Same-sex sexual behaviour reduces reproductive success but mitigates aggression in a subsocial insect

Abstract

Same-sex sexual behaviour (SSB) is widely observed across taxa, yet its evolutionary maintenance remains debated and its fitness consequences remain surprisingly understudied. Here, we quantified the fitness consequences of male SSB in the burying beetle Nicrophorus vespilloides and tested three prominent hypotheses about its function: (i) that it provides practice for future mating, (ii) that it reduces tension through dominance assertion, and (iii) that it arises indirectly from indiscriminate mating. Males were observed in same-sex pairings to quantify SSB and then paired with females to measure their subsequent mating behaviour and reproductive success. Same-sex sexual behaviour was common, occurring in 55% of same-sex pairs, and was associated with reduced reproductive success driven by higher brood failure rates. Both males that initiated, and those that received, mounting attempts from other males suffered reduced fitness. Contrary to the practice hypothesis, SSB did not increase mating speed with females and was associated with reduced mating frequency. However, SSB was associated with reduced aggression, supporting the hypothesis that SSB reduces tension through dominance establishment. Our findings show that SSB can reduce reproductive success but may be maintained due to a beneficial reduction in aggression.