One further interesting theory that originated in Baker and Bellis’s book ‘Human Sperm Competition‘ concerns penis size; the duo looked at the human penis, which although it is not extraordinary by general mammalian standards, is the largest amongst all the primates.

Baker and Bellis suggest that its shape and size may have been formed as a result of ‘sperm competition’. And on that note, they draw attention to the fact that in the seconds after ejaculation, semen forms a soft spongy coagulate that is vulnerable to being removed by a subsequent sexual partner; in the instance that he were to meet the woman quickly enough, and subsequently have sex with her…

The length of the penis and its enlarged bulbous tip, according to Baker and Bellis, suggests that it functions as a ‘suction piston‘ for sperm removal (a sort of natural ‘plumber’s snake’) which is designed to break up and perhaps even remove coagulated sperm deposits from previous males; an effect that would be assisted by the often quite vigorous thrusting that characterises sexual intercourse and ejaculation among humans (and which is really rather difficult to explain or justify otherwise).

A plumber’s snake is used to push a blockage farther down the system, which in this context appears counterproductive, and could even promote fertilization by the earlier male’s sperm, except that the female reproductive tract is essentially a dead end, so this ’suction piston’ hypothesis might have some validity. Although there are some further problems with this theory: If sperm competition has been so important in designing man’s anatomy (especially the relatively large penis) then, why do men have proportionately smaller testicles than chimps? This could be because sperm competition is even more important for chimps; and no matter how often multiple mating might occur among humans, it is even more frequent amongst Chimpanzees. But then, why don’t chimps also have comparatively large penises? Nobody seems to know the answer to this.

The penis and vagina are closely matched in all species, rather like a lock and key. So, human penis size could have been largely determined by the size of the vagina. Vagina size, meanwhile, seems to have been determined by the size of the human baby (specifically, its head diameter) which needs to pass through it in childbirth. As the size of the human brain, and thus the human head increased during evolution, most probably, so did the size of the vagina. And this, in turn, could have generated pressure for the evolution of larger and more competitive penises.

You might also enjoy reading:-

• Robin Baker and the early history of Sperm Wars
• Sperm Wars: The Science of Sex – Reviewed and appraised
• Sperm competition and the Kamikaze Sperm Hypothesis
• Genes determines premature ejaculation in men
• Women don’t care about penis size, but many men suffer from ‘Small-Penis Syndrome’

Posted by Jonathan in Anthropology, Sociobiology

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