You know that "fish don't think/cause fish know everything".

It seems that the line corresponds to the reality.

The new behavioral study showed that fish are able to remember the fighting outputs of their rivals and to figure out their rank on a hierarchy scale based on this.

By watching a limited number of fights between select pairs, fish males understood their position in the group, an astonishing social ability if we compare it to the well known impression that fish are totally destitute of intelligence. Assessing the strength of your rivals is crucial on deciding when to fight and when to flee.

The study was made on Astatotilapia burtoni, an African cichlid in which the males fight viciously over territories in lakes and rivers, because a good area is essential in finding quality food and a good nesting place. "How many fights they win or lose ends up determining their reproductive success," said Logan Grosenick of Stanford University. "This results in a strong evolutionary pressure on them to use social information to choose their fights wisely", he added.

An ability of assessing the rival and its hierarchal rank has been observed till now in a few other animals, like monkeys, rats, and pigeons.

Fish could learn the group's hierarchy on their own just by watching, while in other studies the researchers had to reward the animals.

The team put one fish, the observer, in a central tank from which he could see in surrounding tanks, in which they set up a series of fights between other fish. They introduced stressed fish males into tanks that were the home territory for other males.

These were already lost fights for the new fish. "The home fish chased and bit the invaders, and the invaders consistently fled," Grosenick said. "The observer fish sat there, attending to the fights, like they're watching television," he added.

After several fights, the researchers let the observer choose between two males that they'd seen fighting before. The observers preferred to swim close to the defeated males as they may have 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."