Sperm competition

Robin Baker and Mark A. Bellis have written a book entitled Human Sperm Competition: Copulation, Masturbation and Infidelity (published in 1995) which aims to explain why one human ejaculation contains so many sperm (over 300 million, to be exact). This seems a trifle over the top, bearing in mind that only one sperm will ever be able to fertilize an egg (with the rare exception of non-identical twins). Baker and Bellis came up with the idea of sperm competition, which means that sperm from one man are equipped to engage in a battle with sperm from another man if they meet inside a woman's vagina - the prize being to fertilize the egg and produce a child. Sperm can live for about ten days in the vagina, so any woman who has had sex with two different men in her fertile period may offer a ripe opportunity for a war between sperm with the prize of a successful conception.....

It is certainly a fact that many children have a different biological father to the one who is the putative father. Baker and Bellis suggest that around ten percent of British children are conceived by sperm that has won some kind of "sperm war" inside the mother's vagina. But how can sperm do battle? The answer may lie (according to this highly speculative theory) in the observation that there are many different types of sperm in a man's ejaculate - Baker and Bellis claimed that the sperm in a man's ejaculate have at least eight different forms of head; and as if that were not enough, there are apparently four types of tail and various other shapes and sizes as well. Just what is all this variation about? And what's also rather odd is the fact that a lot of these sperm cells have strangely misshapen heads or tails. The theory holds that some of these sperm are actually soldiers, which have the job of killing other men's sperm.

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Sperm competition

In theory, sperm might emerge from the testicles misshapen because of the difficulty of gametogenesis, the process by which sperm cells are manufactured. It takes about eleven weeks to make sperm cells, during which time much can go awry. Gametogenesis begins in the seminiferous tubules where the head of the sperm is made, and then continues in the epididymis (the tubes lying on top of and alongside the testicles). The sperm cells develop certain qualities while they spend time in the epididymis: they gain motility and the capacity to fertilize the egg. Errors in this whole process are of course inevitable, so perhaps both the number of misshapen sperm and the high total number of sperm in a man's ejaculate are factors that take account of the difficulty of gametogenesis.

Baker and Bellis have a different view - they claim that sperm cells are so massively varied in size and shape because they have so many different function. The newer, most well-formed ones, which have large heads, are the ones whose job is to seek out and fertilize the egg, while some of the others kill the sperm of any other man who might have been fortunate enough to ejaculate inside the woman's vagina. These "soldier sperm" have the job of killing other men's sperm by injecting them with the same substance which allows sperm to penetrate the egg.

What is even more extraordinary is that both the number and proportion of each type of sperm varies according to whether or not a man suspects his partner has been unfaithful to him - he will, say Barker and Bellis, ejaculate more sperm in total and a higher proportion of soldier sperm if he thinks his partner may have had sex with another man. They even suggest that the purpose of masturbation is to ensure the sperm ready for ejaculation are young, fit and healthy and have not had time to decay in the testicles of the man concerned.

if there is anything in this hypothesis, then it follows that women would have a biological incentive to "double-mate" so that they could promote sperm competition and ensure their child was fathered by the fittest and strongest sperm (which may be linked to the fact that fit and healthy men make fit and healthy sperm!) whose offspring would in turn pass on the ability to make strong sperm and so continue to be reproductively successful.

Of course you can take this idea even further. If a woman has an orgasm during sex, she retains more semen; if she does not have an orgasm, her vagina expels the semen of the man with whom she has just had sex in a process called flowback. This is not random leakage, but active expulsion of semen, occurring from ten minutes to an hour after sex. Could it be that the woman has an orgasm when she wants to aid the sperm from the man with whom she subconsciously wishes to fertilize her eggs?

In response, since nature always seeks to even things up in any evolutionary arms race, the shape of the male penis may have evolved so that it effectively scoops out the semen of any other male which happens to be left in the vagina of the woman with whom a man is having sex. When you have sex with a woman, the shape of the head of your penis, the thickness of the shaft, and the thrusting motion of intercourse all serve to suck out the semen left there by any male who happens to have been there before you.

All this may seem very unlikely, but ......Baker and Bellis suggested that the penis evolved its current size and shape so as to remove other males' semen: this is why the human penis is shaped like a piston, straight and thick, topped by a smooth, acorn-shaped glans. They also suggested that this is why men like to thrust so much when they get their penis inside a woman's vagina, regardless of her pleasure. In fact, according to these authors, the motion of the thrusting penis was not designed to bring about female pleasure; it was designed to remove a mating competitor's soft plug and then work as a "push-pull-push-pull-scrape" mechanism which would remove competing sperm.

Thrusting back and forward with a long, thick penile shaft in a distended vagina would indeed remove semen left by another man. And the shape of the glans does seem to be well-adapted to such a function: and, of course, if the penile shaft is thicker than average, it may generate much more suction pressure, which might in turn remove part of the cervical mucus with any sperm contained in it. The penis becomes a physical tool for sexual pleasure; a battering ram, even; not one used in a sense of misogyny or to exercise domination over a woman, but as tool in an evolutionary arms race, one which will advance a man's own evolutionary agenda. Of course, women use similar evolutionary strategies to further their own evolutionary ends. Thus the battle of the sexes is fought at a genetic level, and the penis is a major participant in that war. But socio-political issues have nothing to do with it.

It would be a wild exaggeration to claim that Human Sperm Competition has been received with or achieved universal acclaim. An important element of scientific progress is the generation of new hypotheses, especially when you are working in new areas of knowledge. But while Baker and Bellis have been very successful in coming up with new ideas, they have failed to tell readers that the book and the ideas it puts forth have no factual basis - they are not supported by research, in other words - so though they have written a fascinating book on the penis and the vagina, on ejaculation and sexual strategies, they have been charged with misleading the public on a grand scale.