Although mostly solitary, bears sometimes aggregate in large numbers at important food sources and form family foraging groups. In these cases, a dominance hierarchy involving aggression is established. While it is large adult males that are the highest-ranking, the most aggressive individuals are females that have young. The latter two are also the only ones that form social bonds.
Young born bears are vulnerable, being blind, naked and weighing only 340 to 680 grams. Cubs grow quickly, reaching 25kg by 6 months, and continue lactating for 18 to 30 months while eating a variety of foods. Cubs usually remain with the mother until the third or fourth year of their life. Although they mature sexually between 4-6 years of age, the species continues to grow until 10-11 years old. In the wild, the brown bears can reach 20 to 30 years of age. Despite this long life expectancy, most brown bears die very early.
Sometimes males may fight over females, and once they have won, they tend to guard them for 1 to 3 weeks. Brown bears mate from May to July, and a gestation of 180 to 266 days follows, with births occurring from January to March, usually while the female is still in hibernation. She generally lays down two to three offspring, and breeds again 2 to 4 years later.
Bown bears are omnivorous, and their diet varies with the season - from grass and shoots in the spring to berries and apples in the summer, nuts and plums in autumn. All year round they eat roots, insects, mammals (including moose and elk in the Canadian Rockies), reptiles, and of course, honey. In Alaska, grizzlies feed on salmon during the summer.