Onan, in an effort to avoid siring children, practiced coitus interruptus when sleeping with his brother’s wife. So it is a little puzzling that “onanism” has become associated with more solitary sexual pleasures. Contraception seems to have been mistaken for masturbation.
The truth of the matter, though, may be the other way round – masturbation may have evolved as a way of improving fertility.
In any species where monogamy is not universal, a male’s sperm will sometimes find itself having to compete with other sperm from a previous or subsequent copulation. This may explain why mammals produce sperm in such enormous numbers – in order to improve their chances in such competitions.
The Blocking Hypothesis
Mark Bellis and Robin Baker at the University of Manchester believe that the task for a lot of sperm is to run interference, leaving a small amount to get on with the job. With the aid of volunteer couples, mostly students, they have been testing some of the implications of the idea. The couples are asked to provide sperm samples from copulation and masturbation, and to answer questions about the circumstances that gave rise to them.
One of the findings seems to confirm the blocking hypothesis. If a couple makes love twice, the woman will retain little of the second ejaculate. It seems plausible to think of this as an effect of the sperm in the first ejaculate, since if a condom is worn the first time, the effect disappears.
Other findings seem to show that evolution has provided for some control over the nature of the ejaculate, as well. If competition is the reason for producing a lot of sperm, a male in a monogamous relationship should not need to go to the bother of producing so much.
This seems to be the case. Dr Bellis and Dr Baker found that sperm count at copulation gets lower the more time a couple spends together; it does not seem to depend on how often they have sex.
In masturbation, this is not the case. The sperm count depends mostly on the time since the previous ejaculation. But numbers are not everything. As sperm age, their tails coil up and they become somewhat sticky. This reduces their swimming ability, their energy levels and, perhaps, their ability to fertilise an egg.
After they are made, the sperm join the back of an orderly queue to wait for their moment of glory, so the sperm ejaculated first will be the oldest. If sex is frequent, the oldest sperm are not that old. If it is rare, one way to guarantee fresh sperm might be to masturbate.
Although men in a stable relationship do not necessarily masturbate often, Dr Baker and Dr Bellis found that most will, at least once, after 11 days without sex. But it is not the time since the last copulation which is the crucial factor – it is the time until the next one.
More than half of all masturbation will take place within 48 hours of the next copulation. To Dr Baker and Dr Bellis, it looks possible that the masturbation is a way of getting rid of old, tired sperm.
Women Have the Final Say
But despite man’s best efforts to produce a suitable ejaculate, and to limit the chances of any other sperm, evolution may have given the final say to women. Dr Bellis and Dr Baker’s latest results show that a female can exercise some control over the number of sperm retained after copulation.
Women’s copulatory orgasms – occurring around the same time as the man’s ejaculation – increase the amount of sperm retained. However, an earlier orgasm, during foreplay perhaps, significantly reduces the number of sperm retained at a subsequent copulation. These inter-copulatory orgasms may therefore provide the woman with a way of regulating sperm retention – a control which can be over-ridden by a copulatory orgasm.
From their questionnaires, Dr Baker and Dr Bellis find that women, unlike men, masturbate more in stable relationships than they do when single, and that they masturbate yet more during periods of infertility.
At the same time, they are more likely to have a copulatory orgasm with their clandestine partner than with their primary partner. If the evolutionary aim of adultery is to have children by more than one male, this behavior – giving an advantage to the lover’s sperm – makes sense.