A male fish can size up potential rivals, and even rank them from strongest to weakest, simply by watching how they perform in territorial fights with other males, according to a new study by Stanford University scientists. The researchers say their discovery provides the first direct evidence that fish, like people, can use logical reasoning to figure out their place in the pecking order.
The study, published in the Jan. 25 edition of the journal Nature, is based on a unique experiment with cichlids (SIK-lids), small territorial fish from Africa. "In their natural habitat, male cichlids are constantly trying to ascend socially by beating each other up," said study co-author Russell D. Fernald, professor of biological sciences at Stanford. "It would be really valuable for them to know in advance who to pick a fight with."
The Nature experiment was designed by lead author Logan Grosenick, a graduate student in statistics at Stanford, and Tricia S. Clement, a former postdoctoral fellow. Their goal was to determine whether territorial fish use a type of reasoning called "transitive inference," in which known relationships serve as the basis for understanding unfamiliar ones.
"Transitive inference is essential to logical reasoning," Fernald explained. "It's something that kids generally figure out by age 4 or 5--Mary is taller than Fred, Fred is taller than Pete, therefore Mary is taller than Pete. It's been demonstrated in primates, rats and some bird species, but how and why it evolved in animals is a matter of debate."
In the experiment, the Stanford team used a popular laboratory fish called Astatotilapia burtoni, one of many cichlid species that inhabit Lake Tanganyika in eastern Africa. A. burtoni males are extremely territorial and regularly engage in aggressive fights, the outcome of which determines who gets access to food and mates.