Reduced male pheromone production and an increased female sex ratio promote same-sex sexual behaviour in male Indian meal moths

Same-sex sexual behaviours (SSSBs) are often deemed detrimental to fitness because they carry the same costs as sexual interactions with the opposite sex without the reproductive benefits. Multiple theories have therefore been proposed to explain the presence of this phenomenon in many animal species. For example, the mistaken identity hypothesis suggests that SSSBs result from inaccurate sex recognition, either due to a disruption in sexual signalling or because of an indiscriminate mating strategy by members of one sex. We tested whether the frequency of male SSSBs is dependent on male pheromone production by hair pencils, the adult sex ratio and sexual selection history in the Indian meal moth, Plodia interpunctella . Increased sexual selection on males may increase the costs of inaccurate sex recognition and therefore reduce the number of SSSBs. We found that the frequency of SSSBs was highest when arenas contained at least one female. We also found that males missing at least one hair pencil were more likely to receive at least one mating attempt from a rival male. The sexual selection history of a male's population did not affect the frequency of SSSBs. This result suggests that male sex pheromones are important for sex identification in this species, at least at close range, and can influence behaviour in the context of mating and sexual selection. Our findings also contribute to the growing number of studies showing that the expression of SSSBs is dependent on the social environment and may be an unintended outcome of a promiscuous male mating system, combined with poor close-range sex recognition. • Experimental evolution was used to study male same-sex sexual behaviour (SSSB). • Male SSSB frequency was highest when females were present. • Males with reduced pheromone production received more male mating attempts. • Findings highlight the role of male pheromones in close-range sex identification. • SSSBs may be an outcome of a promiscuous mating system.