There’s little doubt that male humans find ejaculating to be an inherently rewarding experience—of that reality, there is abundant R-rated evidence. But are other male animals, from mammals to insects, also motivated to copulate with females based on the promise of an orgasm alone?
To answer that question, a team of scientists from Bar-Ilan University in Israel and HHMI Janelia Research USA got down with some sexy fruit fly science. Their research, led by Shir Zer-Krispil and published Thursday in Current Biology, honed in on a particular neuropeptide called corazonin (CRZ) that plays a role in activating the release of sperm and seminal fluid in male fruit flies.
Using optogenetics, a technique in which test animals are genetically engineered to have neurological responses to light-based stimuli, the researchers were able to induce ejaculation in the flies using, appropriately, a red light. The idea was to isolate the pleasure response to the tiny fly climaxes from the rest of the sexual encounter, according to study co-author Galit Shohat-Ophir.
"We wanted to know which part of the mating process entails the rewarding value for flies," she said in a statement. "The actions that males perform during courtship? A female's pheromones? The last step of mating which is sperm and seminal fluid release?"
It may be that all of those factors play into the reward response of male fruit flies looking for action, but the study suggests that semen release, by itself, is a significant part of the attraction. When the CRZ-rigged flies were put in an environment that contained an “activation zone” with the red light, they preferred standing near it, seemingly enjoying the fact that it triggered ejaculation. In contrast, female flies, which lack the same CRZ sexual expression patterns as males, showed no preference to hanging around the red light.
To explore those results further, the team trained the male flies to associate an odor with the red light, and found that the males also liked that scent compared to other odors.
But wait: There’s more. The study also delves into how the rewards associated with ejaculation affect the flies’ likelihood of pursuing other pleasures, like ethanol (alcohol). After a few rounds of getting off on the red light, the engineered males had a similar neurological buzz—specifically, increased levels of the courtship-related Neuropeptide F—to males that had genuinely copulated with females.
Both groups of flies that had recently released sperm preferred non-alcoholic liquids over alcohol-spiked liquids when offered. In contrast, control variable males who had not been stimulated by the red light or given the opportunity to mate opted for the alcoholic choice. This result suggests that the pleasure response associated with ejaculation is strong enough that flies deprived of it will turn to booze to compensate, while post-orgasm flies will lay off the drinks.
“Successful mating is naturally rewarding to male flies," said Shohat-Ophir. "Male flies that are sexually deprived have increased motivation to consume alcohol as an alternative reward."
In this way, the study may have implications for substance addiction research, in addition to its insights about the underlying mechanics of sexual motivations in male flies. While sex and drugs are often mixed in our own human culture, it seems that fruit flies tend to opt for the latter when they fail to secure the former.
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