Reza Azanchi

Male fruit flies mate more frequently in the presence of unrelated males.

Fruit Flies Feel Peer Pressure

Liz is a staff writer for Science.

There are things you do and say when out at a bar with friends that you never would at a family dinner—and it turns out fruit flies, though not known to be social animals, may not be much different. New research shows that the insects alter their behavior depending on who they hang out with, even mating more frequently in the company of strangers. Experts say the findings provide clues to the evolution of social behavior.

Instead of talking, fruit flies socialize by emitting compounds called pheromones from their abdomens. Researchers have identified 23 varieties of these chemicals in males, which the insects can blend into various combinations that may mean everything from "Back off, Jack!" to "Hey, do you want to come over to my place after dinner?" Knowing that fruit flies change their sleep and other behavior patterns depending on the number and relatedness of the other flies they are with, neurogeneticist Joel Levine of the University of Toronto, Mississauga, wondered whether social context could also affect what flies "say" to each other.

In one set of experiments, Levine and his colleagues divided the flies into vials of 40 males each. Some vials contained flies that were nearly genetically identical, while other, "mixed" vials, contained 32 of that kind of fly along with eight of a different genetic makeup. Some vials never saw the light of day; others were on a regular day and night cycle. The researchers hourly extracted and analyzed the pheromone blends from individual flies from each vial.

Graduate student Clement Kent's analysis revealed that the amount of daylight, the genetic makeup, and the living arrangements influenced the blend of pheromones produced. But, in particular, pheromones important for courtship and mating were affected most by the social context, the group reports online today in Current Biology. Identical males in homogeneous groups produce one blend of compounds, but their counterparts in a mixed group produced a different blend, one more like the eight individuals who were dissimilar, they report. "What this says is that how a fly reacts [pheromone-wise] to another fly depends on who he has been hanging out with," Levine explains.

Levine's postdoctoral fellow, Joshua Krupp, and his colleagues next tested whether males in mixed groups behave differently when in the company of the opposite sex. They put six genetically identical males in vials with six virgin females and in other vials, four identical and two different males in with the females. The presence of males unlike themselves sent the male fruit flies into sex overdrive: They mated 33% more often than did the sets of six identical males. The data show that mating behavior is not as hard-wired as researchers had thought, says Levine.

"This is a creative piece of research that reveals fruit flies to be more sensitive to their social environment than previously imagined," says Gene Robinson, a neurobiologist at the University of Illinois, Urbana-Champaign. Russell Fernald, a neuroethologist at Stanford University in Palo Alto, California, and his colleagues would like to see the work followed up with studies under more natural conditions, as it's unknown whether the fruit flies experience such crowds in the wild. But discovering plasticity in what was once thought to be a hard-wired behavior is quite exciting, says Fernald. "They may be seeing the spectrum of variation that may be important to the evolution of social behavior."

Posted in Biology, Brain & Behavior