Male mate choice contributes to behavioural isolation in sexually dimorphic fish with traditional sex roles

Behavioural isolation between closely related species with exaggerated male mating traits is traditionally thought to be mediated by female preference for conspecific male ornaments. The role of male mate choice in maintaining boundaries between highly sexually dimorphic species is therefore comparatively neglected. However, mounting theoretical and empirical evidence supports the existence of male mate choice both within, and increasingly, between species with exaggerated male ornaments. We therefore tested the role of male mate choice in the maintenance of species boundaries for two sexually dimorphic species of darters (Percidae: Etheostoma). Using dichotomous choice assays, we measured male preferences of sympatric species Etheosotma barrenense and Etheosotma zonale for size-matched conspecific and heterospecific females, thus reducing the possibility that males would select for general indicators of fecundity. Our results show that males of both species strongly prefer conspecific females. A comparison with published data showed that the strength of preference for conspecific mates is just as strong for males as it is for females in E. barrenense. We also estimated the relative contribution of male mate choice, female mate choice and male-male competition to behavioural isolation. We found that male mate choice contributes as much as female mate choice to total behavioural isolation and thus likely plays an important role in behavioural isolation in these sexually dimorphic species. Our results suggest that the contribution of male choice to behavioural isolation should be studied in a larger variety of animal species to appreciate the relative roles of the sexes in the maintenance of species boundaries.