, Volume 17, Issue 3, pp 253-264

Semen displacement as a sperm competition strategy

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Using a sample of 652 college students, we examined several implications of the hypothesis that the shape of the human penis evolved to enable males to substitute their semen for those of their rivals. The incidence of double mating by females appears sufficient to make semen displacement adaptive (e.g., one in four females acknowledge infidelity, one in eight admit having sex with two or more males in a 24-hour period, and one in 12 report involvement in one or more sexual threesomes with two males). We also document several changes in post-ejaculatory behavior (e.g., reduced thrusting, penis withdrawal, loss of an erection) which may have evolved to minimize displacement of the male’s own semen. Consistent with predictions derived from a theoretical model (Gallup and Burch 2006), we discovered that most females report waiting at least 48 hours following an instance of infidelity before resuming sex with their in-pair partners.

Gordon G. Gallup, Jr. is a professor of psychology interested in human reproductive competition.
Rebecca L. Burch is an associate professor of psychology interested in familial and social relationships, semen biochemistry, semen displacement, and other sexual behaviors. Tracy J. Berens Mitchell is currently a graduate student in the Human Sexuality Program at Widener Univeristy.