Mating season is from June through July. Grizzly bear embryos do not begin to develop until the mother begins her winter hibernation, although mating may have taken place up to 6 months before. In January, usually one to three cubs, each weighing only a pound or less, are born. The cubs gain weight quickly and often have reached 20 pounds by the time they come out of the den. 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 usually live to be 15 to 20 years of age, and a few survive for up to 30 years. Between 1800 and 1975, grizzly bear populations in the lower 48 states decreased from estimates of more than 50,000 to less than 1,000. The grizzly was eliminated from much of the West by the late 1800s. As mountainous areas were settled, development contributed to an increase in human-caused mortality. Livestock depredation control, habitat deterioration, commercial trapping, unregulated hunting, and the perception that grizzlies threatened human life were leading causes of the animal's decline.