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EvolutionEvolution

Rabbit study hints at the origins of the female orgasm

Researchers used rabbits and antidepressants to search for a link between orgasm and ovulation in female mammals.

ByKatherine J. WuNOVA NextNOVA Next
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Researchers used rabbits and antidepressants to search for a link between orgasm and ovulation in female mammals. Image Credit: Aubord Dulac, Shutterstock

The list of humankind’s scientific accomplishments is long. We’ve unraveled the human genome, harnessed quantum mechanics, and sent space probes hurtling past Pluto.

Despite decades of research, however, some puzzles remain unsolved—including the female orgasm, whose biological roots have long defied explanation.

Now, with the help of some antidepressants and a fluffle of rabbits, a team of researchers may now be a bit closer to unveiling the origins of this elusive experience. Their study, published today in the journal PNAS, suggests that the female orgasm is an evolutionary relic—one left over from a distant mammalian ancestor that needed clitoral stimulation to release eggs for fertilization.

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This reproductive reflex isn’t present in women of reproductive age, whose ovaries typically pump out eggs monthly, regardless of sexual activity. But mating with a male still triggers ovulation in other, more evolutionarily ancient mammals like rabbits, ferrets, cats, and camels. These animals also happen to have clitorises that reside very near or within their copulatory canals (that is, their versions of the vagina), which could be directly stimulated during mating.

Three years ago, this peculiar pattern led biologists Mihaela Pavličev of the University of Cincinnati’s College of Medicine and Günther Wagner of Yale University to propose a new theory: The clitoris first arose as a sort of reproductive tripwire that, when set off, would alert the brain that ejaculation was nigh, prompting the release of ovulation-inducing hormones.

Eventually, the researchers argued, some mammalian lineages—including our own—branched off from the so-called induced ovulators, instead adopting monthly cycles. As orgasms’ original purpose went obsolete in these animals, the clitoris migrated out of the vagina, becoming more difficult to stimulate during copulation (something that might help explain why orgasm occurs less reliably in women having sex, Pavličev says). And just like that, orgasm and ovulation went their separate ways.

To further test their hypothesis, Pavličev and her colleagues took a more experimental approach. If orgasms were evolutionarily linked to ovulation, they reasoned, then a drug that put a damper on one process should also dial down the other.

That’s where the antidepressants come in. Fluoxetine, the active ingredient in Prozac, has the side effect of delaying or preventing orgasms in many human patients. To test its effects in a mammal that still relies on sex for ovulation, the researchers administered the drug to 12 female rabbits for two weeks, then mated all of them to a single male (a little stud named Frank). A day later, the team opened the rabbits up to count how many eggs had egressed from their ovaries. The drugged rabbits released about 30 percent fewer eggs than a group of untreated females that had also coupled with Frank.

Pavličev’s team next needed to confirm that fluoxetine was exerting its fertility-compromising effects via the brain—the theoretical intermediary in the sex-induced ovulation pathway—rather than acting on the ovaries directly. So they dosed a new group of rabbits with a hormone that triggers the ovaries to release eggs without involving the central nervous system on the last day of their fluoxetine regimen. This time, the antidepressants had little effect on ovulation.

That left just one fertility pathway as a probable culprit: the one with a clitoral jumpstart, brought on by sex with Frank. Whatever fluoxetine was doing to human orgasms, it was doing to rabbit ovulation—likely because the two phenomena use similar machinery, Pavličev says.

Given the myriad differences between species as evolutionarily distant as humans and rabbits, “there were so many reasons this might not have worked,” Pavličev says. “But we got a really clear answer.”

Of course, rabbits aren’t humans, and researchers don’t know if they (or most other non-human animals) orgasm in the same way people do. (There isn’t an easy way to check.) It’s also possible the rabbits’ downtick in ovulation could be attributed to other effects of fluoxetine, which has been shown to alter the physiology of several species, points out Marlene Zuk, an evolutionary biologist at the University of Minnesota who was not involved in the study.

Still, both Zuk and Elisabeth Lloyd, a philosopher of science at Indiana University who also wasn’t involved in the study, support the study’s use of rabbits, which helped the researchers answer questions about an apparently ancestral trait. And of the available induced ovulators, rabbits were a good choice, Lloyd says. “Camels would’ve been awkward.”

While this study is “well designed and executed,” Lloyd says, the female orgasm still has its mysteries. For instance, it’s unclear why the hormones associated with orgasm and ovulation don’t perfectly overlap. Another lingering question is why women still experience orgasms at all. There’s little reason to stick around in a species that’s supposedly retired your primary function.

Lloyd, who authored “The Case of the Female Orgasm,” proposes that modern female orgasms are a byproduct of embryo development, in which the same starting ingredients yield both male and female genitalia. Pressured to retain male orgasms, which go hand-in-hand with ejaculation, our lineage left women with a “fantastic bonus” with no modern function. Female orgasms are to women what nipples are to men, she says.

The two theories aren’t necessarily incompatible, Lloyd says. Perhaps ovulation is intertwined with orgasms’ origins, while genital development explains their persistence.

(Lloyd also points out that there’s no link between orgasms in women and reproductive rate. It makes you wonder, Zuk says, why few have questioned the importance of male orgasms, especially considering that so many other species seem to get by without.)

The debate on this subject will inevitably rage on, Pavličev says. But she hopes the research will alleviate some of the stigma surrounding female orgasms. “The lack of female orgasm during intercourse has historically been interpreted as being a problem of women,” she says. “But it’s not. It’s a consequence of female anatomy, which has removed the clitoris from the vagina. There’s nothing [wrong] with women.”

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