Fruit Flies Have Mind-blowing Orgasms, Israeli Scientists Prove

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Picture shows a petri dish with three flies inside, two of whom are having sex
There is a reason fruit flies do what the two on the left are doingCredit: Reza Azanchi and Galit Shohat-Ophir
Ruth Schuster
Ruth Schuster
Ruth Schuster
Ruth Schuster

In the latest news of fruit-fly fecundity, Israeli scientists have proven that the male of the species enjoys ejaculation. Moreover, if the male can get an orgasm, he scorns the demon alcohol, while the sexually frustrated bugs evince a desire to get bombed.

The ground-breaking study on Drosophila melanogaster's pleasures, titled "Ejaculation Induced by the Activation of Crz Neurons Is Rewarding to Drosophila Males," was published in the prestigious journal Current Biology by lead author Shir Zer-Krispil and researchers from Bar-Ilan University and the Howard Hughes Medical Institute.

One might wonder if that had been a burning question, given that if ejaculation wasn't rewarding, males would hardly go through the trouble of wooing females and vanquishing the competition. Researcher Galit Shohat-Ophir agrees that the outcome was indeed foreseeable, but explains that it could have been that what male flies really enjoy is the courtship, which they constantly engage in; or that what yanks their chain is the scent of a female fly.

“We wanted to know which part of the mating process entails the rewarding value for flies,” Shohat-Ophir says. “The actions that males perform during courtship? A female’s pheromones? The last step of mating which is sperm and seminal fluid release?”  

In rodents, science has shown that female pheromones induce pleasant "appetitive" memories, she says, that is, the memory of a pleasurable experience. Theoretically, it could have been female fly odors that maddened the male, not fond, "appetitive" memories of ejaculation. But it wasn't. It was the orgasm itself.

Red-light district of Bar-Ilan

Really, why did they do this study? The researchers were actually trying to better understand the mechanisms of pleasure and reward in the super complex human brain by studying them in a simpler organism, which however still displays similar physiological responses to basic stimuli.

Sexually satisfied fruit flies eschew the demon alcohol, but ones bereft of sexual satisfaction will opt for the alcohol-spiked meal.

"We are interested in understanding how the human brain works, but it is too complicated: the number of neurons is larger than the number of people on earth," Shohat-Ophir explains to Haaretz. So, they simplified matters by working on flies, which have around 150,000 neurons in their little brains.

Also, Drosophila are the go-to subject for a vast array of genetic and other scientific studies because they proliferate madly, their life span is conveniently short, their genome is known, and there are no fly rights groups.

So, as the fruit fly is a thoroughly known quality, it is easier for scientists to silence genes in specific neurons, inhibit or artificially activate them without need for surgery, she explains.

The study was done using flies genetically engineered so specific neurons could be activated by shining light on them (so-called "optogenetic tools").

CRZ neurons – which make a protein called corazonin – had previously been shown to trigger ejaculation in the fruit fly. The study employed flies in which CRZ neurons would react to red light.

Guess what this genetically engineered, optogenetically stimulated fly is doingCredit: Avi Jacob, Bar-Ilan University Microscopy Unit

In other words, red light rather than sex caused the fruit fly to orgasm, which is convenient when trying to elucidate what exactly the male drosophila is enjoying.

When male flies with engineered neurons were put into an apparatus that emitted red light on only one side, they clustered on the side with the light.  That strong preference firmly indicates that they earnestly enjoy ejaculating.

Next, the team trained the flies – it can be done - to associate the red light and ejaculation with a particular odor. They then tested whether the flies preferred the scent that reminded them of that past experience of ejaculation. They did.

Fainting and the F neuropeptide

Like people, when a male fly ejaculates, the levels of a small peptide in the brain called Neuropeptide F increase. After some days of having their CRZ neurons triggered, the male flies' brains were swimming in F.

The sexually sated males turned down liquid food spiked with ethanol, choosing meals without alcohol. Sexually unsatisfied males - engineered ones not exposed to light, and control flies that hadn't been engineered – went for the alcoholic meal.

Wait, how do we know a fly is drunk? They behave just like we do, explains Shohat-Ophir: "At the beginning they become hyperactive, similar to the disinhibition stage in humans. Then they start to lose coordination, and eventually, they faint."

The upshot is that male brains seem to have an enormous amount in common, from fly to human. “The principles by which the brain processes reward are extremely conserved in all animals; this is a really basic everyday machinery that helps animals survive,” Shohat-Ophir says.

Ejaculation induced by the ontogenetic stimulation of Crz neurons mimics the rewarding value of full copulation in fruit fliesCredit: Shir Zer-Krispil, Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University

Ultimately the team hopes that their study on ecstasy and appetite for drugs will help treat addiction. “Drugs of abuse use the same systems in the brain that are used to process natural rewards," she says. “Our studies suggest that the state of the animal, i.e., undergoing successful mating or being rejected, affects the motivation to consume drug rewards."