Operational sex ratios in seahorses

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.

Vincent, A.C.J. (1994). Operational sex ratios in seahorses. Behaviour 128:153-167. https://doi.org/10.1163/156853994X00091