The cichlid fish feeds itself by stealing scales from other fish. They can have either a left-slanted or right-slanted jaw. This jaw slant is hereditary and both phenotypes persist in the species. Hori proposed that the frequency of one type depended on the relative frequency of the other, because a left-slanted fish would have a strong advantage in a population of mostly right-slanted fish and vice versa (the fish they prey upon would be less vigilant for the less frequently attacked side). Hori found that the frequency of the two traits oscillated around an equilibrium, supporting a:.