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Mate competition and mate choice are not mutually exclusive behaviors. Both behaviors may drive sexual selection in one or both sexes of a population. One of several factors affecting which behavior is exhibited by which sex is the operational sex ratio (OSR) in the study population. The present study combines behavioral observations in the field with controlled experiments in aquaria to investigate social interactions and mate choice in both male and female long-snouted seahorses Hippocampus guttulatus in the context of the population OSR. Compared with the more readily studied pipefishes, data on OSR and mate choice in seahorses are scarce in the published literature. Our field data provide novel evidence of social promiscuity, size-assortative mating, and an OSR that varies from being unbiased early and midseason to male biased at the end of the breeding season. Our mate choice experiments revealed intersexual differences in mate preference with males significantly preferring larger females to familiar ones. Taken together, our field and experimental results suggest that mate choice rather than intrasexual competition could drive sexual selection in seahorses. [ABSTRACT FROM AUTHOR]