Evidence for genetically-based sperm discrimination in the vaginal tract of a primate species

Females influence offspring paternity through diverse pre- and post-copulatory mechanisms. Sperm discrimination-the differential physiological response to ejaculates based on male or sperm characteristics-can bias fertilization outcomes, but in vivo evidence of this process in large-bodied mammals is lacking. Here, in a study of nine females and four males, we tested whether two aspects of female physiology that affect sperm survival-vaginal immune response and pH-are modulated by male genetic makeup in a non-human primate, the olive baboon (Papio anubis). Our findings suggest post-copulatory differences in vaginal gene expression and pH, with the strongest immune responses and largest pH decreases, harmful to sperm, exhibited by females mating with genetically similar males. These findings are consistent with genetically-based post-copulatory mate discrimination, offering new insights into how interactions between male gametes and the female reproductive tract may shape conception probability in primates.