Fruit Fly Study Shows That Ejaculation Feels Good for Evolutionary Reasons

There's an evolutionary reason sex feels so good.

The Drosophila, better known as fruit flies, have between 40 and 50 days on this planet before they pass on into the afterlife. That means they only have about six weeks to do the dirty as often as they can in order to ensure the survival of their species. It’s a lot of pressure and a lot of work, but luckily for these flies, sex is a pretty pleasurable experience, researchers assert in a study published in Current Biology on Thursday. In order to drive copulation, animals have evolved to enjoy mating, and according to this study, the fruit fly is no exception.

Specifically, fruit flies enjoy the process of ejaculation, the researchers from Israel’s Bar-Ilan University write in the paper. Before this study, little was known about what flies find sexually pleasurable because it’s not exactly easy to measure how good a fly feels after blowing its load. In the paper, however, the researchers explain that one molecule in the brain can be used to indirectly measure pleasure — and that pleasure, at least in flies, isn’t derived from the process of courtship or female pheromones, like other studies have suggested. No, the most pleasurable part of sex for flies is — straight up — sperm release.

“Successful mating is naturally rewarding to male flies and increases the levels of a small peptide in the brain called Neuropeptide F,” co-author Galit Shohat-Ophir, Ph.D., explained in a statement released Thursday.

Scientists used red light to trigger the fly's ejaculation. 

Avi Jacob, BIU Microscopy Unit

In the study, the scientists genetically engineered fruit flies so that when they switched on a red light, neurons would produce the neuropeptide corazonin (CRZ), which in turn would cause the flies to ejaculate. Given the option to stay in one part of an enclosure that had a red light versus one that did not, the better choice was obvious: The flies, thinking, “I definitely would rather be ejaculating right now,” spent all their time in the red light zone.

To double check that it was, in fact, ejaculation that the flies found so pleasurable, the scientists trained the flies to associate the red light ejaculation process with a specific odor. They then let the flies choose whether they wanted to fly around the ejaculation-associated smell or another, and, lo and behold, the flies chose the smell that reminded them of that time they got to nut.

Discovering that flies love to ejaculate is one thing, but understanding what they do when they can’t was arguably the more revealing finding. It turns out flies who get lucky don’t care much for alcohol, whereas their peers who don’t get any drown their sorrows in a drink.

Looking at levels of neuropeptide F in the brains of the flies that had spent all their time in the red light zone ejaculating, the researchers found that the flies had the same levels of neuropeptide F that they would have had if they’d actually had sex. When offered either food or booze, the flies that were satiated sexually took the food. The control flies that weren’t sexed up, however, chose to get drunk. This suggests to the scientists that flies can now be used to study how mating affects the motivation to consume drug rewards.

“Male flies that are sexually deprived increased motivation to consume alcohol as an alternative reward,” explained Shohat-Ophir.

Given the human experience of sex — and sexual frustration — none of this is surprising, though it is comforting to know that other members of the animal kingdom share our pleasure and pain. A species can’t survive if its members don’t have sex and reproduce, so it makes sense that even animals would be bummed out when their plans for a sexy night fall through.

“The principles by which the brain processes reward are extremely conserved in all animals; this is a really basic every-day machinery that helps animals survive,” explains Shohat-Ophir. “Drugs of abuse use the same systems in the brain that are used to process natural rewards. This allows us to use simple model organisms to study aspects of drug addiction.”

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