Male–Male Competition Affects Color Phenotype Expression and Color-Dependent Oxidative Stress Levels in a Polymorphic Cichlid Fish

In some animal species, body coloration can change dramatically during the lifetime of an individual in response to environmental conditions. Body coloration is often a target in sexual selection, but how competition for mates influences phenotypic plasticity in body coloration is rarely considered. Here, we provide experimental evidence in the polymorphic cichlid fish Astatotilapia burtoni that male–male competition for mating territories influences the expression of body coloration. In this species, males can express yellow or blue body coloration and may change color. We housed males individually with visual access to a neighboring male and placed halved terracotta pots as defendable structures either near the neighbor (proximal treatment) or further away from the neighbor (distal treatment). We found that males were more aggressive in the distal treatment, and that a higher proportion of males expressed the yellow phenotype in this setting compared to those housed in the proximal treatment toward the end of the six-week experiment. Unexpectedly, we found that males in the proximal treatment who were smaller than their neighbor were more likely to express the blue phenotype while larger males tended to express the yellow phenotype at the beginning of the experiment. We also found that oxidative stress levels were higher in blue males compared to yellow males. As previous studies show that yellow males are better fighters and blue males more attractive to females, our findings suggest that males can adjust their color phenotype depending on the level of competition or resource holding potential. Our results support the notion that male–male competition may be an important factor influencing the evolution of phenotypic plasticity in coloration.