Abstract

Haplochromine cichlid fish have radiated into hundreds of species in East-African lakes, possibly driven by divergent sexual selection on body coloration. We studied the color polymorphic Lake Victoria cichlid Neochromis omnicaeruleus, in which a presumably ancestral phenotype with blue males and brown females co-occurs with two distinct classes of blotched phenotypes in both sexes. Similar blotch polymorphisms occur in other haplochromine species, and in all studied cases blotched females are much more common than blotched males. In N. omnicaeruleus, the near absence of blotched males seems to be partly due to genetic linkage to a dominant female determiner that turns blotched males into females. However, laboratory breeding suggests that blotched males should be much more common than observed. Here we studied whether differential predation on blotched males contributes to their scarcity. First, in a predation experiment with wild birds, blotched fish indeed incurred more predator attacks. Second, underwater observations revealed behavioral differences between the sexes, consistent with an additional predation risk for males. These data suggest that differential predation with regard to color pattern and sex may be an important selective force in the evolution and maintenance of this color polymorphism. However, we also carried out a population census which revealed that blotched males were rare already as juveniles. To explain the scarcity of blotched males in nature, we therefore have to invoke either selection against blotched males early in life, or a more complex genetic model. These results emphasize the need for further research on the ecology and genetics of this widespread color polymorphism in cichlid fish.