Density-dependent courtship interference by neighbors in fiddler crabs: Increased male-male competition but reduced female choosiness at high population density

High population density increases the proportion of unsuccessful males in both male-male competition and female choice contexts. This prediction assumes that these components of sexual selection operate independently; however, they can sometimes interact within species-specific mating systems, with fluctuations in the intensity of one component influencing the other. In this study, I investigated the association between population density and the intensity of sexual selection in the fiddler crab Austruca lactea. In this species, neighboring males often disrupt female assessments during mate choice, suggesting that male-male competition interferes with female mate choice. I compared female mate-searching behavior, the frequency of male courtship interference, and female choosiness between two populations with differing densities. The number of males sampled by females increased in the higher-density population. The frequency of interference by the finally-selected male at the burrow of the penultimate male also increased in the higher-density population. However, while females always paired immediately with the finally-selected male when no interference occurred in the higher-density population, females in the lower-density population sometimes delayed pairing, although this difference was marginal. These results suggest that male-male competition intensifies while female choosiness decreases with increasing population density. Because male interference coerces females to continue mate-searching, the number of males sampled may increase in high-density populations despite a lower threshold for female preference. This study indicates that the relationship between the intensity of sexual selection components and population density may deviate from theoretical predictions due to social interactions with neighboring males in this species.