Why did the female orgasm evolve? Experiment supports theory

In humans, the female orgasm isn't required for reproduction, so scientists have long been puzzled about why it exists. Now, a new experiment on rabbits involving orgasm-suppressing antidepressants confirms an intriguing theory — that the reflex originally evolved to trigger ovulation.

Humans, other primates and rodents ovulate or produce mature eggs spontaneously (humans on a monthly basis). But in mammals such as rabbits and camels, ovulation typically needs to be triggered by mating. 

Some years ago, Mihaela Pavlicev, then a researcher at the Boston Children's Hospital, was cataloguing information about the ovarian cycle in different mammals when she stumbled on a pattern — in animals where ovulation is induced by mating, the hormones involved are the same ones released during the human orgasm.

Further research showed that animals where ovulation is induced by mating had a different anatomy — their clitoris was inside the "copulatory canal" as opposed to outside, in the case of humans. 

And the genetic relationships between the animals were consistent with the idea that both kinds of mammals shared a common evolutionary ancestor whose ovulation was triggered by mating. 

Pavlicev hypothesized that what humans experience as female orgasm originally evolved to trigger ovulation, and it's a "leftover" trait that no longer has the same purpose. 

"It's a good suggestion, a good hypothesis with a lot of support, but we really wanted to test it somehow," said Pavlicev, now a professor at the University of Vienna. 

She reasoned that if orgasm is really similar to induced ovulation in rabbits, then a drug that suppresses orgasm in humans should suppress ovulation in rabbits.

Suppressing orgasms (in both men and women) is a common side effect of antidepressants known as SSRIs. Pavlicev and her team chose to use a drug from that group called fluoxetine, which is sold under the trade names Prozac and Sarafem.

The researchers treated female rabbits with the drug for two weeks, then mated them with a male rabbit. They found that rabbits given fluoxetine had 30 per cent fewer ovulations — that is, they released fewer eggs — than rabbits that were not given the drug.

"We were actually really, really happy that we actually got such a clear result," said Pavlicev, lead author of the study describing the results, published Monday in the journal Proceedings of the National Academy of Sciences.

The orgasm-like reflex triggered by mating acts via the nervous system to release an ovulation-inducing hormone from the rabbit's brain.

To double-check that the Prozac wasn't suppressing ovulation directly, the researchers ran a second experiment where they gave that hormone to the rabbits treated with fluoextine, which restored the normal rate of ovulations.

What do rabbits feel?

While this shows "copulation-induced ovulation" is analogous to the human female orgasm and points to its origins, Pavlicev said, researchers don't know how it feels for the rabbit: "We don't know whether they have the same experience as we do."

Nor does it tell researchers why the trait still persists in humans, even though it doesn't appear to be useful in reproduction.

"Does it really not have any function today?" she wonders. "What is it … that keeps it there?"

Previous theories about the reason female orgasms exist include the idea that it's for emotional bonding, or that it's a consequence of the fact that male and female bodies develop from the same body plan, and the male orgasm is typically required for reproduction.

Pavlicev says her research shows how important it is to look at evolution and animals to answer questions about ourselves: "It's very often not possible to explain human traits just by looking at humans."

She added that the difference in the location of the clitoris between mammals with induced ovulations and those that ovulate spontaneously also explains why some women have trouble reaching orgasm through sexual intercourse — something that some psychologists had previously blamed on mental health issues.

"It's evolution," she said. "It's not mental problems."

The study was supported by the science development fund at Yale University, where the corresponding author of the study, Gunter Wagner, is an evolutionary biology professor, and partially funded by members of the public through the crowdfunding site experiment.com.

Gonzalo Renato Quintana Zunino recently completed his PhD at Concordia University studying how the brain learns from sexual experiences, and is familiar with studies about orgasms based on animal research. Zunino, who was not involved in the study, said in an email the results "do offer a hint as to why women experience orgasm." However, he noted that it doesn't say whether the rewarding aspect or pleasure of sex plays a role in what happens to the rabbits, although experiments with different drugs might be able to answer that question.

"Animal studies are incredibly valuable to understand the origins and mechanisms of different phenomena, such as orgasms," he wrote in an email. But he noted they have limitations. "There are also few animals, including humans, that engage in sex and sexual behaviours that are not conducive towards reproduction."

He added that Pavlicev's hypothesis isn't the only possible explanation as to why women experience orgasm, and it doesn't mean the theory that female orgasm is a byproduct of male orgasm isn't true.