Aggression, but not testosterone, is associated to oxidative status in a free-living vertebrate

Aggression often shows large inter-individual variation, but high intra-individual consistency. Although the physiological basis and direct costs of aggression are generally well known, less is known about the physiological costs such as increased oxidative stress (OS). This can occur via increased leakage of oxidants during high metabolic demands such as physical activity, or by hormones regulating both metabolism and aggression. Here we address this within a natural population of White's skinks, Egernia whitii; a species in which both sexes exhibit consistent aggressive phenotypes, and sex-specific associations between testosterone and aggression. The results reveal that males' aggressive phenotype, independent of testosterone, was positively associated with antioxidant capacity (OXY), while there was no significant association in females. Oxidative damage (ROM) and oxidative stress index (OI), were not influenced by aggressive phenotype or testosterone, but showed borderline positive associations with body size (i.e., age). The results failed to show that high testosterone increases OS. Instead, OS may be related to sex-specific behavioural patterns associated with aggressive phenotype such as territory and mate acquisition. Although experimental work is needed to identify the causal links for these patterns, the results highlight the need to consider proximate mechanisms to understand constraints on phenotypic variation.