Carotenoid pigmentation does not reflect total non‐enzymatic antioxidant activity in plasma of adult and nestling great tits, Parus major

Abstract

Summary Based on carotenoids’ antioxidant and pro‐vitamin functions, carotenoid pigmentation, particularly in birds, has been hypothesized to honestly advertize condition and health. As antioxidants, carotenoids protect the body from oxidative damage caused by internal or external stressors (e.g. growth, infection and anthropogenic pollution). However, the relative importance of carotenoids in the avian antioxidant defence has been questioned, and the positive relationship between colouration and health may be a ‘red herring’. In the present study, we investigate carotenoid‐based plumage colouration and circulating carotenoids in relation to plasma non‐enzymatic total antioxidant activity (TAA) in adult and nestling great tits (Parus major). The study was conducted in urban vs. rural populations, with documented differences in oxidative stress level and plumage pigmentation. First, we found that there was no relationship between TAA and plumage pigmentation (carotenoid chroma) or plasma carotenoids. Second, urban environment significantly influenced TAA; old (2+ years) urban birds had higher activity than old rural birds, and also compared to younger (1 year) urban adults. This is likely to be due to the increased demand for plasma antioxidants as a consequence of urban environmental stress. Third, nestlings’ TAA showed no difference between urban and rural environments. However, it was highly influenced by brood, but not related to parental levels, therefore most likely due to differences in the nestling environment. We suggest that carotenoids are not significant contributors to extra‐cellular antioxidant defence in great tits, and that carotenoid pigmentation may be an overrated health indicator in this respect. Honest carotenoid signalling, when present, is thus likely to be mediated by some other constraint, such as nutrition or uptake, or other health‐modulating mechanisms.