Female black bears in Ontario may produce their first litter when they are 5 years old, but the average age of first reproduction is about 6 years of age in southern Ontario. Cubs are born while the female is in the den, generally in early January but occasionally as late as early February. The timing of birth depends upon when the female enters the den. If the female enters the den late ( generally in response to abundant fall foods ) then the cubs are born later. Litter size varies from 1 to 4 cubs in Ontario, with most litters having either 2 or 3 cubs. Newborn cubs weigh 200 - 300 grams, which is less than 1/300th of the females body weight. Cubs grow rapidly on milk that has higher fat and protein contents than human or cows milk. By 6 weeks of age, cubs weigh 4-7 lbs. By the time the bears leave the den in late April or early May, the cubs weigh about 10lbs.
Cubs stay with the females throughout the year of birth, and den together the following Fall. At the end of their first year together, females and cubs enter the den for a second winter. Females and cubs separate the following May or June when the cubs ( now yearlings) are 17 - 18 months old. This is the point in the reproductive cycle at which the female is in the poorest condition. They need to quickly gain enough weight to reproduce successfully again. Following the break-up of the group a new breeding cycle begins as the adult female comes into estrus and becomes receptive to males again.