It turns out that hordes of drunken frat guys aren’t the only ones who start fights for seemingly no reason.

Scientists at the California Institute of Technology (Caltech) have discovered an aggression-promoting pheromone in flies, which may be the first clue in figuring out how aggression is hardwired into animals’ brains, according to a recent story published in e! Science News.

Pheromones, which are special chemicals that some animals use to communicate messages, have already been found in the scent glands of other insect species such as ants and beetles. However, proving that these insects use their pheromones to control their aggressive behavior has been difficult, until now.

The researchers found that the vinegar fly, Drosophila melanogaster, was the perfect candidate to help answer this question because they were able to identify the fly's receptor molecules that detect these scents as well as the sensory neurons that express these receptors.

"The genetic/molecular architecture of the olfactory system in Drosophila is well understood," explained Liming Wang, a graduate student in Anderson's lab. "Thus, one can easily test whether a specific olfactory receptor, and the sensory neurons expressing it, are involved in a given behavior."

To test their theory, the researchers made a synthetic pheromone that “robustly promotes aggression in pairs of male flies” and released it into an area filled with the insects. It wasn’t long before the flies became aggressive, performing so-called “lunges” where one fly rears up on its hind legs and uses its forelegs to snap down on an opponent.

Even more interesting, by silencing the neurons that turn on this behavior the researchers were able to block the synthetic pheromones' ability to promote aggression. In addition, the researchers found that separating the flies while they were releasing the pheromones didn't keep the flies from becoming aggressive.

"These experiments suggested that the presence of high densities of male flies in a local environment can indeed promote aggression through their release of cVA and its detection by other flies," Wang explained.

In other words, the more male flies there were, the more likely it was that a fight would break out, a phenomenon that people who have been subjected to standing on a full train or in a crowded stadium have no doubt experienced.  

The researchers believe this happens as a way to limit overpopulation issues where flies outnumber the amount of food available. If there are too many flies, a fight will break out where some flies will be killed, leaving more food for the winners.

The researchers believe it’s unlikely that humans can sense this fly pheromone, but that certainly doesn’t mean that humans lack aggression pheromones entirely.

As the article’s authors pointed out, mice, which are evolutionary close to humans, have been found to contain these fighting chemicals.

"Do these pheromones keep the lines from getting too long at a crowded lunch counter, as irate patrons jockey for position in the queue and some walk away in frustration?" asked coauthor David Anderson. "Only time will tell."