Human sperm competition: ejaculate manipulation by females and a function for the female orgasm

Abstract

Abstract. Behavioural ecologists view monogamy as a subtle mixture of conflict and cooperation between the sexes. In part, conflict and cooperation is cryptic, taking place within the female's reproductive tract. In this paper the cryptic interaction for humans was analysed using data from both a nationwide survey and counts of sperm inseminated into, and ejected by, females. On average, 35% of sperm were ejected by the female within 30 min of insemination. The occurrence and timing of female orgasm in relation to copulation and male ejaculation influenced the number of sperm retained at both the current and next copulation. Orgasms that climaxed at any time between 1 min before the male ejaculated up to 45 min after led to a high level of sperm retention. Lack of climax or a climax more than 1 min before the male ejaculated led to a low level of sperm retention. Sperm from one copulation appeared to hinder the retention of sperm at the next copulation for up to 8 days. The efficiency of the block declined with time after copulation but was fixed at its current level by an inter-copulatory orgasm which thus reduced sperm retention at the next copulation. Inter-copulatory orgasms are either spontaneous (= nocturnal) or induced by self-masturbation or stimulation by a partner. It is argued that orgasms generate a blow-suck mechanism that takes the contents of the upper vagina into the cervix. These contents include sperm and seminal fluid if present; acidic vaginal fluids if not. Inter-copulatory orgasms will therefore lower the pH of the cervical mucus and either kill or reduce the mobility of any sperm that attempt to penetrate from reservoirs in the cervical crypts. Intercopulatory orgasms may also serve an antibiotic function. Copulatory and inter-copulatory orgasms endow females with considerable flexibility in their manipulation of inseminates. The data suggest that, in purely monandrous situations, females reduced the number of sperm retained, perhaps as a strategy to enhance conception. During periods of infidelity, however, females changed their orgasm pattern. The changes would have been cryptic to the male partners and would numerically have favoured the sperm from the extra-pair male, presumably raising his chances of success in sperm competition with the female's partner.