Male-male competition and reproductive success of northern elephant seals, Mirounga augustirostris, was studied for six consecutive breeding seasons at Año Nuevo Island, California. The conclusions were as follows: (i) Less than one third of the males in residence copulate during a breeding season. A few males are responsible for the majority of copulations, (ii) The number and age of males copulating varies with: (a) harem location and topography, (b) the number of estrous females in the harem, and (c) the number of males competing for females, (iii) Copulation frequency is related directly to success in male-male competition, i.e., social rank. (iv) The same individuals may dominate breeding for three consecutive breeding seasons. (v) Successful males die within a year or two after their reproductive peak. (vi) The reproductive success of most males is nil or low because many die before reaching breeding age and some of those that reach maturity are prevented from mating by the highest ranking males. (vii) Individual strategies have important consequences for reproductive success, (viii) Male-male competition is a major cause of pup mortality prior to weaning.

The potential reproductive success of males is much greater than that of females. Changes in colony number and composition affect the reproductive success of males as well as females.