If you look really closely next time a group of fruit flies invades your kitchen, you might notice that some flies fight more than others. Male fruit flies, Drosophila melanogaster, are more aggressive than females, and the latest research from David Anderson's Caltech biology lab suggests that's because they have a unique cluster of neurons in their tiny brains that underlie aggression. The females don't have those cells.

In the video above, a typical male fruit fly fight can be seen in slow motion. You can see one fly "lunging" on the second fly, which is the scientific term to describe this kind of attack. The entire interaction occurs in just 50 milliseconds, or one twentieth of a second.

Kenta Asahina, the postdoc who led the research, says that the team found a cluster of neurons which increase aggression when they become activated, which is thanks to a hormone called tachykinin, or Tk. "Tk is present in these neurons," Asahina said, "and aggression goes down when you eliminate Tk, while aggression goes up when Tk is over-expressed." Since this cluster of neurons is only found in males, Asahina suspects that these neurons in particular explain why males are more aggressive. "Tk plays a critical role in setting the level of aggression in these neurons."

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Stained images of fruit fly brains show the male-specific cluster of neurons associated with aggression, indicated with yellow arrows. (Credit: Anderson Lab/Caltech)

To get increased aggression you need two things: activation of those cells, in addition to the expression of the gene that creates the neuropeptide Tk within those cells. Activating the cells by themselves isn't enough to make the flies hyper-aggressive.

Asahina was able to provoke his hyper-aggressive flies into behaving aggressively even in the absence of the cues that typically provoke aggression, such as certain pheromones. The effect was so strong that he even got flies to attack inanimate objects. Normal flies are known to chase inanimate objects, but not to attack them. What that means is the level of arousal was so high in these flies that they did not need any of the normal signals that are usually necessary to elicit an aggressive response. They're in permanent HULK SMASH mode. They're always angry.

Of course it wouldn't be appropriate to suggest that the hyper-aggressive flies are "angrier" than normal flies, but it would be reasonable to say that they have a lower threshold for aggression. It should also be noted that aggressive behavior is not wholly rooted in genetics - aggression isn't "hardwired" - biology simply plays a critical role. There are certainly other factors to consider.

What's exciting about this is that the hormone Tk has also been associated with aggression in mammals like mice and rats. And some other research has recently suggested that humans diagnosed with personality disorders that are characterized by high levels of aggression also have increased levels of Tk.

Given that, these new findings will allow researchers to better understand the mechanisms that underlie aggression at the cellular and genetic levels, with possible implications for better understanding human aggression, at least among males. In a press release, Anderson explained it like this: "If aggression is like a car, and if more aggression is like a car going faster, we want to know if what we're doing when we trigger these cells is stepping on the gas or taking the foot off the brake. And we want to know where and how that's happening in the brain. That's going to take a lot of work."

Asahina K., Watanabe K., Duistermars B., Hoopfer E., González C., Eyjólfsdóttir E., Perona P. & Anderson D. (2014). Tachykinin-Expressing Neurons Control Male-Specific Aggressive Arousal in Drosophila, Cell, 156 (1-2) 221-235. DOI: 10.1016/j.cell.2013.11.045