Same-sex sexual behaviour as a by-product of reproductive strategy under male-male scramble competition

A variety of hypotheses have been suggested to explain the evolution of same-sex sexual behaviour (SSB) in male insects. Males may be poor at discriminating other males from females, inexperienced, or may need to learn how to distinguish females. Alternatively SSB could be a by-product of a plastic reproductive strategy to succeed in scramble competition. Such a strategy could be adaptive if the cost of missing the chance to mate with available females exceeds the cost of mounting single males instead (i.e. SSB). We tested predictions derived from these hypotheses by experimentally varying the sex ratio and thus the intensity of scramble competition in the water strider Tenagogerris euphrosyne. We allowed males to experience female-biased or male-biased sex ratio treatments for a week, and then compared their expression of SSB under standard conditions. We also measured daily mating success of males during the treatment and related this to SSB frequency under assay conditions. Males kept in male-biased sex ratio conditions exhibited more SSB, mounting males and females equally often under assay conditions. Males kept under female-biased conditions only mounted males one-quarter as often as they mounted females in the assay. Male mating experience under treatment conditions did not influence the expression of SSB in the assay. In a second experiment we found that males that were more likely to exhibit SSB enjoyed higher initial mating success in a subsequent mating assay in a male-biased sex ratio. Our results indicate that plasticity in reproductive strategy not only increases mating opportunity but also confers a reproductive benefit. Taken together, SSB in T. euphrosyne is a highly labile trait responsive to sex ratio that delivers increased mating success under intense scramble competition.