In any case, there must be a reason. “This neuro-endocrine reflex is too complex to be an evolutionary accident,” researchers write in a new study published in September in Proceedings of the National Academy of Sciences USA.
The authors recently proposed an alternative hypothesis: the phenomenon evolved from a mechanism for stimulating ovulation. Humans and great apes are known as spontaneous ovulators—they are fertile at certain times during their menstrual cycle, regardless of whether or not they have sex. But for some animals, such as rabbits, cats and camels, ovulation is triggered by sex itself—a process known as “copulation-induced ovulation.” Could female orgasm have developed from a similar mechanism?
To test the idea, Mihaela Pavlicev, a professor of pediatrics at the University of Cincinnati College of Medicine, and her colleagues conducted a series of experiments in female rabbits. For two weeks, they gave the animals daily doses of the antidepressant fluoxetine, a selective serotonin reuptake inhibitor (SSRI). SSRIs have been found to cause sexual dysfunction in both men and women, so Pavlicev and her team wanted to see if the drug would impact ovulation in the rabbits. A set of control rabbits did not receive fluoxetine. The researchers then had the female rabbits mate with a single male—named Frank—and measured the number of times the females ovulated. They found that the rabbits who received the drug had about 30 percent fewer ovulations after mating than the control animals.
But Pavlicev and her colleagues wanted to confirm fluoxetine was reducing ovulation by way of the central nervous system rather than somehow affecting the ovaries directly. So in a second experiment, they gave some rabbits the drug—but instead of having those rabbits mate with Frank, they injected the animals with human chorionic gonadotropin, a hormone that stimulates ovulation. This time the fluoxetine-treated rabbits experienced roughly the same number of ovulations as the untreated ones, suggesting the drug indeed acts on the central nervous system and not the ovaries. At high doses, fluoxetine is known to cause weight loss—which could, in theory, affect ovulation. But the researchers did not find any relationship between the rabbits’ body weight and the number of ovulations caused by copulation.
Fluoxetine works by preventing the reabsorption of serotonin in the synapses of brain cells, as well as other tissues. The team conducted a final set of experiments in which it treated rabbit ovaries with fluoxetine in a lab dish and showed that serotonin does not collect in the ovaries—further supporting the notion that it acts at the brain level.
Julie Bakker, a neuroendocrinologist at the University of Liège in Belgium who studies ovulation in ferrets, was skeptical of the findings, however. A “30 percent reduction in the number of ovulations is very marginal. It would have been much more convincing if there was no ovulation at all,” says Bakker, who was not involved in the work. “It would have been nice if the authors had actually measured serotonin in the brain of their rabbits to determine whether their treatment protocol with fluoxetine” raised levels of the neurotransmitter.
Studying orgasm in animal models in tricky. “There’s no such thing as orgasm in rabbits,” Bakker says—it is more like a light switch, in which male stimulation triggers the brain, which triggers ovulation. In addition to ferrets, she has studied ovulation in mice and rats, and the only kind of orgasmic behavior she has seen is in female rats: It is, she says, “kind of a thrashing behavior—stretching their legs in certain way that might have been uterine contractions.” The rabbit experiment is an interesting idea, she adds, but she would like to see more convincing evidence. “It’s a door opener,” she says.
Raúl Paredes, director of the Juriquilla unit at the National Autonomous University of Mexico’s National School of Higher Education, agrees the study is interesting, but he says it is “very reductionist [to assume] that female orgasm consists of a copulation-induced reflex.” The bigger issue is how one defines orgasm, he adds. “This is a human construct because, aside from the physiological changes that can occur during sex, the definition involves feelings of pleasure,” he says. “This certainly can’t be measured in animals.”
ABOUT THE AUTHOR(S)
Tanya Lewis is an associate editor at Scientific American who covers health and medicine.