Researchers say the female orgasm may have once been involved in human reproduction.
Study co-author Gunter Wagner, professor of ecology and evolutionary biology at Yale University in New Haven, CT, and colleagues publish their findings in the Journal of Experimental Zoology Part B Molecular and Developmental Evolution.
Wagner notes that previous studies that have attempted to uncover the reasons why women orgasm have focused on "evidence from human biology and the modification of a trait rather than its evolutionary origin."
In an attempt to address this research gap, the team conducted an analysis of how orgasms have evolved across a wide range of species.
Specifically, they looked at whether a certain physiological trait associated with the human female orgasm - the release of the hormones prolactin and oxytocin - arises in other mammals with a placenta, and if so, what role it plays.
The team explains that there is an abundance of physiological characteristics that can be traced across mammalian evolution.
For example, they point out that the ovarian cycle in female humans does not depend on sexual activity, whereas ovulation is induced by males in other mammalian species.
Female orgasm may have had a direct role in reproduction
From their analysis, the researchers found that evolution of male-induced ovulation occurred first, while spontaneous ovulation derived from this, evolving later.
As such, the researchers speculate the female orgasm may have once had a direct role in reproduction - in other words, it acted as a reflex to trigger ovulation. They suggest that this reflex evolved to become redundant; it is now simply a feeling of pleasure during sexual activity.
The team also points to research that compared genitalia across placental mammalian species.
It was found that, alongside the evolution of spontaneous evolution, the position of the clitoris - the female sex organ that, when stimulated, can induce orgasm - also evolved; it was once located in the copulatory canal.
Wagner and colleagues suggest that this modification reduced the likelihood that the clitoris gains sufficient stimulation during sexual intercourse, and as such, the release of the orgasm-inducing hormones prolactin and oxytocin through sexual intercourse is less likely.
"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 characterizes a trait that we know as female orgasm in humans. This insight enabled us to trace the evolution of the trait across species."
Study co-author Mihaela Pavličev, Cincinnati Children's Hospital