Colour and levels of aggression in the midas cichlid

Previous work established that gold morphs of the Midas cichlid (Cichlasoma citrinellum) dominate normal ones of about the same size. Left unresolved, however, was whether the gold morphs dominate because they are inherently more aggressive or because the gold colour inhibits aggression. The issue was clarified here by comparing levels of aggression within groups of golds only, normals only, and golds plus normals. At first the groups of golds were the least aggressive. But 2 days later the levels of aggression in the other groups fell to about that of the golds. We conclude therefore that the gold colour inhibits attack, but that this effect is only discernible in groups when they are establishing inter-individual relationships disappearing when the groups stabilize. We suggest that gold coloration inhibits attacking by stimulating fear responses.