Oxidative stress has been suggested as a proximate cost of reproduction and hence as a major constraint in the evolution of life histories, and it is therefore thought that antioxidants alleviate the effects of reproductive effort on oxidative stress. Furthermore, carotenoid-based ornaments have been proposed to mirror male ability to resist oxidative stress. Using a full-factorial experimental design in a natural population of great tits Parus major, we manipulated brood size and supplemented the male parent with either carotenoids or a placebo. We then assessed antioxidant capacity via a measure of whole blood resistance to a free radical attack during the nestling rearing period. Males of enlarged broods showed impaired antioxidant capacity 5 days after the brood size manipulation. However, 13 days after manipulation, they had their antioxidant capacity restored, an effect that may be due to the development of compensatory antioxidant mechanisms or due to reduced investment in the current reproduction in favor of future survival and reproduction. Carotenoid supplementation did not affect male antioxidant capacity nor was the interaction with the brood manipulation significant. Males with stronger carotenoid-based plumage colors did not show higher antioxidant capacity 5 days after the brood size manipulation, but after 13 days, the relationship was highly significant. This study on a natural population shows that larger brood size can induce a transient decrease in antioxidant capacity. It also supports the hypothesis that carotenoid-based plumage may signal male ability to resist oxidative stress, particularly during the energetically demanding nestling rearing period.
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