Mice that watch fights come away battle-ready
Spectators’ brains and behavior look more like those of an experienced fighter after only watching others brawl
Princeton University
People love to watch violent things. From slasher films to boxing, there’s some intrinsic appeal to watching people get pummeled. The dystopian thriller Squid Game, for instance, debuted in 2021 and in three years racked up 330 million streams totalling more than 2.8 billion hours viewed. Even monkeys pay more attention to videos of other monkeys fighting.
Now, researchers at Princeton University have found that there may be something useful gained from all this watching: being better prepared if you find yourself in a similar situation.
In a new paper, Princeton neuroscientists reveal that male mice who merely watched other males fight were better prepared fighters themselves without being any meaner or more aggressive than other mice. Their brains changed in many of the same ways, too, as mice with actual fighting experience.
“They're kind of midway between the truly experienced fighters and the naive animals,” said Annegret Falkner, assistant professor of neuroscience at Princeton and senior author of the paper. “There's something about experience that gets you all the way there, but observation gets you in a similar direction, but only about halfway."
The paper was published on November 04 in the journal Nature.
Learning by watching
Male mice were assigned to one of three different groups: fighters, spectators, or novices.
For eight consecutive days, each group participated in three daily 10-minute training sessions. Fighters had a sparring session each time with a pushover mouse who was easy to defeat.
Spectators sat in the nosebleeds on a raised platform behind a clear barrier, watching a different pair of mice spar below. They could see, smell, and hear the action but couldn’t join in.
Novices got to play with a similar-sized mouse stuffie for the same amount of time and were spared from watching any fights.
On the ninth day, all mice faced three 4-minute rounds with a hard-to-beat opponent.
Across training sessions and on fight night, Jorge Iravedra-Garcia, a postdoctoral scholar in the Falkner lab and lead author of the paper, filmed their behavior from above.
During training, he found that just like people and monkeys, spectator mice were glued to the fights.
“The observing mice don't care so much when the fighting animals were just walking around and interacting,” Falkner said. “But we see them orient and attend once attacks start. Even if an attack starts and they're not oriented, they're more likely than chance to turn and watch what's happening, so there's something very salient and sticky about aggression that's different from other behaviors."
All that keen watching seems to have paid off.
On fight night, both spectators and fighters were quicker to attack the tougher opponent than novices. They also spent less time with their backs turned. No group spent more time attacking than the others, though, which suggests eight days of practice, by doing or by watching, didn’t simply make the mice more aggressive.
The choreography was also similar between spectators and fighters.
"Not only are they emulating some of these behaviors, but the way that they move between these behaviors is also quite similar,” Iravedra-Garcia said. “When you compute the transitions from aggression behavior one to investigation behavior two, experienced animals behave very similarly to observers in those situations as well."
Relaxed inhibition
Watching fights also shifted spectators’ brain activity to look more like a fighter.
Excitatory brain cells were engineered to glow green when active and inhibitory brain cells to glow red. Iravedra-Garcia then implanted optical fibers, each barely wider than a human hair, into a dozen interconnected brain regions. The glow traveling back up each fiber let him track both cell types in every region at once while a mouse watched or fought.
Prior work found aggression “mirror” neurons in one brain region, Iravedra-Garcia tinkered for a year to perfect a device allowing him to measure effects across a network of 12 interconnected brain regions simultaneously.
The hard work paid off: Iravedra-Garcia found that brain activity is "mirrored" not just in the previously studied brain area, but across nearly every region tracked. When they watched, spectators’ inhibitory cells fired in patterns much like a fighter’s, whereas their excitatory activity looked more like a novice's. On fight night, however, activity across their network looked more like a seasoned fighter’s than a novice’s, though not a perfect match.
"Observation's not a replacement for experience,” Falkner said. “But it gets you part of the way there."
Over the eight days, inhibitory cells in fighters and spectators alike gradually loosened their grip on key aggression hubs, like easing off a brake. When the team restored that inhibition, the mice fought like novices again.
“I know kung fu”
Iravedra-Garcia and Falkner are now setting their sights on two new directions. One project will investigate how female onlookers watching male mice brawl changes their interest (or reluctance) with either fighter.
“OK, this guy won, this guy lost. How do you encode the information? What do you do with that information?” Falkner said. “We think females may use this information to make mate choices later."
The ultimate experiment for Falkner, though, is best captured by the classic scene in the film The Matrix when the main character Neo opens his eyes after having been uploaded with various fighting styles, and says, “I know kung fu.”
Now that the group knows the brain activity patterns that change during watching aggressive bouts, they want to see if artificially imparting that same brain activity pattern in an untrained mouse can endow them with the same circuit and behavioral changes that comes from observational learning.
“If you think about the photometry data as the musical staff of exactly what they just saw, we could replay all that activity back down to the circuit and give them the experience of watching, even though they never watched anything,” Falkner said. “If you replay back this sequence of activity and then you test them, do they behave as if they actually watched a fight?"
The findings may also hint at how exposure to violent experiences may shape the way people respond to threats. While it’s unclear how it might translate to humans, in mice at least, exposure didn’t make anyone more violent, just more prepared.
“I do think that it does suggest that if you were exposed to a lot of violence, you might have undergone circuit changes that create behavioral states that might be adaptive if you're in a violent situation, but they're probably not adaptive if you're not in a violent situation,” Falkner said. “I have a hunch that probably some of these circuit changes are conserved in mice and humans."
Authorship & Support
Jorge M. Iravedra-Garcia, Eartha Mae Guthman, Lenca Cuturela, Edgar J. Ocasio-Arce, Jonathan W. Pillow, Annegret L. Falkner. Aggression experience and observation drive shared behavioral and neural changes. Nature. November 04, 2026. DOI: 10.1038/s41586-026-11217-9
This work was supported by grants from the National Institutes of Health (NIH), grant numbers DP2MH126375, and R01MH126035; the New York Stem Cell Foundation; the Simons Foundation; the Klingenstein Foundation; and the Alfred P. Sloan Foundation.
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About the Princeton Neuroscience Institute (PNI)
Founded in 2005, the Princeton Neuroscience Institute at Princeton University brings together researchers across disciplines at Princeton University to investigate how the brain gives rise to perception, cognition, and behavior. Led by Director Mala Murthy, PNI has built internationally recognized strengths in computational and quantitative neuroscience, advanced neurotechnology, and integrative approaches that connect molecular, cellular, and systems-level analyses with human cognitive studies. For more information, please visit: https://www.pni.princeton.edu
Journal
Nature
Method of Research
Experimental study
Subject of Research
Animals
Article Title
Aggression experience and observation drive shared behavioral and neural changes
Article Publication Date
4-Nov-2026
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