To find out, eight different bystanders were tested in the familiar square tank and in a new setting--a rectangular aquarium with three adjacent compartments. In each test, a bystander was placed in the middle compartment between two sets of rivals that he had never seen together--A and E (AE), and B and D (BD). At this point in the experiment, all the rivals had recovered from earlier losses, so their physical appearance was similar, right down to the eyebar. From the point of view of the bystander, therefore, each rival looked like a winner.
Using a video camera, researchers recorded which rival the bystander approached first, and the overall time he spent next to each of them. "Previous experiments in A. burtoni and other fish have shown that time spent in tank quadrants adjacent to a particular male indicates bystander 'preference,' and that bystanders spend more time near the rival they perceive to be weaker," the authors explained.
Losers and winners
The results were dramatic. Virtually all of the bystanders swam to the weaker rival first and stayed near him for a significantly longer period of time. In the AE tests, bystanders preferred E, the wimpiest of all the losers, over A, the top fish in the tank. In the more subtle BD tests, most bystanders chose D over B, even though these two rivals were ranked very close together on the dominance hierarchy.
"These results show that fish do, in fact, use transitive inference to figure out where they rank in the social order," Fernald said. "I was amazed that they could do this through vicarious experience, just by watching other males fight. In Lake Tanganyika, where conditions change all the time, it would be advantageous for a male to know who the new boss is going to be and who his weakest rivals are. Our experiment shows that male cichlids can actuall