Why do women have orgasms? The question has long confused scientists; after all, orgasm isn’t necessary for conception, and women can orgasm even when they’re not having reproductive sex. Now, a new study brings an interesting new theory into the mix: Women’s orgasms could be a vestige left over by evolution itself.
In a literature review recently published in the Journal of Evolutionary Zoology, a pair of developmental evolutionary specialists posit that as the female reproductive system evolved, so did the role of the orgasm. Once necessary for conception, female orgasms now appear to be a bonus for human intercourse. But similar to vestigial organs like tonsils or appendix, the orgasm remained.
The secret lies in ovulation, the mechanism that causes ovaries to discharge eggs for reproduction. In some species, like cats and rabbits, physical stimulation is needed to prompt the egg to be released—a phenomenon called induced ovulation. But in humans, ovulation happens spontaneously (without stimulation), often on a regular schedule. And not only can human females come to orgasm without penetration, but a recent survey of more than 1,000 women suggests that many—only 61.6 percent of heterosexual women—do not orgasm during intercourse at all. Nor is female orgasm associated with a higher number of offspring in humans.
This has long confused scientists, who in the past came up with two hypotheses. Some think that women do need orgasms to reproduce, but researchers have not yet figured out why. Others consider orgasms to be happy accidents associated with the clitoris, the organ responsible for sexual stimulation that is sometimes thought of as the female version of the male penis.
The authors of the new study, however, don't think the human female orgasm is accidental or related to male evolution. Rather, they trace it to ovulation. “By just reading the literature, we found that there is an endocrine surge just following the female orgasm in humans,” the study’s author, Mihaela Pavličev of Cincinnati Children’s Hospital Medical Center, told Smithsonian.com.
This surge of hormones, including prolactin and oxytocin, is similar to other surges observed in animals like rats, who need these natural chemicals to tell their body to ovulate. The surge can also help eggs implant in species like rodents. Some studies even suggest that humans have similar connections between egg implantation with post-orgasmic hormone shifts.
That hormone-orgasm connection in both humans and induced ovulators led Pavličev to believe that they were once connected long before humans became a species. She speculates that spontaneous ovulation likely evolved in the last common ancestor of primates and rodents. Eventually, however, they must have evolved into so-called spontaneous ovulators, but the hormonal reactions associated with orgasms remained.
This concept is supported by a fascinating finding: the development of spontaneous ovulation parallels a shift in clitoris position. Based on the evolutionary ties between a range of animals, the researches found that later-evolving creatures, humans included, ovulated spontaneously. And this change coincided with the clitoris shifting northward, further away from the vagina.
“At that point,” says Pavličev, “the clitoris lost its function for reproduction.”
Pavličev’s work raises other, even more fascinating questions. Why did humans start ovulating spontaneously in the first place? Which came first: spontaneous ovulation or induced ovulation? And what evolutionary pressures sparked these changes in women?
Pavličev is particularly interested in the connection between female orgasms and their apparent association with egg implantation. If there really is an evolutionary case for that adaptation, she says—or if humans simply haven’t evolved past the orgasm-implantation connection yet—further research could one day lead to changes in recommendations for women trying to get pregnant through in vitro fertilization.
Perhaps the most intriguing aspect of Pavličev’s study is the implication that there is an evolutionary reason women don’t always orgasm. “It’s not that there’s anything wrong,” she says. “It’s just how our anatomy is.” Translation: Women who don’t achieve orgasm during sexual intercourse are not defective—just highly evolved.
Editor's Note, August 1, 2016: This article has been changed to clarify that spontaneous ovulation likely evolved in mammals long before humans split off as a species.