Learned aggression biases in males of Lake Victoria cichlid fish

Male–male competition for mating territories can exert negative frequency-dependent selection on a male secondary sexual trait, such as nuptial coloration. This can occur when males bias aggression towards own-coloured competitors, resulting in a fitness advantage for rare phenotypes, thereby promoting the evolution and maintenance of stable colour polymorphisms. It could operate in the extensive radiations of haplochromine cichlid fish in East African lakes. In a previous investigation we studied wild-caught blue and red Pundamilia males from Lake Victoria; males from a location where most resemble blue (referred to as bluish males) biased aggression towards blue stimulus males. In contrast, blue males from a location where blue and red occur sympatrically biased aggression towards red stimulus males. Using lab-bred bluish and blue males, we tested the hypothesis that exclusive experience with blue males (blue treatment) leads to an aggression bias towards blue and that experience with blue and red males (mixed treatment) leads to an aggression bias towards red. Contrary to predictions, blue-treated males did not distinguish between blue and red males, whereas mixed-treated males preferentially attacked blue stimulus males. The data suggest that prior experience can affect aggression biases and that experience with more than one phenotype may be required for the development of biases. Yet, our results cannot explain the direction of differences in specific biases observed in wild-caught males from different populations.