Imagine a crowded dance floor where bodies move to the beat beneath a glittering disco ball. Some of the dancers pair up, drawn together by the heady atmosphere.
For flies, as for humans, the right atmosphere can serve as a potent aphrodisiac. For the fruit fly Drosophila erecta, it is the scent of a fermented fruit that sets the stage, bringing together many flies for their courtship dance. It is this environment that D. erecta males need to trigger mating behavior: packed with other flies, visually stimulating, and rich with the tangy aroma of their preferred fruit. Without this scent, males show no interest in courtship.
These findings, described by Rockefeller scientists in a new paper in Nature, reveal how ecology and social context shape mating behaviors and demonstrate how the neural activity behind it is cued.
"While D. erecta shares the same conserved neural circuitry as other closely related Drosophila species, it's unique among them in that its brain requires an additional environmental input—the aroma of food—to activate neurons promoting sexual arousal," says first author Philipp Brand, a former postdoc in the Laboratory of Neurophysiology and Behavior and now a research group leader at the Max Planck Institute for Brain Research in Frankfurt. "Without that scent, the males are completely uninterested in mating, in contrast to its close cousins."
"Drosophila courtship behaviors are really beautifully tuned to the environment, which is especially interesting in light of fruit flies not being considered a social species," says lab head Vanessa Ruta . "This study nicely demonstrates an emerging principle: that courtship is shaped by ecological and social context and can't really be divorced from it. Intuitively, it makes a lot of sense. We humans are very sensitive to how we respond to others depending on social context, so it's not surprising that other animals have evolved to do the same."
A more natural setting
While the complex tango of Drosophila courtship has long provided a model for studying the sensory and circuit mechanisms controlling sexual arousal, most studies have focused on single pairs of males in females in stripped-down laboratory settings that do not reflect the sensory-rich natural world where these matings take place among hundreds of flies jostling for space on the same fruit.
"The observation of courtship and social behaviors in flies has occurred in this very abstracted environment," Ruta says. "Trying to replicate a more naturalistic setting provides a very different framing for thinking about the structure of these behaviors."
Brand, an evolutionary biologist who conducted extensive field work in Central America for his Ph.D. before joining Ruta's lab, aimed to explore social and host environments in a more native setting for the current study.
Because some initial findings suggested that olfactory circuits might be involved in social and courtship behaviors among fruit flies, the researchers constructed different environments reflecting a number of potential scenarios that a male fruit fly might find himself in: alone or with many others, with the scent of apple cider vinegar (ACV) as a proxy for the bouquet of fermenting fruit, or without any odor.
"We used large assay chambers with a central food patch of ACV on agar to mimic the presence of a fermenting fruit in the wild," Brand describes. "We found striking differences in the dependency on food odors for mating behaviors across species—and D. erecta emerged as quite unique."
Male D. melanogaster flies and several other closely related species, for example, explored the testing arena and mated with female flies whether or not ACV was present. In stark contrast, when there was no ACV, male D. erecta essentially sat around, uninterested in either their surroundings or other fruit flies. The aroma of ACV, however, utterly transformed their behavior: They clustered onto the ACV patch and continuously approached any nearby fly—female or male—until they found a successful match. Over 90% of D. erecta matings occurred on the ACV patch, while D. melanogaster's couplings were more spatially distributed.
"As soon as food is there, it becomes a very vigorous interaction between all of the flies," Brand says. "It looks kind of chaotic because everyone is interacting with everyone."
Indeed, D. erecta males displayed more vigorous courtship when part of a larger group than when alone with a single female, suggesting that the motion of nearby flies increased their arousal. The constant movement dramatically altered each male's visual experience, expanding the fraction of his visual field occupied by other flies and increasing the visual motion sweeping across his retina.
Tracking environmental and social influences
To find out what was going on in D. erecta's brain in the presence of ACV, the researchers placed males in a virtual reality setup built in Ruta's lab in which a fly walks on an air-supported ball, able to turn and move forward or backward in place, while its brain activity is monitored.
Earlier research from the lab had uncovered a conserved neural circuit in the higher brain region among several close relatives of the model organism Drosophila melanogaster involving P1 interneurons and LC10a visual neurons, which orchestrate visually guided courtship pursuit. So, too, was that circuit active during D. erecta's mating behavior—but with a novel twist.
"In D. erecta, these P1 and LC10a visual neurons were only activated when food odor was present," Brand explains. That elevated visual acuity draws the males to other flies—a "social foraging" that leads to the crowded dance floor, so to speak, of the ACV patch. This increases visual motion, which in turn increases the male's arousal, creating a feedback loop.
"This feedback loop gives rise to collective behaviors of the kind you see in schools of fish, flocks of birds, or swarms of locusts," Ruta notes. "These typically nonsocial creatures are actually strongly shaped by their social environment and ecological setting."
That aligned with what the researchers know about the species in the wild. There, D. erecta's mating season is short: Its host fruit, the West African screwpine, is only in season for two to four months of the year, which creates a highly competitive reproductive setting operating under a serious time crunch. In contrast, D. melanogaster has a wide variety of food sources throughout the year.
Interestingly, D. erecta's unique behavior doesn't seem to have emerged from the evolution of an entirely new arousal pathway in the brain. "Instead, it's the change in how sensory inputs are weighed that's giving rise to new functions. For example, in D. erecta, the olfactory information serves as this very strict gate that changes the weighting of how visual cues are used to mediate courtship behavior," says Brand. "It's a sort of tinkering with a conserved neural substrate rather than a rewiring of it."
The work reframes courtship as not a ritual between one male and female but a practice deeply embedded in its natural social context, Ruta says. "We have to think beyond courtship being a beautiful duet between a male and a female. There are many other selective pressures that shape the way these behaviors unfold."
