From June through July is mating season. Until the mother begins her winter hibernation, grizzly bear embryos do not begin to develop, although mating may have taken place up to 6 months before. As with other bears, if the mother has not accumulated enough fat to sustain herself as well as developing cubs, the embryos may not implant.
In January, usually one to three cubs, each weighing only a pound or less, are born. The cubs gain weight quickly and by the time they come out of the den, they often have reached 20 pounds. As many as half of all cubs may not reaching breeding age - a leading reason for the grizzly's low numbers.
Males on mating
After the musk ox, the grizzly has the second slowest reproduction rate of all North American mammals, making it harder for it to rebound from threats to its survival. Females do not reach breeding maturity until they are 4 to 9 years old and generally give birth to two cubs every 3 years. On occasion, one or three cubs may be born to a female, but two is the more common number.
Cubs remain dependent upon their mother's milk for almost a year, stay with their mother for 2 to 3 years, and reach breeding maturity at about five years. In some cases they may not breed until 8 or 9 years of age. When they do reach breeding age, females only breed every 3 or more years. Males compete with each other for breeding opportunities and seek females each year. Grizzlies should live to be 15 to 20 years of age, and a few survive for up to 30 years. Of course, with expanding human populations and diminishing secure habitat, a bear's life span continues to be limited below what one should biologically expect from the species.