Food gathering is a top priority in the life of grizzly bears. They feed extensively on both vegetation and animal matter. Their claws and front leg muscles are remarkably well adapted to digging for roots, tubers, and corms. They may also dig to capture ground squirrels, marmots, and pocket gophers. Brown bears are strongly attracted to succulent forbs, sedges, and grasses. In spring and early summer they may ingest up to 90 pounds of this high-protein forage per day. Bears gain their fat reserves to endure the 5- to 7-month denning period by feeding on high-energy mast (berries, pine nuts) or salmon. The 2 1/2- to 3-month summer feeding period is particularly crucial for reaching maximum body frame and preparing for the breeding season and winter.
Being ultimate opportunists, brown bears feed on many other food items. For example, the Yellowstone grizzlies have clearly become more predatory since the closure of the garbage dumps in the Yellowstone area. They are exploiting the abundant elk and bison populations that have built up within the park. They hunt the elk calves in the spring, and some bears learn to hunt adult elk, moose, and even bison. The ungulate herds, domestic sheep, and cows also provide an abundant carrion supply each springâ€”the animals that die over winter thaw out just when the bears need a rich food source.
Grizzly bears are adept at securing food from human sources such as garbage dumps, dumpsters, trash cans, restaurants, orchards, and bee yards. Some bears learn to prey on livestock, especially sheep that graze on open, remote rangeland.