Mice that only watch other mice fight can become more aggressive themselves, and their brain activity begins to resemble that of actual fighters, researchers at Princeton have found, NPR reported. The study was published in the journal Nature. Annegret Falkner, an associate professor at the Princeton Neuroscience Institute, and colleagues set up what she described as a "gladiator arena": a spectator mouse watched from a platform above two mice fighting below. "Mice fight a lot in the wild," Falkner said. "Mice form hierarchies, normally. They defend their territories." In the lab, the fights were a kind of performance. "The fight is always fixed," she said. Some mice were trained to lose, others were experienced winners.
The mouse on the platform watched a series of live, four-minute cage fights over about a week, purely as a spectator. Then the observers were tested in their own fights. Jorge Iravedra Garcia, a postdoctoral researcher in Falkner's lab, said the mice that had watched mimicked what they saw. "They were the ones that were starting the fights. They were pushing back. And in between, they held these sorts of postures that meant that they were ready to engage." In other words, the observer mice learned to be more vigilant and aggressive. The researchers also tracked brain activity through the training and the fighting. "When we did this in fighters and watchers alike, the way that activity lights up across this suite of brain regions was actually very similar," Iravedra Garcia said. Simply watching fights, the team argued, rewired mouse brains to look more like those of fighters.
Brains have excitatory and inhibitory neurons, like the gas and brakes in a car. A population of inhibitory neurons acts as a brake on aggression. All mice, including those that had never seen a fight, were "gassed up and ready to go" when placed in a brawl, Iravedra Garcia said, but mice that had fought or watched fights had fewer of those brakes. The observers' moves were not as smooth or regular as those of experienced fighters, but they showed some strategy and retreated less. "They don't fully recapture the sort of behavioral effects of experience, but it gets you somewhere in the middle," he said.
Moriel Zelikowsky, a neurobiologist at the University of Utah who was not involved in the work, called it "very creative." The findings, she said, go beyond simple mirroring and show how the brain can be reshaped by watching, in ways that might help an animal survive. Similar learning, she said, may apply to other crucial behaviors such as mating and feeding. Falkner noted that humans also learn physical skills by watching, as in sports, though watching the NFL will not make someone a professional athlete. The verified result is a controlled lab finding about observational learning in mice, published in Nature, not a claim about human violence.