Flies Love to Ejaculate, Experiment Finds

By Ryan F. Mandelbaum on at

If there’s one thing I know about human males, it’s that they love ejaculating. It can sometimes seem like their entire existence is motivated by a need to shoot sperm and seminal fluid from their genitalia. But have you ever wondered from where they acquired such a desire?

A team of scientists, led by graduate student Shir Zer-Krispil at Bar-Ilan University in Israel, found further evidence that the frequently studied Drospophila fly finds ejaculating rewarding, too. It’s pretty nuts to think about how this trait stuck around in both flies and humans.

Now, the scientists hope to apply the research to understanding how the brain rewards certain behaviours more generally, even things like drug use that may have negative consequences.

“As neuroscientists, we want to understand how the brain works,” Galit Shohat-Ophir from Bar-Ilan University told Gizmodo. “Anything that helps us understand these principles is interesting.”

How do you study male fly ejaculation? You need to find a way to separate it from the entire multi-step process of fly sex—you’ve got to make them finish without a mate. The researchers already knew that activating certain nervous system cells in fly abdomens, called corazonin-expressing neurons, or CRZ neurons, led to the flies releasing semen. So they just needed to stimulate these neurons and figure out whether the flies actually liked it.

The researchers genetically modified 12 male flies so that the CRZ neuron would trigger under red light, which the flies could not otherwise see. They gave both modified and unmodified flies a choice of rooms where they could fly, one with red light and one without. They found that these flies generally preferred hanging out under the red light (where they subsequently ejaculated), while unaltered flies did not. They also trained flies to associate certain smells with the red lights—and, lo and behold, the altered flies preferred to be near those smells.

Aside from other tests to understand the biological machinery behind the results, the researchers found that genetically modified females did not show a preference for either room, according to the paper published today in Current Biology. They also found that repeatedly activating the CRZ neurons on the abdomen increased the level of a molecule called neuropeptide F in the brain. These flies preferred not to eat fruit spiked with ethanol, something they’d normally find rewarding. Maybe since they’d already been rewarded, they didn’t want the ethanol—though the authors offered other possible explanations.

Shohat-Ophir told Gizmodo it’s hard to tell what specifically the fly found pleasurable, whether it was ejaculation or the activation of the abdominal neurons. But it’s clear that the flies prefer ejaculating to not ejaculating, given the choice—which could help explain why humans like it, too. “It really shows us that the reward machinery is an ancient machinery that probably evolved very early to help animals survive and reproduce,” she said. “It’s very conserved from flies to eventually mammals and people,” so perhaps it’s something that evolved first in some ancient ancestor that flies and humans shared.

She was also excited about the effect of activating the neurons on desire to consume ethanol. Perhaps next they could look at different responses between individual flies and how ejaculation changes the levels of molecules in the brain. She hopes this could help them learn about reward behaviours more generally, specifically those related to drugs or addiction.

Anyway, we’re all animals here. At our most primal, we’re not so different. [Current Biology]