Interracial sperm competition
Sperm competition is a form of post-copulatory sexual selection  whereby male ejaculates simultaneously physically compete to fertilize a single ovum. Physiological evidence, including testis size relative to body weight and the volume of sperm in ejaculations, suggests that humans have experienced a low-to-intermediate level of selection pressure for sperm competition in our evolutionary history. Evidence suggests that, among the great apes, relative testis size is associated with the breeding system of each primate species. The volume of sperm in ejaculates scales proportionately with testis size and, consistent with the intermediate weight of human males testis, ejaculate volume is also intermediate between primates with high and low levels of sperm competition. Researchers have suggested that males produce more and higher quality sperm after spending time apart from their partners, implying that males are responding to an increased risk of sperm competition,  although this view has been challenged in recent years.