black bear cubs weigh less than a pound, and nurse from their mother in the winter den until the whole family emerges in the spring in search of food.
Black bears are highly promiscuous, and a boar’s (male bear) strategy is to keep an eye on as many females at one time as possible. He will find a female and stay for an hour or two while he determines if she is receptive to breeding, quickly moving on to the next female if she is not. Once a male determines a female is ready to mate he will move in and after some very perfunctory courting he will mount her and begin the mating process. If he misreads her cues and attempts to approach too early the female will aggressively drive him away with bites and blows. Copulation may last as long as 30 minutes, and the male will stay with the female afterwards. The bears may mate several times over the course of a single day or even several days, and then the male will move on to seek other females who are receptive to breeding.
Both males and females will couple with multiple mates, and many females will conceive multiple times and have cubs from different fathers in the same litter. Litter size typically ranges from 1 to 4, and is mostly determined by the mothers early winter fat stores. The more fat the more cubs.
Black bears are expanding their range in the eastern US and are becoming a more common sight for wildlife watchers and camera trappers alike so good luck catching a glimpse of one out in the woods.
Elbroch, Mark and Kurt Rinehart. Behavior of North American Mammals. New York: Houghton Mifflin, 2011. Print.
Lariviere, S. 2001. Ursus americanus. Mammalian Species 647:1-11.