The fruit fly fight club

Rebecca Su | Oct. 7, 2015

In the evolutionary scramble for food, territory, and mating partners, all animals fight.

The male fruit fly is no exception. With females or an especially ripe banana around, it’s only a matter of time before a brawl breaks out: the champion, perched on the edge of a penny-sized ring, fans out his wings in warning as a challenger approaches. Goaded, he rears up on his hind legs and lunges. They wrestle for a few dramatic seconds before the opponent escapes.

Fruit flies boxing. (Courtesy of the Kravitz Lab)

Ed Kravitz, George Packer Berry Professor of Neurobiology at Harvard University, has studied fighting fruit flies for over a decade. He believes that winning is more than just a matter of luck; it’s a matter of biology — hormones and neural circuits within the brain that drive aggressive behavior. His research group, also known as the Fruit Fly Fight Club, studies these six-legged combatants to understand the neural underpinnings of aggression.

Whereas female fruit flies tend to share resources, the males fight to establish a hierarchy. They flare their wings, charge at each other, and tussle in tiny, heated skirmishes. In the aftermath of these contests, Kravitz has found that behavior emerges in predictable patterns. “Males learn during these fights,” he explains. “A ‘loser mentality’ develops in the losing flies, and they go on to lose their second fights.” The victors, on the other hand, are more likely to win their upcoming rounds.

Moreover, by inbreeding winners for 35 generations, Kravitz’s team has created a line of “bullies.” “In the wild type parent stock of flies, there’s a wide distribution of aggression levels,” says Kravitz. “By inbreeding winners, we select the extreme that shows the highest levels of aggression.” These hyper-aggressive males start fights more readily, escalate fights more quickly, and always beat opponents from the original parent line. However, once a bully loses a fight (to another bully), it loses its mean streak, and goes back to being a regular fly.

Fruit flies tussling. (Courtesy of the Kravitz Lab)

These findings suggest that though there may be a genetic basis for aggressive behavior, patterns of winning and losing are also linked to learning and memory. In order to investigate these patterns, Kravitz is currently studying circuits of neurons within the brain that can affect fruit fly behavior. In particular, he is interested in neurons that release and respond to a set of key molecules: serotonin, octopamine (also known as norepinephrine), and dopamine.

When it comes to aggression, serotonin is involved in making fights more intense. Kravitz has found that flies with serotonin removed from their nervous systems still fight with one another, but these seldom escalate beyond minor scuffles. Dopamine has more subtle effects; though it’s involved in arousal, it also seems to counteract the effects of serotonin by holding aggressive behaviors in check. Octopamine mediates decision-making, affecting whether a fly chooses to fight or mate.

Unraveling the web of several hundred neurons that interact with serotonin, dopamine, and octopamine in the fly brain is no trivial task. Using a combination of genetic techniques, Kravitz’s team is working to pick out individual neurons that control aggression.

“We’ve found two pairs of neurons in the brain,” says Kravitz. “If we change the firing patterns of the those neurons, it does the same thing as if you remove serotonin from the entire system.” In other words, flies with the altered neurons are less aggressive, starting fights but less eager to escalate them. These neurons also seem aggression-specific, with little effect on other behaviors. To better understand how certain neurons control behavior, the team is now working to investigate the circuitry underlying this phenomenon.

Kravitz, who originally studied aggression in lobsters, switched to fruit flies about 15 years ago. He explains that they reach adulthood more quickly and they can be studied with more advanced genetic tools. And, he adds, “Every millennium, you should change the animals you work on.”

Though Kravitz speaks fondly of his fighting fruit flies, he’s especially interested in what they can reveal about aggression in more complex species. “The interesting thing is that the same chemicals, or closely related chemicals, that are involved in flies are suspected to be involved in human behavior as well,” he notes. And though humans and flies have different central nervous systems, Kravitz maintains that researchers can still uncover important parallels.

As he puts it, “What we’re looking for are the principles, and not the details.”

Rebecca Su is a senior in Silliman College. Contact her at rebecca.su@yale.edu.

Click here to watch more footage of the fruit fly fight club. (Featured image courtesy of the Kravitz Lab.)