Sex has always had its fair share of complications, for biologists and lovers alike. But some of the latest research on the subject adds a startling new dimension to the proceedings. When a man ejaculates, for example, the make-up of his semen may reflect the amount of time he has spent with his partner since they last made love. If they have been apart, he may release more sperm, perhaps because he must try and allow for the possibility that she has been unfaithful. And the woman? The timing of her climaxes may influence how many of her partner’s sperm she retains inside her body after sex – an effect which could influence her reproductive fate.
These are just two of the subtle forces at work during lovemaking, according to Robin Baker and Mark Bellis of the University of Manchester. The team’s findings (to be published this autumn in Animal Behaviour) also bring new evidence to bear on ancient sexual conundrums such as the biological significance of masturbation. Their conclusions are bold and extraordinary and will be eagerly scrutinised by other biologists. Two researchers I spoke to were respectful but sceptical; another two were more positive. All felt that the new findings were intensely interesting – a fact reflected in the large audiences that gather to hear Baker and Bellis lecture at conferences. ‘I just hope that somebody else will have a go at doing the same sort of thing,’ says Baker.
The team’s research focuses on ‘sperm competition’, a phenomenon that looms large in the lives of many animals. Whenever a female animal has sex while carrying living sperm from a previous partner inside her body, the scene is set for sperm competition. The two rival batches of sperm, each intent on fertilising her eggs, must compete for the privilege (see ‘Faithless female seeks better genes. . .’, ¿ìè¶ÌÊÓƵ, 4 July 1992). Many species of insect and bird go to astonishing lengths to give their sperm a head start in this most vital of races .
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Does sperm competition also happen in humans? And could it have shaped our evolution? Many researchers would like to answer those questions, even though they involve formidable practical and ethical problems. The first hurdle is to establish the chances of fertile sperm from two or more men coming into contact inside women’s bodies. Direct evidence for this most intimate of details is, by its very nature, difficult to come by, although women, like men, obviously indulge in sexual adventures. And then there are widely quoted figures from genetic studies, which suggest that around 10 per cent of children in certain samples have been fathered by someone other than their putative father.
A few years ago, Baker and Bellis collected some more direct evidence of ‘double matings’ by women – instances where sperm from one man could find itself rubbing shoulders, so to speak, with sperm from another. (The life span of sperm inside a woman’s body is reckoned to be about five days.) The evidence came from a questionnaire distributed to women across the country with the help of Company magazine. Of around 3000 copulations on which the team gathered detailed information, 76 were double matings. Among women who had had sex more than a thousand times in their lives, nearly three-quarters claimed to have ‘double mated’ at least once.
If sperm competition has had a hand in human evolution, one would expect it to have left some indelible marks on our sexual physiology. Consider men first. The number of sperm a man sheds during sex is probably a compromise between the need to fertilise eggs and the need to keep biological costs as low as possible. If there is a risk that his sperm will have to compete with those of a rival, he should unleash more sperm than normal to try to defeat his rival by sheer force of numbers. How does he assess that risk? One way is to monitor the amount of time he has spent away from his partner since they last made love, working on the assumption that absence makes the fond heart wander. That assumption would sometimes seem well-founded. ‘We have data from our nationwide survey that shows that the proportion of time a couple spends together really does correlate with the risk of sperm competition,’ explains Baker.
Do men take that risk into account? Baker and Bellis recruited a band of volunteers – staff and students from the university – and asked them to collect semen in condoms during lovemaking with their regular partners. Sure enough, a correlation did emerge: men who had spent more time with their partners released fewer sperm when making love (although not when masturbating). These results were promising, but they fell short of establishing that individual men adjust their performance according to their social calendars. However, the team’s most recent research – assessing a series of ejaculates from individual men – does address that question. ‘It’s the individual male that’s adjusting numbers,’ says Baker, ‘it’s not just differences between males who spend different proportions of their time with their partners.’
The team’s findings also offer insights into some of the finer points of ejaculation, notably the way men allocate sperm to successive bouts of lovemaking with their regular partners. Men, it seems, neither shed the same number of sperm on each occasion, nor dispense each and every sperm they have produced since they last made love. Rather, they act as if they are trying to keep the sperm population inside their partners at some optimum level, allowing for losses and topping up accordingly. If the risk of sperm competition increases, then so does that optimum number.
Other factors may also affect how many sperm a man lavishes on his partner. Studies on animals suggest that males may devote more resources to sperm competition if the female they are courting is of higher ‘reproductive value’; that is, if she is more fertile, likely to bear healthier babies and so on. Baker and Bellis argue that in humans such qualities often go hand in hand with greater body weight, up to a point. So do men with heavier partners shed more sperm during lovemaking? Apparently they do, although the researchers’ results only provide evidence of a correlation, not a causal link – and as such are open to various interpretations. ‘What we would like are samples from men who change partners, to look and see if sperm numbers are adjusted according to size of partner, but we haven’t got enough data for that yet,’ says Baker.
Ejaculation is clearly a much subtler affair than most men would have guessed. Those subtleties also extend to male masturbation, a practice puzzling to biologists because it combines apparent futility with near-universal popularity. If men do not make love for three days or so, they turn increasingly to masturbation. Baker and Bellis wondered how this activity fits into the sperm economy, given that wasting sperm would seem to threaten a man’s ability to compete with his rivals.
Their analysis led them to some striking conclusions. Suppose a man masturbates and then makes love a day or two later. When he makes love, he releases fewer sperm than he would have done had he not masturbated, but those sperm seem particularly competitive. That competitive edge is revealed by what happens to the sperm inside the woman – whether she keeps them within her body or ejects them in the ‘flowback’, globules of semen and other fluid that issue from her vagina soon after sex. When a man masturbates and then makes love, say the researchers, his partner retains just as many sperm as usual, even though there were fewer sperm in the ejaculate – hinting that his sperm are in some sense more tenacious than usual.
And the explanation of this curious improvement? 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.
Flowback and think of England
The decision to study flowbacks cannot have been an easy one, for all sorts of reasons, but it has been rewarded by a cluster of new ideas about female sexual physiology. 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. Baker and Bellis asked women to collect flowbacks in glass beakers after intercourse. By analysing these samples, the researchers aimed to work out how many sperm women keep inside them and how many they discard – and the factors that influence this most important ratio.
This is by no means a simple affair. Ideally one would want to measure both the number of sperm ejaculated (by collecting sperm in a condom) and the number of sperm in the flowback; obviously this is not possible. To get over this hurdle, the team used a predictive equation, developed in their previous work, to calculate the number of sperm ejaculated. They then subtracted the number of sperm in the flowback to estimate the number retained by the woman.
With the help of this procedure, the team found that their subjects usually expelled about a third of the sperm left inside them by men, but sometimes they achieved near total ejection. What factors influence the quantity of sperm in the flowback? A major one, say Baker and Bellis, is the female orgasm. 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. (A point to bear in mind: retaining relatively few sperm is not a barrier to conception. One woman who became pregnant during the study retained very few sperm around the time of conception.)
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.
Women take control
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. Evolution may have provided both sexes with some subtle tricks, but women, it seems, currently have the upper hand. ‘The female is well ahead, I think,’ says Baker.
1: Rough and tumble
Male animals of many kinds go to enormous lengths to win the fight to pass on their genes. Some ‘guard’ their mates to try to ward off the competition, while others attempt to seal their partners’ reproductive tracts with plugs. Others adopt a different approach, removing sperm already deposited within potential mates by their rivals. Among damselflies and dragonflies, for example, the male is equipped with an ingeniously shaped penis that scoops out sperm from inside the female’s body. With his rival’s sperm removed, he makes his own deposit.
When the male of a species of cricket inseminates a female, he simply flushes out the sperm left by her former lovers – which adhere to his penis. He then eats the sperm, showing that his stomach is as strong as his determination to be a father. Some of the sperm produced by many butterflies and moths have no nuclei – and hence cannot fertilise eggs. Some zoologists think that these misbegotten gametes may be designed to displace or destroy rival sperm in the female tract and so ease the progress of their better-endowed relatives.
A few years ago Robin Baker and Mark Bellis at Manchester University 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. This intriguing theory has yet to gain wide acceptance.
2. Pass notes on sperm
Size: Head 0.005 millimetres; tail 0.045 millimetres.
Swimming speed: 3 millimetres/minute.
Life span inside female tract: Estimates vary from a brief span of two days, through five days (the figure used by Robin Baker and Mark Bellis) to a protracted career of nine days.
Time taken for development in testis: 74 days.
Rate of production: As many as 300 million a day, according to some estimates.
Number in the ejaculate: Between 200 to 500 million in about 3 millilitres of semen. The liberality of individual men varies from ejaculate to ejaculate.
