Measuring same-sex sexual behaviour: the influence of the male social environment

Same-sex sexual behaviour (SSB) is attracting increasing research attention, but quantifying and interpreting SSB present unique challenges. Chief among these are (1) partitioning the expression of SSB into separate influences from interacting partners, (2) distinguishing between same-sex behaviour, same-sex preference and same-sex orientation and testing for correlations between them, and (3) evaluating how the social environment modulates the expression of SSB. We used sexually mature male Drosophila melanogaster in staged encounters to address these aims. The expression of SSB was not consistent across choice and no-choice experimental trials, indicating that a tendency to display SSB when females are absent does not correlate with greater same-sex preference when a choice is available. The expression of SSB was sensitive to the social experience of males. Experience with other males and experience with females both decreased the incidence of male SSB, suggesting that both avoidance learning and mating experience mitigate its expression. SSB in D. melanogaster appears to be a highly labile trait susceptible to varied influences from the social environment. We suggest that SSB expressed in different social contexts probably represents different physiological origins, which is a potentially important consideration in studies examining its genetic architecture and evolutionary maintenance.