When A. burtoni males fight, it's easy to spot the winner. Mature males have a menacing black stripe, or eyebar, on their face. After a fight, the winner retains his showy appearance, but the loser's eyebar temporarily disappears as he tries to flee his more aggressive opponent.
To test for transitive inference, the Stanford team took advantage of this reversible change in appearance by staging a series of short fights between male cichlids of equal size. In the experiment, fish that lost their eyebar during one-on-one combat were declared the loser.
After each bout, the loser was separated from his opponent and put back in his original tank. Within minutes, his eyebar returned, and he looked like all the other dominant males again.
Bystanders and rivals
The fights were staged in a square tank divided into several compartments. A lone male observer--the "bystander"--was placed in a cubicle in the center of the tank. Surrounding him were five smaller compartments, each with a solitary male rival identified simply as A, B, C, D and E. Researchers made sure that the bystander had never met any of his five potential rivals.
Although the bystander remained alone in his cubicle and never swam with the others, he was allowed to observe a series of fights between rival pairs--A vs. B, B vs. C, C vs. D, and D vs. E. Researchers manipulated the fights so that A would dominate B, B would dominate C, and so forth down the line.
"These fights, taken together, imply the dominance hierarchy A>B>C>D>E," the authors wrote. But did the bystander really comprehend this intricate pecking order, and if so, wo