Why Males are More Aggressive Than Females: Special Cells Promote Fighting

A group of fruit flies may just look like they're fighting over a tasty meal, such as a piece of fruit, but that's not all that's going on. Scientists have discovered that male flies put up more of a fight and have learned exactly why that is. It turns out that special cells in their brains, which are absent in females, promote fighting.

"The sex-specific cells that we identified exert their effects on fighting by releasing a particular type of neuropeptide, or hormone, that has also been implicated in aggression in mammals including mouse and rat," said David Anderson, one of the researchers, in a news release. "In addition, there are some recent papers implicating increased levels of this hormone in people with personality disorders that lead to higher levels of aggression."

In order to learn more about fruit fly aggression, the researchers created a small library consisting of different fly lines. In each line, a different set of specific neurons was genetically labeled and could be artificially activated, with each neuron type secreting a different neuropeptide. Forty lines were tested for their ability to increase aggression when their labeled neurons were activated. In the end, the scientists found that the one that produced he most dramatic increase in aggression had neurons expressing tachykinin (Tk).

The scientists weren't done yet, though. They then used a set of genetic tools to identify exactly which neurons were responsible for the effect on aggression and see if the gene that encodes for Tk also controls aggressive behavior by acting in that cell.

"We had to winnow away the different cells to find exactly which ones were involved in aggression-that's how we discovered that within this line, there was a male-specific set of neurons that was responsible for increased aggressive behavior," said Kenta Asahina, one of the researchers, in a news release.

The findings reveal a bit more about the circuitry that controls aggression. This, in turn, could lead to future research in mammals and could represent a way to treat specific conditions, such as personality disorders in humans.

The findings are published in the journal Cell.