No simple measures for antioxidant status in birds: Complexity in inter- and intraspecific correlations among circulating antioxidant types

Recently, ecologists have shown interest in examining antioxidant protection in wild animals; carotenoids in particular have received attention as antioxidants that play an important role in mediating health, life-history trade-offs and sexual selection. However, we know almost nothing about the relationships among levels of different antioxidants in wild animals or whether variation in antioxidants can be viewed as a single coherent system. 2. Here we use a data set of 903 individuals from 99 bird species to examine covariation among concentrations of three types of antioxidants (uric acid, vitamin E and four carotenoids), and their relationship to a summary measure, Trolox-equivalent antioxidant capacity (TEAC), both inter- and intraspecifically in 30 species. 3. Three axes were necessary to adequately describe variation in nine antioxidant measures, and these axes corresponded to the basic categories of antioxidant measured: uric acid, vitamin E and carotenoids. There was substantial heterogeneity in the correlations across species. 4. TEAC covaried strongly with uric acid levels, both interspecifically and in 23 of the 30 species. Concentrations of different carotenoids covaried both inter- and intraspecifically, but there was also substantial variance explained by each carotenoid independent of the others. Vitamin E concentration did not robustly correlate with any other antioxidant parameters. 5. Overall, we show that although antioxidants tend to covary with those of similar type, these correlations demonstrate evolutionary lability and/or ecological heterogeneity. Even the most general associations are absent in some species, suggesting that conceptualization of a single antioxidant system is oversimplified and that ecological studies attempting to examine antioxidant function should use multiple measures.