The female orgasm has always provoked biologist’s curiosity. From an evolutionary perspective, the male orgasm has a clear purpose: it is required for ejaculation and the subsequent transfer of sperm. Explaining the female orgasm is more difficult, since fertilization and reproduction will occur whether or not a female orgasm occurs. In fact, female orgasms occur more frequently during masturbation or homosexual intercourse than during heterosexual intercourse. Natural selection, the driving process of evolution, favors traits that yield a survival or reproductive advantage to a species, so why has the female orgasm evolved if it provides no apparent survival or reproductive advantage?
Yale’s Günter Wagner and his colleague, Mihaela Pavlicev, recently identified that the female orgasm—like the male orgasm—predates the primate lineage. Thus the human female orgasm likely evolved from an older, functional trait and was passed down through generations. Here we will explore the history of this trait and how its function has changed since it originated.
For species whose ovulation is induced by copulation, it is thought that orgasms stimulate the release of hormones such us prolactin and oxytocin, triggering ovulation. In contrast, women are spontaneous ovulators. Although they too release prolactin and oxytocin during orgasm, their ovulation cycles do not depend on these hormones.
Differences in ovulation cycles between species may have evolved in tandem with anatomical differences. Wagner’s study predicted that for spontaneously ovulating animals, the distance between the clitoris and the vaginal opening can be larger than for animals with copulation-induced ovulation, whose clitorises should be within (or near) their vaginal openings. In accordance with this prediction, external, non-penile stimulation of the clitoris is required for many women to achieve orgasm because the clitoris is relatively far from the vaginal canal.
Looking deeper into anatomical evolution, Wagner and his colleague studied databases of veterinary literature comparing female animal anatomy. In spite of a dearth of accurate studies on female genitalia, they found enough data to support their hypothesis. They discovered that in most species of reptiles, birds and mammals, a single canal is used for urination and copulation. In these animals, the clitoris is often within or nearby the copulatory canal. However, in the ancestors of humans and other primates, the urogenital canal–the canal used for urination and copulation–shortened until the urethra became an entirely independent canal. As these two canals separated, the distance between the copulatory canal and the clitoris also increased.
Wagner realized copulation-induced ovulation occurred most often in animals in which the clitoris was located within or near the copulatory canal. Spontaneous ovulators, in contrast, evolved to separate the clitoris and vagina. Together, this information suggests that the common ancestor of many mammals was a copulation-induced ovulator. However, as spontaneous ovulation evolved in humans and primates, clitoral stimulation as a means to induce ovulation became useless, and evolution distanced the clitoris from the copulatory canal.
Though researchers yet to prove that copulation-induced ovulation is triggered by clitoral stimulation and orgasm, the theory is pharmacologically testable. Future studies, could use drug induced anorgasmia–the inability to achieve orgasm–and the subsequent monitoring of an animal’s ovulation cycle, to begin to answer this question.
In the context of spontaneous ovulation, clitoral stimulation leading to female orgasm may serve another purpose. For example, orgasm might improve pair bonding. Wagner emphasized that although the female orgasm may not make a clear contribution to reproductive fitness, this does not reflect its modern importance. “Maybe the way to think about the female orgasm is as a type of art . . . which doesn’t have to have a specific purpose but still has value,” Wagner explained, referencing the way we value our ability to admire art despite its non-existent connection with reproduction or fitness.
Wagner’s research has powerful and liberating implications for both men and women, freeing them from preconceived notions about the meaning of female orgasm during heterosexual intercourse. The research will hopefully discourage unhealthy notions, such as Sigmund Freud’s labeling of the clitoral orgasm as “infantile,” or theories that claim the female orgasm occurs more frequently with higher-quality-males as a mechanism for retaining larger quantities of their sperm. Wagner and Pavlicev explain the difficulties associated with achieving female orgasm during heterosexual intercourse, as an effect of evolution–without any intrinsic implications regarding their value.