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.
