Social rank influences relationships between hormones and oxidative stress in a cichlid fish

An individuals social environment can have widespread effects on their physiology, including effects on oxidative stress and hormone levels. Many studies have posited that variation in oxidative stress experienced by individuals of different social ranks might be due to endocrine differences, however, few studies have evaluated this hypothesis. Here, we assessed whether a suite of markers associated with oxidative stress in different tissues (blood, plasma, liver, or gonads) had social rank-specific relationships with circulating testosterone or cortisol levels in males of a cichlid fish, Astatotilapia burtoni. Across all fish, blood DNA damage (a global marker of oxidative stress) and gonadal synthesis of reactive oxygen species (as indicated by NADPH-oxidase (NOX) activity) were lower when testosterone was high. High DNA damage in both the blood and gonads was associated with high cortisol in subordinates, but low cortisol in dominants. Additionally, high cortisol was associated with greater production of reactive oxygen species (greater NOX activity) in both the gonads (dominants only) and liver (dominants and subordinates). In general, high testosterone was associated with lower oxidative stress across both social ranks, whereas high cortisol was associated with lower oxidative stress in dominants and higher oxidative stress in subordinates. Taken together, our results show that differences in the social environment can lead to contrasting relationships between hormones and oxidative stress.