Social rank, color morph, and social network metrics predict oxidative stress in a cichlid fish

Dominance hierarchies are a fundamental part of social systems in many species and social rank can influence access to resources and impact health and physiology. While social subordination is a profound stressor, few studies consider the social stress experienced by dominant males due to constantly needing to defend their dominance status through costly aggressive displays. Recent studies suggest that in species that use body coloration to signal status, these costs may also be color morph-specific. Our study examines the link between the social rank, intensity of territorial defense, body coloration, and oxidative stress in males of the color polymorphic cichlid fish Astatotilapia burtoni where males are either blue or yellow. We studied behavior in naturalistic communities and examined circulating reactive oxygen metabolites (ROMs) and antioxidant defenses. We found that dominant males experience higher concentrations of circulating ROMs without notably increasing their antioxidant defenses, but this effect was not related to color morph. Aggression and social network ties predicted oxidative stress in a morph-specific manner, with yellow but not blue males showing signs of increased oxidative damage with increasing agonistic effort. In contrast to expectation, oxidative stress was not influenced by cortisol or testosterone levels. We conclude that oxidative stress is instrumental to understanding the costs and benefits of high social rank. Living in a social hierarchy can be stressful even for those on top. While past work has established the stress involved in being a low-ranking member of a hierarchy, we show that while high-ranking males attain definite advantages in terms of decreased stress hormone levels and increased reproductive opportunities, they also pay for their increased activity level with increased risk of cellular damage.