Human female orgasm mystery cracked

An oversexed rabbit called Frank, aided by a veritable harem of bunnies on antidepressants, may well have cracked the mystery of the human female orgasm.

And to get why that’s major it helps to know two reasons the orgasm has been such an enduring enigma.

The first is that women don’t need to have an orgasm to conceive, which puts a big question mark over why it evolved in humans at all.

Second, orgasm rates vary wildly among women, ranging from the many to the few and right down to zero – US comedian Remy Kassimir even started her podcast How Cum to work out why she hadn’t had one.

Evolution says that’s weird. If a trait has hung around because it helps survival, it should be much more stable across the species.

Enter a team of researchers, led by evolutionary biologist Gunter Wagner from Yale University in the US, and a bunch of rabbits ready to get down and fluffy for the cause.

Wagner’s team had a theory. Rabbits, along with camels, ferrets and cats, all trigger ovulation – release of eggs by the ovary – when they have sex. But human females don’t. Ovulation is instead dictated by the hormonal tides of the menstrual cycle.

Maybe, the researchers conjectured, the orgasm is a remnant of an earlier time when sex did bring forth the eggs.

To find out they turned to the ubiquitous antidepressant fluoxetine, which many will know as Prozac. It’s an SSRI drug that increases serotonin levels. It also causes anorgasmia, the technical name for not being able to orgasm despite putting in the hard yards.

Wagner and colleagues dosed female rabbits with fluoxetine for two weeks then corralled them with Frank who, true to species, obliged repeatedly. Proof of sex was verified by the presence of sperm on a vaginal swab.

The next part was less convivial. The female rabbits were killed and their ovaries removed to count the eggs.

Prozac, it turns out, is something of a downer when it comes to ovulation.

Female rabbits given fluoxetine released 30% fewer eggs than control rabbits that also had sex with Frank but didn’t get the drug.

What does that actually mean?

Well, not a lot about rabbit orgasms because the team didn’t, and probably can’t measure them. What it does say is that a drug that stops women orgasming also puts the brakes on rabbits ovulating after sex.

Which suggests, the authors argue, the two are “homologues”; they share a physiological pathway and, in all likelihood, evolutionary origins.

“[F]emale orgasm consists of a copulation-induced reflex that originally had a role in triggering ovulation,” they propose.

Now, ovaries have serotonin receptors, so a big question is whether Prozac was working directly on them or somewhere else.

Wagner and co called on Frank to step up again. This time, the ever-ready bunny went to work with rabbits that got an injection of something that mimics the effect of luteinising hormone, a trigger for ovulation made in the human brain.

With that agent on board, fluoxetine caused an insignificant drop in egg release. This suggests, the authors say, fluoxetine is working via the brain. By extension, sex in female rabbits (and quite possibly orgasms once upon a time in women) triggers ovulation through a brain-based effect.

Which may not come as a surprise to people in the know.

“We conclude that the effect of fluoxetine on copulation-induced ovulation rate supports the ovulatory homologue model of female orgasm, suggesting that female orgasm has very deep evolutionary roots among the early [placental] mammals,” the authors conclude.

The study appears in the journal PNAS.