Phenotypic plasticity in mating behavior can be an asset to reproductive success and is observed in a wide variety of species. In particular, the presence of a rival male in the pre-mating arena can elicit behavioral responses from males that directly increase their reproductive success. Although many studies have demonstrated a recurring suite of behavioral responses by males to rivals, they have focused on the sex ratio, number and the density of competitors. However, a key factor not examined so far is how the relatedness of rival males influences the expression of plastic mating behavior, as individuals gain less by competing against their siblings than non-related rivals. I tackle this issue here by exposing full sibling males of Drosophila melanogaster to a range of different social environments in the sensitive period between eclosion and first mating. This is a prime period of sexual maturation during which males perceive the level of sexual competition they may face in the mating arena and respond accordingly. I found that the copulation durations of males exposed to a rival male were significantly longer than males kept in isolation. However, the copulation durations of males kept with a sibling or non-related rivals were indistinguishable. In contrast to this, there were no significant differences in mating latencies between any of the males that had matured in different social environments. Together these results confirm that the relatedness of rival males does not affect the plasticity of copulation duration that directly benefits reproductive fitness. The role of relatedness in the plasticity of mating latency or ‘attractiveness’ of males to females requires further study.

rival males, mating behavior, phenotypic plasticity, Drosophila