Males’ aggression toward each other is an old story throughout the animal kingdom. It’s not that females aren’t aggressive, but in many species, male-on-male battles are more common.
Take fruit flies. “The males are more aggressive than females,” said David Anderson, a California Institute of Technology neuroscientist who knows their tussles well. Anderson runs a kind of fight club for fruit flies in his lab at Caltech, with the goal of understanding the deep evolutionary roots of very fundamental behaviors.
Anderson and his colleagues recently identified one gene and a tiny group of neurons – present only in the brains of male fruit flies – that can control aggression.
The gene is also found in mammals and has also been associated with aggression in some mammalian species, perhaps even in humans, although that is not clear.
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The discovery, reported in the journal Cell last month, does not tell the whole story of fly aggression. Some fighting is naturally linked to food and mating, while the mechanism the scientists found is not. But it is a striking indication of how brain structure and chemistry work together, as well as a reminder that as different as humans and flies are, they are not always very far apart.
The process of discovery, recounted step by step in the paper, gives a glimpse of modern brain research and the lengths to which scientists must go if they want to get down to the level of how neurons control behavior.
The research began, Anderson said, with the hypothesis that neuropeptides – a kind of hormone in the brain – had a role in controlling aggression as they do in some other fundamental behaviors like feeding and mating.
When the researchers compared the brains of male and female flies, they found a few neurons, present only in the male, that produced tachykinin. When these neurons were silenced, the researchers were able to decrease aggression.
Mammals have several different kinds of tachykinin, including substance P, which has been connected to aggression in rodents and has a variety of suspected roles in human beings, including a possible link to aggression. The researchers had not set out with this particular brain hormone in mind, but the parallel in mammals made sense.
They had identified a cluster of neurons, as few as three, that caused an increase in aggression. Those few neurons were only in males, and they were active when males were fighting each other.
The researchers did more genetic manipulation, deleting and adding copies of tachykinin genes, so that the neurons would produce more or less of the chemical. They found that with enough tachykinin produced by these few neurons, flies became more or less aggressive. They could even make small flies attack bigger flies.