It is one of the few mammals known to use a tool. While floating on its back, it places a rock on its chest and then employs the rock as an anvil for breaking the shells of mussels, clams, and large sea snails in order to obtain the soft internal parts. It obtains about 23 percent of its water needs from drinking sea water and most of the rest from its food. Animals of opposite sex usually come together only briefly for courtship and mating. At most times there is sexual segregation, with males and females occupying separate sections of coastline.
Males usually occur at higher densities.
Such behavior has been documented both in Alaska and California, and males have been observed to return to the same place for up to seven years. The boundaries of these territories are vigorously patrolled, and intruding males are repulsed, but serious fighting is rare. The owner seeks to mate with any female that enters, though sometimes a pair bond is formed for a few days or weeks. Male territories are usually about 20-50 ha. Breeding occurs throughout the year, but births peak in late May and June in the Aleutians and from December to February off California.