Testing Wickler's hypothesis: cichlids are unable to distinguish eggs from egg spots in the wild

Cichlid fish have undergone explosive radiation in a process aided by sexual selection. Thus, phenotypes that contribute to reproduction may partially explain the rate of radiation. One such phenotype is the presence of circular spots on the anal fin of the haplochromine cichlids. In 1963, Wickler postulated this phenotype acts as an egg mimic which enhances male fertilization success. However, it remained unclear whether cichlids can distinguish between eggs and egg spots in their natural environment. Further, studies have found differing effects of egg spot characteristics in female mate preference. In this study, we quantified the color of eggs, egg spots, and anal fins of two haplochromine species (Metriaclima benetos and Astatotilapia burtoni). Using a receptor noise limited (RNL) visual model, we found that these species are unable to distinguish the colors of eggs and egg spots in the lighting of their natural habitat. Further, only M. benetos could distinguish the color of egg spots from anal fins. This study supports Wickler's egg spot mimicry hypothesis and potentially explains species differences in the roles of egg spots as sexually selected traits. It further points out the importance of avoiding human biases in quantifying what colors fish can and cannot distinguish.

Automated summary

Cichlid fish have experienced rapid radiation, aided by sexual selection and their ability to mimic the appearance of eggs on their anal fins. However, it was uncertain whether cichlids can actually distinguish between eggs and egg spots in their natural environment. This study found that while the two haplochromine species could not distinguish the colors of eggs and egg spots, one species was able to distinguish egg spots from anal fins.