The existence of women’s orgasms has given scientists and philosophers a lot to chew on over the centuries. The pleasurable climax is neither required for reproduction nor particularly easy to achieve during heterosexual intercourse, based on simple mechanics. Yet it inexplicably evolved and persists.

Researchers have come up with a variety of theories to try to explain women’s big ‘O’ mystery. Some hypothesize that it does, in fact, subtly benefit reproductive success. Others put forth the “by-product” theory, which suggests that women experience orgasms only because they share developmental stages with men, in whom orgasms are an explosive adaptation critical for human reproduction.

Now, evolutionary biologists Mihaela Pavličev, of the University of Cincinnati, and Günter Wagner, of Yale, offer an entirely different theory that they argue fits with the evolution of fellow mammals. They suggest that female orgasms used to be the trigger for readying eggs for fertilization but became obsolete and then co-opted to serve primate-specific roles—such as enabling bonding and partner choice—after cyclical egg-releasing evolved in ancestors.

In outlining their theory, published in the Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, Doctors Pavličev and Wagner begin with the simple idea that successful baby-making requires the well-timed meeting of a sperm and an egg. For many male mammals, unleashing sperm isn’t necessarily a carefully orchestrated event. Females, on the other hand, are more methodical about egg-readying—aka ovulation. They have several strategies, including timing it with having sex, using environmental or seasonal cues (perhaps times of the year when temperatures and resources are favorable for rearing offspring), establishing independent, “spontaneous” cycles, or some mix of those three.

For instance, ovulation in mares is timed by day length, with breeding generally restricted to spring and summer, while women ovulate independently on a monthly cycle. Ovulation in female hedgehogs is directly induced by having sex.

The authors note that copulation-based ovulation seems to be commonly distributed among placental mammals and has a deep evolutionary root. This, Pavličev and Wagner argue, suggests that sex-triggered ovulation is the ancestral method for regulating egg offerings, making women’s cyclical strategy a younger, evolutionary offshoot.

Regardless of the initial trigger—sex, cycles, seasons, etc—the next step toward ovulation is a common biochemical reflex: a spurt of hormones released from the brain. These hormones include prolactin and oxytocin, which both spur an egg to mature and help ready a female's body for pregnancy. In mammals with sex-induced ovulation, that gush of baby-making hormones is specifically thought to be triggered by exciting the erectile bit of female genitalia known as the clitoris. That fits with anatomical comparisons, Pavličev and Wagner note. In mammals that have sex-induced ovulation, the clitoris tends to be in a place where it will be easily stimulated during penetration—either in or very near the vaginal canal.

In women and other female primates, however, the clitoris sits relatively far from the vaginal canal and is not as easily stimulated by heterosexual intercourse (masturbation and homosexual intercourse are more reliable ways of reaching the peak). Not coincidentally, women also don’t have sex-induced ovulation—women’s egg-cueing hormones are doled out in a monthly cycle independent of sex. Still, when a woman orgasms, there’s a detectable surge of prolactin and oxytocin accompanied by euphoric sensation.

Pavličev and Wagner see a clear explanation of what happened: the female orgasm began as a trigger for ovulation, but its role was nixed with the evolution of women's current cyclical ovulation strategy. This led to the de-coupling of reproduction and orgasms, which “likely opened the potential to co-opt the clitoris into new, primate- or human-specific roles,” they argue. And those new roles for the female orgasm, such as helping females bond with or select partners, “may explain its maintenance, but not its origin.”

Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 2016. DOI: 10.1002/jez.b.22690