Masturbation female

Following Smith (1984) I have argued that the risk of sperm warfare has helped to shape the evolution of just about every aspect of human sexuality - from testis size to penis shape, and from the wet sheet phenomenon to masturbation and the female orgasm (Baker and Beilis 1995 Baker 1996). 

COPULATION, MASTURBATION AND INFIDELITY SPERM COMPETITION AND FEMALE SEXUAL STRATEGY... 

Figure 7. Influence of the time interval between a male masturbating and then inseminating a female on the number of sperm inseminated and the number of spenn retained. Although longer abstinence times (masturbation to copulation) increases the number of sperm ejaculated into the woman, the number of sperm she retains does not increase (Baker and Beilis 1993a,b). If the competitiveness of an ejaculate is a trade-off between the age and number of sperm, ejaculates preceded by masturbation a day beforehand may well be the most competitive for an EPC male (Baker 1996).

Men inseminate fewer sperm if they have masturbated beforehand, the amount depending on how recently the masturbation occurred (Baker and Beilis 1993a). The flow-back study shows, however, that the number retained by the female does not decrease (Fig. 7). Inseminates preceded by masturbation are retained in higher proportions and similar numbers as inseminates not so preceded. Yet the sperm inseminated are younger. They should therefore live longer inside the female and, whether the sperm competition proceeds as a lottery, a race or as warfare, could be at a competitive advantage. 

In contrast to this ever-nearer dead-end for the direct investigation of ejaculates, the possibilities for investigating the way that testis size and symmetry influence male (and, for symmetry, female) reproductive strategy at different stages of their lives seem to be widening. There is still much more to discover about human copulation, masturbation and infidelity than is discussed in this interim description of the state-of-the-art. 

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