Same‐Sex Sexual Behaviours in a Wasp: Links With Reproductive Success and Potential Chemical Cues for Sex Recognition

Same‐sex sexual behaviour (SSB) in animals, once considered an evolutionary dead end, is surprisingly widespread, including in male insects. These behaviours may arise from either adaptive or non‐adaptive processes. One hypothesis suggests that SSB can occur when courtship costs are low or when missing a mating opportunity is costly leading to a reduced behavioural threshold for courtship—a ‘broad mating filter’, as predicted by threshold acceptance theory. We investigated SSB in males of the parasitoid wasp Venturia canescens (Gravenhorst, 1829, Hymenoptera: Ichneumonidae), a species where encounters between partners are infrequent, females are monandrous and male courtship is not energetically costly at least in terms of longevity. We hypothesised that males exhibit a broad mating filter, accepting conspecifics for courtship regardless of sex. First, behavioural assays assessed whether SSB correlated with higher reproductive success. 18% of males displayed SSB, and these behaviours had no detectable effect on either courting or mating success. Thus, males expressing SSB did not experience any reproductive advantage. Since literature shows that cuticular compounds can be cues for sex recognition, we compared, in a second experiment, cuticular hydrocarbon (CHC) profiles between sexes. Our chemical analyses suppose quantitative CHC differences between males and females, indicating that males would ignore those potential cues for sex recognition when expressing SSB. Our findings support the hypothesis of a broad mating filter in V. canescens males. In this species, where conspecific encounters are rare, SSB appears to arise from reduced discrimination rather than adaptive benefits.