In 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.
Contrary to all expectations based on their male pregnancy, recent work shows that seahorses exhibit conventional sex roles in mating competition: male seahorses compete more to obtain eggs than females do to give their eggs away. This suggests that the operational sex ratio (OSR: the relative number of males and females available to mate) might be male-biased despite the length and limitations of male pregnancy. I here report a study of a laboratory population of the Sri Lankan seahorse Hippocampus fuscus. The OSR amongst mated seahorses is unbiased because males and females effectively have equal reproductive rates, although some evidence suggests that a female may have limited potential for a higher reproductive rate than her partner. In contrast, the OSR among mate-seeking seahorses is male-biased, because unmated (empty) males can prepare to mate more quickly and can remain ready to mate for longer than can unmated females. Thus, the OSR is male-biased in exactly that segment of the population which should compete for mates. These findings are compatible with the sexually monogamous mating pattern of many seahorse species.
The sex with the higher potential reproductive rate is expected to mate polygamously unless there are temporal or spatial constraints on mate availability. We investigated whether such constraints were evident in a population of the monogamous seahorse Hippocampus whitei (family Syngnathidae). Across the whole study site, breeding was more asynchronous than expected by chance. Our findings are thus compatible with the hypothesis that asynchronous breeding may promote and/or maintain monogamy. Asynchrony per se was unlikely to explain monogamy entirely, however, as temporal opportunities for polygamy remained and the males that were nearest one another had the lowest level of asynchrony. Moreover, each animal's home range overlapped with home ranges of potential mates other than their partner, implying a lack of spatial constraints on polygamy. We suggest that H. whitei mated monogamously because the benefits of polygamy were reduced by (1) only small differences in the potential reproductive rates of males and females and/or (2) a mate familiarity effect that increased reproductive success in successive matings. Further research could investigate relationships between mating pattern and varying intersexual differences in potential reproductive rates across syngnathid species.