Watching cage fights altered mouse brains, study finds

Observer mice mixed aggressive and defensive behaviors, suggesting learning from both sides

Experimental design and observed fighting behavior

NPR host Scott Detrow introduced the findings, saying brain scientists were finding that a mouse’s brain could be primed for battle simply by watching others fight. Pien Huang then reported the study, beginning with two mice swiping at each other in a cage while another mouse watched from above.

The study in Nature placed observer mice on a platform above staged cage fights and found that watching other mice fight changed the observers’ later behavior and brain activity. Observer mice became more vigilant and aggressive, mixed aggressive and defensive behaviors, and showed brain activity similar to that of fighters. But watching did not reproduce the full effects of fighting experience.

Annegret Falkner, an associate professor at the Princeton Neuroscience Institute, said, “We set this up to look a little bit like a gladiator arena, so the mouse is kind of above the two mice fighting on a platform.” Falkner noted that mice fight a lot in the wild, where they normally form hierarchies and defend their territories. She said each laboratory fight was fixed: some mice were trained to perform losing behavior, while very experienced mice displayed winning behavior. Pien Huang described the laboratory contests as a kind of performance, or a rodent wrestling match.

A spectator mouse on the platform above a pair watched a series of live, four-minute cage fights over about a week. It remained purely a spectator during that period. At the end, observer mice tried their skills in their own cage fights. Jorge Iravedra Garcia, a postdoctoral researcher in Falkner’s lab, said the mice that watched fights 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.” Huang said this meant observer mice had learned to be more vigilant and aggressive.

Huang said the observer mice fought with a mix of aggressive and defensive behaviors, “like they had learned tactics from both sides.” Falkner said that combination might not happen through experience alone. “You only have your own perspective and experience,” she said.

Brain similarity and the limits of observation

Researchers tracked brain activity through the training and fighting. Iravedra Garcia said, “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.” Huang said the similarity meant that simply watching fights rewired mouse brains to look like that of a fighter.

Huang compared excitatory neurons to a car’s gas and inhibitory neurons to its brakes. Iraavedra Garcia said a population of inhibitory neurons has the job, when it comes to aggression, of braking, or stopping. Huang said all mice, including those that had never even seen fighting, were “gassed up and ready to go” when placed in a brawl, and that those that had fought or watched fights before had “fewer inhibitions, fewer brakes.”

Watching still did not make an observer indistinguishable from an experienced fighter. Iraavedra Garcia said, “They don’t fully recapture the sort of behavioral effects of experience, but it gets you somewhere in the middle.” He said the observers’ movements were less smooth and less regular than those of mice with real fighting experience, but they had a sense of strategy and retreated less.

Zelikowsky considered broader applications

Moriel Zelikowsky, a neurobiologist at the University of Utah who was not affiliated with the paper, called the work “a very creative work by the authors there.” Zelikowsky also said, “I’m staging these kinds of fight club and even observers of the fight club.” Huang said the findings shed light on how animals learn by watching. Zelikowsky said the work went beyond mirroring what an animal saw or heard and might show how observational experiences reshape the brain in ways that could increase an animal’s chance of survival.

Zelikowsky said the same type of observational learning may extend to other behaviors, including mating and feeding, which she described as critical to get right. The reported experiment concerned mouse aggression, while the extension to mating and feeding remained Zelikowsky’s proposed implication.

Falkner compared the mouse findings with human skill learning. She said humans can learn physical skills by watching others, including sports: “The question is not whether you can be a professional athlete by watching the NFL, but are you better at football for watching sports than if you had never watched anything? And the answer is absolutely yes.”

Pien Huang said, “So at least for mouse fights, watching others pummel others is a safer and maybe smarter way to learn.”