Sperm competition risk and male genital anatomy: comparative evidence for reduced duration of female sexual receptivity in primates with penile spines

Abstract

Selection pressures influencing the wayin which males stimulate females during copu- lation are not well understood. In mammals, copulatorystimulation can influence female remating behaviour, both via neuroendocrine mechanisms mediating control of sexual behaviour, and po- tentiallyalso via effects of minor injuryto the female genital tract. Male adaptations to increase copulatorystimulation maytherefore function to reduce sperm competition risk byreducing the probabilitythat females will remate. This hypothesis was tested using data for primates to explore relationships between male penile anatomyand the duration of female sexual receptivity . It was predicted that penile spines or relativelylarge bacula might function to increase copulatorystim- ulation and hence to reduce the duration of female sexual receptivity. Results of the comparative analyses presented show that, after control for phylogenetic effects, relatively high penile spinosity of male primates is associated with a relativelyshort duration of female sexual receptivitywithin the ovarian cycle, although no evidence was found for a similar relationship between baculum length and duration of female sexual receptivity. The findings presented suggest a new potential function for mammalian penile spines in the context of sexual selection, and add to growing evidence that sperm competition and associated sexual conflict are important selection pressures in the evolution of animal genitalia.