Adult sex ratio influences courtship behaviour, contest behaviour and breeding success in Onthophagus taurus

Social interactions (e.g. aggression, mating and parenting) often depend on the social environment, and recent studies suggest that the adult sex ratio (ASR, the proportion of males in the adult population) modulates these social interactions. While observational studies and phylogenetic analyses provide strong support for the proposition that the relative frequencies of adult males and females impact social decisions, the experimental evidence for the impacts of ASR on social interactions is moot. In this study, we manipulated ASR in dung beetles, Onthophagus taurus, to test the responses of males and females in contest behaviour, courtship behaviour and breeding success. We found that ASR influenced the intensity of both contest and courtship, with both contests and courtships being more frequent in male-biased ASRs than in female-biased and unbiased treatments. The weight of brood balls did not differ significantly between ASR treatments, suggesting that ASR does not affect the breeding success of dung beetles by changing the parental investment of females and males. Moreover, the mean number of brood balls per male was fewer in male-biased treatments than in female-biased and unbiased treatments, while there was no significant difference in the mean number of brood balls per female between treatments, supporting the hypothesis that a male-skewed ASR can impair the breeding success of organisms by influencing the reproductive capacity of males but not females. Our results extend the understanding of the influence of ASR on the reproductive strategy decisions of males and females in breeding success. Future analyses are needed to understand the physiological and genetic implications of mate choice and contest behaviour in the context of a variable social environment, as represented by ASR.