Context-dependent relationship between a composite measure of men's mate value and ejaculate quality

Abstract

Secondary sexual traits in males are recognized as having arisen in order to gain access to reproductive opportunities, through their effects on the outcome of male-male competition and female choice. The phenotype-linked fertility hypothesis proposes that ejaculate quality is honestly advertised via secondary sexual traits. Alternatively, if males have limited resources to allocate to both pre- and postcopulatory traits, males possessing attractive phenotypic or behavioral traits may produce poorer quality ejaculates. Sperm competition theory also predicts that the female phenotype will influence ejaculate quality, with males increasing investment as female attractiveness increases. However, the extent to which the male and female phenotypes interact in affecting ejaculate quality has not been widely studied. Here, we examine how male and female phenotypes influence ejaculate quality in humans. Eighty-one men, for whom we had a composite measure of overall male mate value, produced a semen sample in response to images of either highly attractive or less attractive women. We found a significant relationship between male mate value and ejaculate quality that was context dependent. Sperm motility and concentration increased with male mate value but only when men viewed images of highly attractive women. Context dependence may contribute, in part, to the often conflicting patterns of variation found in studies that test the phenotype-linked fertility hypothesis.