Intrasexual competition and mate choice in assassin bugs with uniparental male and female care

Paternal care has the potential to lead to female-biased operational sex ratios and sex role reversal. Whereas this issue has been repeatedly investigated in fish, we present the first study on insects. We investigated two species of assassin bug: Rhinocoris tristis, which shows male care of eggs, and Rhinocoris carmelita, which shows female care of eggs. In both species, the potential reproductive rate of males exceeded that of females. Since the population sex ratio was unbiased and there was no difference in mortality rates between the sexes, we predicted that the operational sex ratio would be skewed in favour of males in both species and thus males would be the more competitive sex. Field and laboratory observations upheld this prediction, with males competing for females in both species. In R. tristis, males also competed for ownership of eggs, whereas females discriminated between males according to ownership of eggs. However, choosiness was not restricted to females in R. tristis. Observations suggested that R. tristis males discriminated between females according to their weight: heavier females, which were likely to oviposit sooner, were guarded for longer by males in a period of postcopulatory riding. We suggest that the primary function of postcopulatory riding is to prevent females from mating with conspecifics, thus reducing the possibility of sperm competition. This behaviour might also be an important prerequisite for the evolution of male care.