The human penis is a device that can "scoop out" semen from the female reproductive tract as well as deposit it there, US scientists have discovered, writes Natasha Gilbert.
In a study that used artificial human male and female genitals to simulate sexual encounters, researchers from the State University of New York found that the head (or coronal ridge) of a man's phallus can displace existing semen from a woman's cervix before a new load is released.
Gordon Gallup, lead author of the study, and his colleagues at the department of psychology speculated that this morphological feature evolved as a tactic to maximise resident males' likelihood of paternity. The study states that in the event of extra-pair copulation, the resident male can rid the female reproductive tract of semen left by other males.
"The resident male's penis can draw semen left by other males away from the female's cervix preventing conception," Professor Gallup said. "He can then replace it with his own."
The researchers tested two latex phalluses, equipped with a coronal ridge but differing in girth, and one plastic penis without a head that acted as a control. They inserted each model at several different depths into a plastic vagina holding simulated semen.
The study reports that the two latex phalluses with a coronal ridge displaced about 91 per cent of the imitation semen, outperforming the plastic penis, which managed to remove only 35.5 per cent. It also states that the deeper the penises were thrust into the model vagina, the greater the semen displacement.
Videotapes of the simulated sexual encounters revealed that when the latex phalluses were inserted 75 per cent or more into the vagina, the semen was forced back around the shaft and then collected behind the coronal ridge.
The findings are published in the journal Evolution and Human Behaviour .