Female orgasms aid human reproduction by stimulating ovulation, scientists have found, apparently solving a problem that has been debated for thousands of years.
The purpose of the female orgasm is a mystery because it does not appear to serve any obvious role in reproduction. It is not needed to conceive and is surprisingly uncommon during heterosexual sex.
Mostly, research dedicated to finding out the point of the female orgasm has focused on humans. By looking at other mammals, scientists now believe it does have a part to play.
"Prior studies have tended to focus on evidence from human biology and the modification of a trait rather than its evolutionary origin," said Gunter Wagner, from Yale University. He and Mihaela Pavličev from Cincinnati Children's Hospital have now published a study on the role of the female orgasm in the journal JEZ-Molecular and Developmental Evolution.
In it, the team wrote: "Aristotle noted the main obstacle in explaining the role of female orgasm: human females can conceive without it ... Equally suggestive is the statistics, showing human female orgasm during sexual intercourse is uncommon, in particular without additional clitoral stimulation."
To look for the evolutionary purpose, they focused on the physiological trait of an orgasm - the neuro-endocrine discharge of prolactin and oxytocin. They looked for this in other mammals and found this trait is involved in ovulation. In humans, ovulation is not dependent on sex but in some other species, it is induced by the male. This trait evolved first and cyclical ovulation (that we have) emerged later.
Researchers believe the female orgasm may have evolved for the reproductive purpose of triggering the reflex and inducing ovulation. Later on this became redundant, leaving the female orgasm to become the perplexing subject it is today.
The scientists said that while they may not be able to prove ovulation is the reason for female orgasms, "the fact that these surges are enhanced by the female orgasm, rather than general arousal, indicates that orgasmic peak is an important part of the mechanism".
Pavlicev added: "Homologous traits in different species are often difficult to identify, as they can change substantially in the course of evolution. We think the hormonal surge characterises a trait that we know as female orgasm in humans. This insight enabled us to trace the evolution of the trait across species."