There's no question humans find sex enjoyable. Some studies in rodents suggest that this might be similar in other mammals, but what about the rest of the animal kingdom?
New research shows that for male fruit flies, the process of ejaculation might actually be quite pleasurable.
The work implies that sexual pleasure can occur in "simple animals, not as it's been assumed only in mammals," says study leader Galit Shohat-Ophir, a neuroscientist at Bar-Ilan University in Israel. (Read why female dragonflies go to extreme lengths to avoid mating.)
Shohat-Ophir's previous research found that male fruit flies seem to find something about mating rewarding.
But it wasn't clear which part they seemed to be getting a kick out of: the lengthy courtship ritual, the pheromones communicated by the female, or copulation itself.
Red Light District
To find out, Shohat-Ophir and colleagues from her university and Janelia Research Campus in Virginia genetically engineered male flies so that a distinct group of nerve cells in their abdomen would be activated by a red light. These neurons produce a protein, called corazonin, that turns on the ejaculation process.
The team then placed the altered flies in an arena, and flicked on the red light at one end. Most flies headed straight for the red-light zone, where their abdominal neurons were activated, causing them to ejaculate.
"The preference to that zone was quite immediate," Shohat-Ophir says. "Some of them just stayed there, as well."
In another experiment, the researchers examined the modified insects' brains. After a few days of having their corazonin neurons repeatedly stimulated, the flies appeared to have high levels of a protein called neuropeptide F, which spikes during rewarding situations, like eating sugar. (See "Wild Romance: Weird Animal Courtship and Mating Rituals.")
Yet another indicator for fruit flies enjoying sex is their reaction to alcohol, according to Shohat-Ophir’s previous research: Sexually deprived male flies prefer an alcohol drink over a non-alcoholic one, seemingly as an alternative reward. But if they mate or have their corazonin neurons activated, they shun alcohol.
Overall, Shohat-Ophir says it makes sense that ejaculation would be pleasurable: Any feelings that motivate an animal to keep on mating should be favored by evolution.
Sex and Drugs
David Anderson, a neurobiologist at the California Institute of Technology, agrees with this explanation. "Mating is very important for any species to survive, and so any mechanisms that increase the robustness of mating, as a behavior, would be positively selected in evolution."
However, he's unconvinced that the flies find ejaculation rewarding per se. For instance, there are other corazonin neurons located in the brain that may not have anything to do with sex.
These brain neurons may have been also activated by the red light and could have had their own effect on the brain's reward system, says Anderson, who was not involved in the new research.
He says that experiments the team undertook to ensure that the effect is caused by the abdominal neurons weren’t sufficient.
But both experts agree such studies have value beyond understanding the sex lives of bugs, such as deciphering the basic neurophysiology of drug addiction. (Read how science is unlocking the secrets of addiction.)
Cocaine and heroin become addictive because they tap into our brains' reward centers, which evolved to make animals engage in behaviors necessary for survival—like mating, Anderson says.
If we can't figure out such brain mechanisms "in a simple organism like a fruit fly," he says, "how are we ever going to understand them in something as complicated as a human?”