The mystery of the female orgasm is a cliché as old as time—or at least as old as Aristotle, who wrote a scientific treatise on reproductive pleasure, some 2,400 years ago. For centuries, academics have seemingly pored over women's anatomies, trying to divine the ways they get off. The results have been pretty anticlimactic.
Now, a new paper claims to describe the ancestral purpose of the female orgasm. Much like the human tailbone, female orgasms once served an important function, according to the study whose findings were published today in JEZ-Molecular and Developmental Evolution. The evolutionary role of orgasms, the authors theorize, was to induce ovulation during sex, allowing women to become pregnant.
"Prior studies have tended to focus on evidence from human biology and the modification of a trait rather than its evolutionary origin," said co-author Gunter Wagner, a professor of ecology and evolutionary biology at Yale University, in a statement.
Induced ovulation is literally the stimulation of egg production through the physical act of sex. Most felines, for example, are induced ovulators, and the benefit of this trait—in cats, at least—includes higher chances of single paternity for males, and the assurance of fertilization for females. Among primates, however, induced ovulation has never been observed, though some studies suggest that human semen contains a potent ovulation-triggering protein.
Since the female orgasm isn't inherent to successful reproduction, the team of biologists at Yale University and the Cincinnati Children's Hospital hoped to find a physiological reason for its existence.
When women have orgasms, they found, their bodies are flooded with "feel good" chemicals called prolactin and oxytocin, which is an effect that has already been observed in men. Similar chemical releases happen during induced ovulation, the authors claim, which led them to believe that orgasms once provoked fertilization in ancient humans.
"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," said co-author Mihaela Pavličev, a resident at the Center for Prevention of Preterm Birth at Cincinnati Children's Hospital, in a statement.
The study also found that induced ovulation arose first in mammals, with spontaneous ovulation (i.e. having a menstrual cycle) only appearing 75 million years ago. Wagner and Pavličev say that once animals abandoned male-induced ovulation, the clitoris coincidentally moved away from the copulatory canal—where it's more likely to be stimulated during sex—and toward the outward genitalia.
Basically, orgasms stuck around, but are as useful today as our stubby little tailbones.
Whether a Darwinian approach should be applied to the female orgasm is subject to debate. For years, scientists tried to find an analog for the male orgasm in women, leading critics to denounce their findings as biased and patriarchal. The mystification of female sexuality was further encouraged by the field of evolutionary psychology, which proposes psychological traits as adaptive characteristics. In a male-centric world, female orgasms surely can't be for pleasure alone. But if they aren't for making babies, they must serve another purpose—vestigial or otherwise.
Elisabeth Lloyd, a professor of biology at Indiana University, told The Guardian that while the study's findings are thought-provoking, they leave a lot of evidence by the wayside. For example, the authors failed to consider the neurological and muscular effects of an orgasm, as well as orgasms in other mammalian species.
"It all seems to be rather purposeless—except for the enjoyment, obviously," she said. "It doesn't mean it is not important, it just means it doesn't have an evolutionary purpose."