Semen Displacement Theory

How does the penis deal with sperm competition?

The mushroom-cap shaped glans at the tip of a human penis is very special and unique.  Other great apes have a much simpler penis shape (feel free to use Google images for this one), and while we’re so caught up on the complexities of the human vagina I think we often overlook and undermine the intricacies of dick. If we pay closer attention to penile anatomy we can often stumble upon great meaning.

Where the mushroom head joins the shaft we find the coronal ridge, the rim around the circumference of the penis that is created by the glans having a wider diameter than the shaft itself. 

The marvellous Prof. Gordon Gallup reckoned that the distinctive shape of the human penis, especially the coronal ridge, might have evolved as a result of paternity competition, i.e. rivalry between men to make it be one of their own sperm that fertilises a woman's egg.

Excellent semen removal service van

Excellent semen removal service van

The human penis is chiselled in a way that it can effectively function as a semen removal service. The coronal ridge and other features of the penis can work together to dislodge and remove sperm in the female reproductive tract that may have been previously left there by someone else.  Sperm can survive in there for up to several days, so even if the women was last ejaculated in yesterday or the day before, human penis morphology is still equipped for potential sperm competition.

Other characteristics of the human penis that seem to have evolved for maximal reproductive success in the face of sperm competition include; the boundless force and distance at which ejaculation happens, and the tendency for the penis to expand and fill the vaginal tract during penetration.  The pumping/thrusting action of sex also aids the displacement of semen via the coronal ridge, by drawing the other men’s semen away from the cervix.

 Studies have found that men and women report that sex with their partners involves deeper and quicker thrusting than usual when there has been time apart, and/or when there has been suspicion or allegations of infidelity on the female’s part.  This suggests that when there is any chance of another mans sperm lurking inside a guys girlfriend they are motivated (either consciously or unconsciously) to use their penis to full semen removal effect.

Just in case you were wondering, Gallup did in fact devise a study to check if the penis really was capable at displacing semen from the vagina.  Unfortunately the research was carried out on artificial genitalia rather then real live humans (he probably didn’t want to push his luck with the ethical approval committee).

Gallup and his team of researchers got a bunch of realistic prosthetic dicks of various lengths and diameters, both with and without coronal ridges. They made batches of artificial seminal fluid (see recipe here) and loaded it up into a ‘life-like’ latex vagina.

Next the fake dicks were used to penetrate the fake vag at varying depths and the researchers simulated the thrusting motion.  After each trial they measured how much semen had been displaced from the fake vagina. 

As expected, the penises with the coronal ridge displaced around 91% of the ‘semen’ where as the control dick only removed 35.3%.  The deeper the simulated thrusting – the more fluid was displaced, and shallow thrusting didn’t manage to catch any semen at all.

The results of this inspired research and the data collected from the surveys strongly support the principal points of semen displacement theory.

The study of evolutionary willy morphology leads to questions about our ancestors sexual behaviour.  Humans tend to view ourselves as naturally monogamous, or at a stretch some people allow the idea that men are designed to ‘spread their seed’, while women are looking for ‘the one’.

But semen displacement theory suggests that our lady ancestors may have been enjoying multiple sexual partners also.  What would be the point of a semen-removing penis if there were no semen to remove?

Of course this theory and its corresponding studies are by no means proof that humans are supposed to be non-monogamous, but it’s certainly a topic for discussion.  And if a side effect of having multiple partners is more energetic pumping and thrusting…   maybe it's something to look in to.