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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.
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
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?
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