Trending Science: New theory suggests the female orgasm is an ‘evolutionary leftover’

One of the most iconic movie scenes of the 20th century was Meg Ryan’s hilariously effective fake orgasm in a busy deli in front of a mortified Billy Crystal in the 1989 smash hit, ‘When Harry Met Sally.’ However, the specific evolutionary functions of the female orgasm have baffled scientists for generations, as the euphoric sensation is not actually necessary for conception. Now scientists are arguing that instead of having an evolutionary purpose, the pleasure of female orgasm is merely a vestige of how ovulation was induced in ancestral mammals by the hormonal surges that accompany it.

Humans and other primates do not need intercourse to trigger ovulation - they have evolved to a point where this happens on its own. However, the hormonal changes that take place during intercourse persist and fuel the female orgasm, the scientists hypothesise. As these hormonal surges no longer confer a biological advantage, orgasms during intercourse may thus be lost in some women as a result.

The new work addresses what David Puts, a biological anthropologist at Pennsylvania State University, University Park, calls ‘one of the most contentious questions in the study of the evolution of human sexuality: whether women’s orgasm has an evolutionary function.’ Scientists have proposed dozens of theories on the evolution of female orgasm, most being suggested decades or even longer ago. These have included arguments that women have orgasms because their reproductive systems derive from the same origins as those of men, who biologically require orgasm to release sperm. Another popular theory was that orgasms persist because they encourage loyal partners. Others have proposed that female orgasms allow for optimal physiological conditions to increase the chances of conception.

Writing in the journal ‘JEZ-Molecular and Development Evolution’, co-authors Mihaela Pavilcev from Cincinnati Children’s Hospital and Gunter Wagner from Yale University describe how they delved into the anatomy and behaviour of a host of placental mammals to uncover the evolutionary origin of female orgasm. In mammals, such as cats and rabbits, hormonal surges occur during sex and play a crucial role in signalling for eggs to be released from the female’s ovaries – in essence, they are an absolutely essential component of the reproductive process. By tracing these mechanisms of ovulation across the evolutionary tree of mammals, the scientists found that so-called ‘male-induced ovulation’ predates spontaneous ovulation that was likely to have arisen in the common ancestor of primates and rodents about 75 million years ago.

The authors argue that their theory is supported by a comparison of the position of the clitoris, which gives rise to orgasm, in different mammals. Species that rely on hormonal surges to trigger ovulation tend to have a clitoris inside or near the female sex canal, making it more likely to be stimulated during sex. Mammals that ovulate spontaneously - including humans - have theirs further away. This could also help to explain, according to Pavilcev, why many women do not experience orgasms during sex.

David Puts insists that the new theory is plausible, though he does stress that it only examines the hormonal component of female orgasm. ‘Of course, it is difficult or impossible to investigate sexual pleasure in nonhuman animals,’ he commented. However, he does state that the new research goes a long way towards boosting our understanding of the ancestral form of human female orgasm. ‘From there, we can explore how these components have changed over evolutionary time in response to putative selection pressures.’

However, the research has received some criticism from other experts in the field, with Elisabeth Lloyd, professor of biology at Indiana University arguing that the theory does not take into account neurological and muscular aspects of human female orgasm. Also, due to the fact that it’s very difficult to assess whether other mammals feel the pleasure associated with orgasms, the work can only ever address the evolution of some of the components of female orgasm.

One of the positive outcomes of the continued interest in this mystery could be an improvement in reproductive medicine. However, whilst the new theory does propose an intriguing answer to one of the great mysteries of human sexuality, it appears that scientists will continue to further test both this theory and their own hypotheses on the real raison d’être behind the female orgasm.