The penis is one of nature’s great curiosities. Why would a man, presumably with a female partner in a stable, monogamous relationship, need a particularly large appendage with an elaborate, flared glans or head, and oversized testicles?
If it is accepted that the average male penis is between 5 and 7 inches in length, there is no doubt that it is far larger than one would expect of a species our size. In this we are similar to the chimpanzee which, like humans, has an outsized penis and testes for its overall size. If we are meant to be monogamous, as many would have us believe, why would we need such large genitalia, which point to a sperm-making factory?
The answer can be found in our closest relatives, the chimp and the gorilla. Although male chimps are well-endowed, gorillas are exactly the opposite. For such large, physically intimidating creatures, they are disappointingly puny with respect to their reproductive organs. Scientists put this down to a simple fact: chimps, which are wildly promiscuous and therefore require plenty of sperm to spread around, need larger genitalia in order to make, store and deposit their sperm effectively.
Gorilla society, on the other hand, revolves around an all-powerful male who has a harem, and therefore has no need to compete with other males for sexual access to females. The result is that his genitalia – unlike his physical appearance – are small.
Where do humans fit into this equation, and what bearing does that have on the shape of man’s penis? In terms of penis size relative to body size, humans fall in between chimps and gorillas, which would seem to suggest our ancestors were more promiscuous than modern man but not quite as frisky as chimps. On the other hand, we were certainly more sexually adventurous than the gorilla, and human females clearly had more sexual freedom than their gorilla counterparts.
But that doesn’t explain the whole story. The most unusual – some would say downright bizarre – feature of the human penis is the bulbous glans or head, which flares out elaborately, creating a coronal rim that resembles a carefully designed motorcycle helmet. Nothing in evolution happens by accident, and scientists have always been baffled by the penis’ design. Even chimps have a straightforward penis without a prominent head, which would seem to suggest that human penis shape doesn’t have much to do with promiscuity.
But it turns out that the flared glans and coronal rim have everything to do with promiscuity. Scientists now believe that the shape is perfectly designed to remove the sperm of other males from the female vagina during sex. Using sex toys as a research tool, demonstrations showed that more than 90% of sperm from a previous ejaculation was ‘scooped’ out by a thrusting penis.
Sceptics argued that if a man was ‘scooping’ out rivals’ sperm during thrusting, he would also likely remove most of his own sperm. But research shows that immediately after ejaculation, the head of the penis shrinks dramatically, meaning a man removes very little of his own sperm. In addition, the head becomes almost painfully sensitive after climaxing, which deters further thrusting. This means his sperm – and therefore his chance of ensuring paternity – is mostly preserved.
There are precedents in nature: some insects have barbed penises that are designed to draw out rivals’ sperm; male dunnock birds actually peck at their partner’s cloaca in order to remove the sperm of rivals. But why doesn’t our closest relative, the chimp, have the same penis design? Science suggests their sperm works differently: it acts as a plug to the female’s reproductive tract. This ‘plug’ stops rival sperm from reaching the all-important egg.
Man has unwittingly interfered with the way in which nature has designed the penis, however. Circumcision, which is widespread in some parts of the world, results in men losing sensitivity on the glans; the loss of crucial nerve endings means sex may be less pleasurable for a circumcised man who then thrusts harder and deeper in order to derive maximum pleasure. Although there is no conclusive scientific evidence to prove this, numerous anecdotal accounts from women who have had sex with both circumcised and non-circumcised men report that the thrusts of the former tend to be rougher and more frantic.
The result, some believe, is that – all other factors being equal – a circumcised penis would be more effective at removing a rivals’ sperm, giving it the edge in fertilising the egg. Because modern man is generally monogamous, however, and because human reproduction is traditionally a carefully controlled process within a stable relationship, circumcision is largely irrelevant as a reproductive factor in society.
Nonetheless, no prospective father should feel completely at ease. The occasional incidence of hetero-paternity, a phenomenon in which twins have different fathers, shows that a tiny minority of women still subconsciously respond to the evolutionary call for sperm competition. This attitude of ‘letting the best man win’ by encouraging rival sperm to fight it out in the reproductive tract shows that humans are still vulnerable to the echoes of our evolutionary past, as the shape of the penis proves.