Seahorses Exhibit Conventional Sex Roles in Mating Competition, Despite Male Pregnancy

AbstractIn seahorses, only males undergo a pregnancy. It had been tacitly and explicitly assumed that seahorses were sex role reversed (that females competed more intensely than males for access to mates), on the basis that male pregnancy so limited male reproduction as to produce a female-biased operational sex ratio (OSR). However, this supposition had never been investigated. The laboratory experiments in this paper demonstrate that, contrary to expectations, seahorses exhibit conventional sex roles: male seahorses compete more intensely than females for access to mates, on both the first and final days of courtship. Competing males are more active than competing females in those courtship and competitive behaviours common to both sexes, and only males exhibit uniquely competitive behaviours (wrestling and snapping). Males which succeeded in copulating are heavier than their rivals and copulating seahorses of both sexes generally are more active in courtship and competition than are their unsuccessful rivals. The finding that seahorses maintain conventional sex roles requires us to reconsider the impact of male pregnancy on the OSR.