A possible explanation for one of biology’s greatest mysteries, the female orgasm, has been bolstered by research showing that rabbits given antidepressants release fewer eggs during sex.
The human female orgasm has long proved curious, having no obvious purpose besides being pleasurable.
The scientists behind the study have previously proposed it might have its evolutionary roots in a reflex linked to the release of eggs during sex – a mechanism that exists today in several animal species, including rabbits.
Since humans have spontaneous ovulation, the theory goes that female orgasm may be an evolutionary hangover. They say the new experiment supports the idea.
“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?” said Dr Mihaela Pavličev a researcher at the University of Cincinnati who co-authored the study.
To explore the question the team gave 12 female rabbits a two-week course of fluoxetine (trade name Prozac) – an antidepressant known to reduce the capacity for women to orgasm – and looked at the number of eggs released after the animals had sex with a male rabbit called Frank.
The results, published in the Proceedings of the National Academy of Sciences, showed that rabbits given the antidepressants released 30% fewer eggs than nine rabbits that were not given Prozac but still mated with Frank.
Further experiments backed the theory that the drug was affecting ovulation through a mechanism involved in copulation and entailing the nervous system, rather than acting directly on the ovaries or other routes.
The team said the results fit their theory that rabbits needed to experience something akin to an orgasm to have a hormonal surge and ovulate, although it is not known if it gives the animals sexual pleasure.
They also said their theory was supported by a previous finding that animals that rely on sex-induced hormonal surges for ovulation tended to have a clitoris – the organ behind the female orgasm – in a position that meant it was more likely to be stimulated during sex.
Pavličev said it remained unclear exactly how Prozac affected the reflex in rabbits, and thatfurther experiments with other animals was necessary.
She added that even if the theory was right, an important question was why is the organism still there – could it be that it had acquired a different function, that there was a function on female health?
Elisabeth Lloyd, a professor of biology at Indiana University and author of The Case of the Female Orgasm, praised the study and said it offered support for the team’s theory.
She said questions remained, including whether the same muscles and nerve fibres involved in the reflex in rabbits were present in the female orgasm.
She also noted that ovulation and orgasm were linked to different hormones in humans, though it was unclear why – if female orgasm had its roots in ovulation – orgasms in men and women showed so many overlapping features.
Such features, she said, had generally been thought to support the theory that orgasm evolved to reward males for fertilisation, with the female orgasm simply a bonus arising from the way embryos of different sexes develop from a common form.
Lloyd and Pavličev said it might be that the theories could be reconciled, with one potentially explaining the origin of female orgasm and the other indicating why it still existed.
Pavličev added that understanding the role of female orgasm remained important.“Maybe it can direct a little bit more friendly focus on female sexuality rather than just being in [the] service of childbearing,” she said.
