Robin Baker and Mark Bellis at Manchester University have made the startling suggestion that the human ejaculate contains 'kamikaze' sperm, whose role is to wage war on sperm shed by other men. On this view, fewer than 1 per cent of human sperm may be 'egg-getters' - sperm specialised for fertilisation. The remainder may be designed to sit in strategic spots in the female tract and interfere with the passage of rival sperm, or to meander through the tract, bent on seeking and destroying those rival sperm.
Sperm have a limited shelf life inside a man's body. Any ejaculation, be it during lovemaking or masturbation, removes ageing sperm from the store, allowing the next ejaculate to contain a particularly youthful, high-quality stock. And so, in the absence of lovemaking, masturbation is a sensible ploy for a man faced with the constant threat of sperm competition. 'Putting in sperm that are younger means that they are going to be inside the woman - and be more competitive - for longer,' says Bellis.
'Flowbacks' usually emerge about half an hour after sex, when women expel - sometimes quite forcefully - three to eight white globules, containing semen and female secretions.
If the woman climaxes at any time up to a minute before the man, or does not have a orgasm at all, she retains relatively few sperm. If she climaxes at any time from a minute before the man to around 45 minutes after, she retains a relatively large number of sperm.
These findings have allowed the Manchester team to adjudicate between two competing theories of the female orgasm. The poleaxe theory - which portrays the orgasm as a device to make women lie down and sleep after sex so as to minimise loss of semen - gets no support from the data. Its main competitor, the equally inelegantly named 'upsuck' theory, fares much better. On this view, the orgasm is a virtuoso muscular performance aimed at sucking vaginal contents, sperm included, towards the womb - just as one might fill a pen with ink. (In an extraordinary experiment performed forty-odd years ago, scientists showed that a mare's uterus could suck up 80 millilitres of fluid in 5 seconds.) If this is indeed what happens during an orgasm, then one would expect a woman to retain more sperm if her orgasm coincided with, or followed, ejaculation, which is exactly what the researchers found.
But there is also another side to all this. If the timing of a woman's orgasm - and whether or not she has an orgasm at all - affects the number of sperm she retains, she is anything but a passive participant in sperm competition. Suppose she has two lovers whose sperm are competing for the chance to fertilise her eggs. In theory, she could affect the outcome of that contest - using her orgasms to retain the sperm of one lover and reject those of his rival. Force of numbers might then help her favourite's sperm win the race to fertilise an egg.
And this is not the only such technique at her disposal. Baker and Bellis also found that the number of sperm a woman retains after intercourse depends on her sexual behaviour in the days leading up to that encounter. Imagine she has sex on two occasions separated by a few days - on Monday and Friday, say. The number of sperm she retains on Friday is apparently influenced by sperm left inside her body as a result of Monday's episode. It is as if sperm from Monday's copulation somehow manage to block retention during Friday's. Although the force of this block declines with time, women can halt the decline in its tracks by having a 'non-copulatory' orgasm - by masturbating on Wednesday, say. The mechanisms behind these effects have still to be discovered, but Baker and Bellis argue that women can exploit them, albeit unconsciously, to influence the number of sperm they retain when making love.
Taken together, the team's findings imply that women have some extraordinary skills. Do they actually use those skills to influence sperm competition? Drawing once again on their survey of female sexual behaviour, Baker and Bellis argue that they do. A woman who is having an affair, they say, tends to act in ways that favour her lover's sperm over her regular partner's.
For more than a hundred more unusual stories and ideas about men, women and sex, see The Global Ideas Bank's Relationships section.
Summarised from an article entitled 'The subtle side of sex' by Stephen Young in the New Scientist (August 14 '93).
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