Origin of female orgasm revealed by rutting rabbits

By Dr. Ananya Mandal, MDSep 30 2019

The female orgasm has been a complex and difficult-to-study and understand phenomenon for centuries. Now researchers have come up with an easier way of studying female orgasms using rabbits. They speculate that the evolution of the female orgasm could be related to the mammalian reflex linking ovulation to intercourse.

The study titled, “An experimental test of the ovulatory homolog model of female orgasm,” was published in the latest issue of the journal Proceedings of the National Academy of Sciences.

Image Credit: Victoria Paladiy / Shutterstock

The same authors had published their work on female orgasms in 2016 in the journal Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. Their work was titled, “The Evolutionary Origin of Female Orgasm,” where they had studied the connection between the hormone and neuronal reflex circuits that connected intercourse with ovulation. The authors of the study had written, “The orgasm in women does not obviously contribute to the reproductive success, and surprisingly unreliably accompanies heterosexual intercourse.” In this new study they write, “The existence of female orgasm is intriguing for 2 reasons: On the one hand, female orgasm is not necessary for female reproductive success, and on the other hand, this neuro-endocrine reflex is too complex to be an evolutionary accident.”

This means that while early during evolution, intercourse and ovulation were connected, orgasms could still be the connecting force between intercourse and female reproduction. They added that in this study they, “...provide experimental evidence, strengthening the likelihood that female orgasm evolved from copulation-induced ovulation. This finding helps interpreting otherwise difficult to explain aspects of female sexuality, such as the low rate of female orgasm during intercourse.”

The team of researchers write that there are several theories regarding the actual use and role of female orgasms in reproduction of female mammals. Some researchers have said that the uterus or womb contracts during an orgasm and this facilitate the movement of the sperm within in so that fertilization can occur. However females do conceive without having an orgasm or may have an orgasm without an intercourse. This debunks the womb contraction theory. Some researchers have said that orgasms in females help them enjoy the intercourse which may facilitate more such leading to conception.

According to lead author Professor Mihaela Pavličev from the Department of Paediatrics, University of Cincinnati College of Medicine, the actual evolution of the female orgasm has still not been understood. They believe the ovulation or release of the egg from the ovaries could be tied to intercourse and orgasms. She said, “We know there is a reflex [in rabbits], but the question [is] could this be the same one that has lost the function in humans?”

According to Pavličev among certain mammals the ovulation is triggered by certain hormones and these are released during intercourse. Among human females, monkeys, mice, sheep and some other mammals though, ovulation occurs once a month - mid cycle. The human females also have a hormonal surge during intercourse like other mammals, the authors of the study wrote.

The team looked at 41 species of mammals including 15 species of cats, camels and koala bears that have reflex ovulation as a result of intercourse in their 2016 study. The researchers explained that the earlier mammals could have similar reflex ovulation that ensured fertilization of the eggs after intercourse.

In this new study the team explained that antidepressants reduce the ability to have an orgasm. One of the antidepressants commonly used in fluoxetine sold as Prozac. Twelve female rabbits were given fluoxetine for two weeks as part of the experiment, the team wrote. By the third week, the rate of ovulation of the rabbits decreased, the team found. They were allowed to have sex with a male rabbit called Frank and results showed that their rate of ovulation had fallen. The rate of decline in ovulation of 30 percent more among the Prozac treated rabbits compared to the nine rabbits that were not treated with Prozac but allowed to mate with Frank.

Pavličev explained that the same reflex arc that controlled brain hormones and nerves that stimulate the ovulations could be involved in orgasms. With evolution the reflex circuit of the brain that triggered ovulation among females could have changed to a circuitry that led to the female orgasm triggered by intercourse she said. Pavličev added that what is not known is if the rabbits that ovulate after intercourse also experience orgasms.

Study co-author Günter P. Wagner, Ph.D., a professor of evolutionary biology at Yale explained the need for female orgasms despite its seemingly no role in ovulation and reproduction. He said, “Thus the question shifts from ‘how did female orgasms evolve’ to ‘why is [the] female orgasm maintained, given that it lost its function in reproduction [ovulation]?’” He added, “It also shows, with the work of other researchers, that there is no correlation between female orgasms and reproductive success; that the female orgasm has to have some other, non-reproductive role.”

The authors of the study wrote in conclusion, “We conclude that the effect of fluoxetine on copulation-induced ovulation rate supports the ovulatory homolog model of female orgasm, suggesting that female orgasm has very deep evolutionary roots among the early eutherian mammals.”

Wagner said, “In particular the implication is that female orgasm has likely no function in reproduction, but probably still exists because of the benefit of the women herself, and other primate females, that have a similar biology. The question now is: “what is that non-reproductive functional role of female orgasm?” We do not have answers for that yet” Pavličev emphasized the role of studying the evolution of female orgasms saying, “Maybe it can direct a little bit more friendly focus on female sexuality rather than just being in [the] service of childbearing.”