Sex differences in survival costs of homosexual and heterosexual interactions: evidence from a fly and a beetle

Studies on the costs of sexual reproduction have focused primarily on the costs of heterosexual courtship and mating, whereas the costs of homosexual interactions, such as male–male or female–female displays and mounting, have been relatively neglected. This may reflect an implicit assumption that heterosexual interactions are more costly in most species, but this assumption has never been verified. We tested this assumption experimentally by comparing the effects of hetero- and homosexual interactions on life span in two distantly related insects with contrasting mating systems: the seed beetle Callosobruchus maculatus, and the sexually dimorphic carrion fly Prochyliza xanthostoma. Despite pronounced behavioural and morphological differences between these species, results were remarkably congruent. Relative to individually housed virgin controls, male life span was reduced to a similar degree in males maintained with other males and males maintained with females. In contrast, female life span was strongly reduced relative to controls when females were kept with males, but was affected very little when females were maintained with other females. Thus, the costs of homosexual and heterosexual interactions are similar for males, but highly dissimilar for females. Our results suggest that the costs of homosexual interactions can be considerable, and may have important consequences for the evolution of mating systems.