Polygyny is a widespread and evolutionarily significant mating system in vertebrates. The southern elephant seal, Mirounga leonina, has often been cited as being extremely polygynous, thus providing an important reference point for studies on mating systems. During the breeding season, these animals form terrestrial harems in which one dominant male controls tens to hundreds of females. Our current understanding of polygynous mating systems seems to imply that, unlike males, females are not under selection pressure to adopt alternative mating strategies, and in the case of the southern elephant seal, the possibility of mating at sea has not been considered. Furthermore, elephant seal females are thought to breed annually. Using a 25-year mark–recapture data set, we found that elephant seal females skipped breeding seasons, often returning to pup in the following breeding season. Females did not need to haul out on land in order to breed in the following season, thus providing evidence for mating at sea by virgin and multiparous females. Nonpolygynous, opportunistic mating at sea could be an important alternative mating strategy in a supposedly strictly polygynous species. This has implications for our understanding of elephant seal ecology, demography and behaviour and of the evolution of vertebrate polygyny in general. If polygyny does not preclude females from adopting alternative mating strategies, the term ‘polygyny’ may be misleading. Traditional concentration on male strategies has hampered our understanding of mating systems, in assuming that females capitulate to these strategies. We suggest similar misinterpretations could occur in other polygynous species.