Human sperm competition: ejaculate adjustment by males and the function of masturbation
Abstract. Sperm competition theory argues that the number of sperm inseminated into a female by a male is a trade-off between two opposing pressures. On the one hand, the risk that sperm may find themselves in competition with the sperm from another male favours the male inseminating more sperm. On the other hand, ejaculates are costly to produce and males are favoured who economize over the number of sperm inseminated. This paper analyses: (1) sperm numbers and other ejaculation data for 35 human couples; and (2) data relating to the most recent copulation reported by 3587 women. Three sets of predictions based on sperm competition theory are tested. These are that the number of sperm inseminated should be a function of: (1) risk of sperm competition; (2) female reproductive value; and (3) optimum partitioning of ejaculates between successive in-pair copulations. During in-pair copulation the male used successive inseminations to 'top-up' the population of sperm in his female partner. In accordance with sperm competition theory: (1) individual males inseminated more sperm when the pair had spent a smaller proportion of their time together and hence risk of sperm competition was greater; and (2) larger females wore inseminated with more sperm than smaller females. In apparent contradiction to sperm competition theory, the number of sperm inseminated did not vary according to female orgasm pattern or the probability of conception. This apparent failure of the theory may instead be due to the male's lack of necessary information. Paradoxically, male mammals seem to waste huge numbers of sperm through spontaneous emission and self-masturbation. Such shedding of sperm could be adaptive if it led to more competitive and/or more fertile inseminates at the next copulation. The data showed that a recent male masturbation reduced the number of sperm inseminated at the next copulation but not the number retained by the female. It is concluded that masturbation is a male strategy to increase sperm fitness without increasing sperm numbers in the female tract. The possibility that, in the absence of sperm competition, the probability of fertilization decreases if too many sperm are inseminated is discussed. This latter factor may be more important than ejaculate cost in favouring male restraint over the number of sperm inseminated.
