Structure of northern elephant seal population breeding on San Nicholas Island, California, in 1971.

The structure of the population of northern elephant seals, Mirounga angustirostris, on San Nicolas Island, California, was studied during the 1970 and 1971 breeding season. At the population peak on 31 January 1971, there were 77 males, 306 females, 315 pups, and 6 yearlings on shore. The breeding population was subdivided into 15 groups, containing from 3 to 75 breeding animals. Fourteen of the groups were considered territories defended by individual males, and one group, the largest, was controlled by three males in a social hierarchy. The relatively small size of the San Nicolas population and the large amount of available beach space allowed the existence of numerous small breeding groups. Female elephant seals, although gregarious, apparently prefer to be in small groups when conditions permit it. It is likely that the same males that were territorial would have formed the nucleus of a social hierarchy if space had been limited enough to cause all of the females in the population to congregate in one large group.