This new study, on the other hand, showed the fish could learn the hierarchy on their own, just by watching.
To figure out what was going on, Grosenick and colleagues kept one fish, the observer, in a central tank with windows onto other tanks.
Then they staged a series of rigged fights between other fish.
For example, they stressed a fish and then put it into another's tank, setting it up to lose to the fish that was in its home territory.
The home fish chased and bit the invaders, and the invaders consistently fled, Grosenick said.
And when the fights were on, the observer fish "sat there, attending to the fights, like they're watching television," he added.
After the observer fish watched a bunch of these fights, the researchers tested them on their understanding of the hierarchy.
The researchers let the observer fish choose between two fish that they'd never seen fight before.
The observers preferred to swim near those that were lower in the hierarchy. Presumably, the observers felt safer away from the more dominant males.
"This [study] shows possible evolutionary origins" of this kind of inference, said Thomas Zentall of the University of Kentucky in Lexington.
"It's saving the fish from getting attacked as much as they might otherwise," Zentall said.
"It must have considerable value," he added. "Even though they have a small brain, they're able to process this kind of information."
This ability may be widespread, study leader Grosenick added.
"I would expect this in most species that have dominance hierarchies."
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