Grizzly bears usually become sexually mature between 5 and 7 years of age. The age of maturity is dependent on habitat quality and the corresponding vitality of the bears. In the Mackenzie Mountains, the quality of forage is low and although some bears reproduce between 6 and 10 years of age, they do not reach peak reproductive potential until 10 to 16 years. Females continue to produce cubs until their late teens and early 20s.
The oldest recorded age for a female to give birth is 25 years. However, even though grizzly bears are potentially long-lived, a long inter-litter period of approximately 3 years and a small litter size (average 2 cubs) means that they are not particularly prolific. Under the best possible conditions, a female, living to age 25, could experience six reproductive cycles and produce about 13 cubs.
Male and female grizzlies are polygamous and mating occurs from late spring until early summer. "Delayed implantation," however, ensures that the fertilized egg does not implant in the uterus until October or November. This is believed to be a survival mechanism in that the embryo will not develop unless the bear is healthy and has sufficient fat reserves to last the winter and to look after the cubs. Delayed implantation and the corresponding brief period of development (6 – 8 weeks) means that the cubs are surprisingly tiny at birth.
Grizzly bear cubs are born in January or February while the female is still in her den. Most litters contain two cubs, but one, three, or even four is possible. The cubs are about the size of a small squirrel, and weigh between 350 – 700 g. They are blind and hairless. Their total helplessness at birth is ideally suited to the lethargic, fasting mother bear who is in no condition to feed and cope with more advanced offspring. The cubs grow rapidly and weigh 15 kg at 3 months and 25 kg at 6 months, but some nurse until they are almost 2 years old. The cubs remain with their mother for at least their first year. They are highly dependent on her for protection from all dangers, including large male bears, which occasionally kill and eat cubs. The mother spends much time teaching the cubs hunting and feeding techniques. They also learn to recognize predators and to locate suitable denning areas. The mother is a strict disciplinarian and quickly discourages misbehaviour with a sharp blow from her forepaw. The cubs den with their mother for a second winter and then are chased off in the spring when she goes into oestrus. If for some reason the mother does not go into oestrus, the family may den together for the third winter.
The first year of independence for the cubs is difficult. Without the solicitous attention of the mother, they seek out their own home range and denning site. Females often use a portion of the mother’s home range, while males travel farther afield to establish a new territory.