Mutual mate choice in the potbellied seahorse (Hippocampus abdominalis)

Models of sexual selection often assume dichotomous sex roles, with one sex competing for access to mates, while the other sex is choosy. However, it is well known that mating decisions are realized by integrating information across multiple traits, the relative importance of which may be sex specific. While a large body of work has investigated the influence of sexual signals on mating behavior, such traits have typically been studied in isolation, oversimplifying the multimodal communication associated with natural mating behavior. We investigated the impact of 2 key traits (Major Histocompatibility Class II beta-chain [MHIIb] olfactory cues and body size) on mate choice decisions in the potbellied seahorse (Hippocampus abdominalis), a species considered to have female competition and male choice. We used a hierarchical experimental design (1. olfactory cues only, 2. olfactory and visual cues, and 3. free interaction) to investigate behavioral preferences and mating success of female and male seahorses under increasing levels of multimodal stimulation. Our data show that female seahorses prefer and mate with MHIIb-dissimilar males, while male seahorses mate randomly with respect to this trait. Conversely, males prefer and mate with large females, while females show no size-based mating preference. The multimodal integration of sex-specific mate preferences in mating behavior of the potbellied seahorse suggests the existence of mutual mate choice in this species. The results presented here suggest that more comprehensive studies of mating behavior, considering both female and male preferences for multiple traits, may lead to a more nuanced understanding of how sexual selection operates in natural populations.