Copulation, Masturbation, and Infidelity

Abstract

When a woman copulates with two or more different men within five days, the sperm from those men compete for the ‘prize’ of fertilising any egg she may produce. This ‘sperm competition’ is probably both a lottery and a race, but more than anything it could also be a war, with sperm of different morphologies playing different roles. The risk of sperm competition has been argued to shape the evolution of almost every aspect of human sexuality: from testis size to penis shape; from the ‘wet sheet’ phenomenon to masturbation and the female orgasm. Most male behaviour can be seen as an attempt either to prevent sperm competition or to win any competition that occurs if he fails. Most female behaviour can be seen — as she ‘shops around’ for resources and genes — as a continual attempt to optimise any advantageous opportunities for promoting sperm competition. Here, I summarise and test some hypotheses as to how the risk of sperm competition has shaped male and female sexuality. In particular, I evaluate suggestions that men with larger testes, men of greater bilateral symmetry, and bisexual men are morphs adapted to greater involvement in sperm competition. The timing of copulations which could lead to sperm competition varies with the risk of conception in different ways in different phases of a woman’s reproductive ontogeny. So too does the woman’s retention of sperm as determined by her orgasm pattern. Direct evidence is presented that men with larger testes and more symmetrical men are more likely to become involved in sperm competition. Both also inseminate women with more sperm during copulation and shed more sperm during masturbation. Bisexual men, however, ejaculate fewer sperm. Heterosexual men inseminate established partners with more sperm when the risk of sperm competition is high but inseminate extra-pair women with fewer sperm. Low sperm numbers during extra-pair copulations and in the ejaculates of bisexual men are achieved via masturbation and may be strategies for success in sperm competition. If ejaculate competitiveness is a trade-off between sperm numbers and sperm age, smaller but younger ejaculates may be a better compromise when the male has only a limited opportunity to inseminate a particular woman, whereas larger, albeit older, ejaculates may be a better compromise when a male has more frequent opportunity to inseminate.