Variation in mate competition favors phenotypic plasticity in male coloration of an African cichlid

Sexual selection is thought to be a potent evolutionary force giving rise to diversity in sexual traits that enhance mating success, such as ornament, sexual display, and weapons. The expression of sexual traits is often influenced by environmental conditions, suggesting that phenotypic plasticity may precede and facilitate evolutionary divergence in sexual traits by sexual selection. However, the mechanisms that promote plasticity in sexual traits remain poorly understood. Using the cichlid fish Astatotilapia burtoni, we show that sexual selection may promote plasticity in sexual traits. In this species, males change between yellow and blue color and exhibit intense male contest competition over breeding territories to attract females. We found that experimentally increased competition over territories led to a higher proportion of males expressing the yellow phenotype. Furthermore, we found that expression of yellow coloration is beneficial to males in territorial disputes. Specifically, yellow males won more staged dyadic contests than blue males, and yellow males had a lower level of oxidative stress than blue males, especially when engaging in more territorial display. However, females spawned more with blue males than with yellow males, suggesting that expression of blue coloration is sexually more attractive. The ability to adjust color phenotype according to the local competitive environment could therefore promote the persistence of plastic changes in coloration. Our findings challenge the view that phenotypic plasticity sets the stage for the evolution of genetically fixed changes via sexual selection, and instead suggest that sexual selection may favor plasticity in sexual traits.