In early spring, grizzlies emerge from their dens. After a week or two they begin to eat grass, various roots and winter-killed mammals. By May, more nutritious plants are available along creeks and in open areas created by snow slides. Grizzlies also eat elk calves and deer fawns in late spring. Bears usually mate in early June.

Most of a grizzly's summer is spent eating to regain weight lost during the previous winter. Berries form the bulk of their diet, with huckleberries preferred west of the divide and buffalo berries to the east. Bears start to eat berries at lower elevations by early July, and follow the berries upslope as they ripen.

Adults often gain 100 lbs in the last few months before denning. In the fall, bears may move back to the valleys to find food. Fall foods include mountain ash berries, and the bulbs and roots of various plants, like glacier lilies. If less food is available, bears tend to den earlier.

Grizzlies use their long claws to dig dens about eight feet horizontally into steep slopes and are usually settled in by mid-November. Snow seals the den and insulates it from cold air. Dens are usually above 6,000 feet elevation. Pregnant females and females with cubs den earlier (late October) and stay in the den longer.

In January, from one to three tiny cubs are born. Blind and covered only with fine hair, they weigh less than one pound at birth. Nursing on their mother's milk, they grow to weigh 15-20 lbs by the time they emerge from the den in early May. Cubs are raised entirely by their mother. They stay with their mother through the next two winters but are usually on their own at 2½ years when their mother is ready to breed again.