Sex wars and the great egg race: Masturbation may help couples to conceive, reports Chris Barnard

ACCORDING to folklore, it inflicts blindness, saps strength and leads to sterility and impotence. The evils of masturbation are as legion as the pejorative names deriving from it. Despite its alarming mythology, however, masturbation turns out to be a useful mechanism for sharpening reproductive performance - in both sexes.

While copulating with the partner of one's choice might seem like the pinnacle of sexual success, negotiating the minefield of social skills is only the half of it as far as reproductive success is concerned. Once sperm are shed into the vagina, a new, more subtle battle of the sexes ensues. There are two aspects to this: first, there is a struggle between the sperm of rival males to fertilise the egg (sperm competition); second, the female may be able to manipulate the outcome of that competition.

Biologists at the University of Manchester have discovered that patterns of masturbatory and copulatory orgasm are important weapons in the conflict.

In common with many other superficially monogamous species, the apparent stability of human pair bonds is regularly undermined by sex with other partners. With the help of volunteers who were supplied with condoms, Robin Baker and Mark Bellis have studied changes in the number of sperm ejaculated in relation to the risk of sperm competition from rival males. The possibility of his partner mating with a rival poses a problem for the 'regular' male. If he has been away from his partner for any length of time, there is a chance that when he next copulates with her she may already be carrying the sperm of a rival. One way to deal with this is to up the number of sperm in his ejaculate and swamp the opposition.

Condom samples from copulations with regular partners confirmed this; men ejaculated more sperm the longer they had been away from their partners. The number of sperm in control masturbatory samples showed no such variation, so the effect was not due simply to replenishment.

However, ejaculating a great quantity of sperm has its drawbacks. The more sperm that are inseminated, the more are retained in the female reproductive tract. If this number becomes too great, the chances of successful fertilisation can drop because development of the egg is impaired by multiple fertilisation. So if the risk of sperm competition is low, increasing output may not be such a good idea. Variation in ejaculate content can thus amount to a flexible 'topping-up' strategy, with a male attempting to maintain an optimum number of sperm in his partner's tract - where the optimum depends on the likelihood of competition.

This situation is further complicated by the female. According to Baker and Bellis's evidence, far from being passive receptacles for incoming sperm, females actively manipulate the sperm content of their reproductive tract by using different patterns of copulatory and masturbatory orgasm. Baker and Bellis found that during sex with their regular partner, the timing of female orgasm relative to the male resulted in low levels of sperm retention.

Adultery is different, however. During copulations with a male other than her regular partner, the timing of orgasm shifted so that the suction effect of the contractions drew sperm deeper into the tract. However, almost by definition, the regular partner is still likely to to have left residual sperm in the tract. In addition to the timing of orgasm, females pursuing extra-pair relationships stepped up their frequency of masturbatory and involuntary nocturnal orgasms.

The likely effect of this was to increase the acidity of the cervical environment and thus kill or reduce the mobility of the regular partner's sperm retained in the tract. Such additional orgasms could be an effective means of loading the dice in favour of an extra-pair male's sperm at the next copulation. This fits with the finding that women indulging in extra-pair matings tend to do so at a point in their cycle when they are most likely to conceive.

While there's not much a male can do about this surreptitious tweaking of the odds, he can go some way to redressing the balance during in-pair copulation. Baker and Bellis discovered that masturbation adjusted the content of ejaculates in favour of sperm that were more likely to be retained by the female. These were, on average, younger sperm made available by the tendency for males to masturbate less than 48 hours before copulation, so getting rid of older, less competitive stock.

Far from being an unproductive avenue of self-relief, therefore, masturbation may be a powerful customising agent, allowing both sexes to manipulate opportunities for fertilisation in a complex struggle for reproductive advantage. The conflicts of interest between the sexes so obvious at the social level thus extend to the internal female environment, fine-tuning the race for the egg among competing ejaculates.