Oxidative physiology of reproduction in a passerine bird: a field experiment

Organisms face resource trade-offs to support their parental effort and survival. The life-history oxidative stress hypothesis predicts that an individual's redox state modulates the trade-off between current and residual fitness, but this has seldom been tested experimentally in non-captive organisms. In this study, we manipulated the brood size in breeding pairs of barn swallows (Hirundo rustica) and found that females tending enlarged broods had increased levels of plasma oxidative damage (malondialdehyde concentration). This effect, however, was not accompanied by either a depletion, or defensive upregulation in antioxidants (glutathione, total antioxidant capacity, and uric acid) that may explain the increase in oxidative damage. Brood size manipulation and the level of plasma oxidative damage during brood rearing are not translated into decreased annual return rate, which does not support the oxidative stress hypothesis of life-history trade-offs. On the contrary, we found that female's oxidative damage and total glutathione levels, an important intracellular non-enzymatic antioxidant measured at hatching decreased and correlated positively, respectively with annual return rate, suggesting that oxidative condition at hatching might be a more important contributor to fitness than the oxidative physiology measured during chick rearing. We also show that individual traits and ecological factors, such as the timing of breeding and the abundance of blood-sucking nest mites, correlated with the redox state of males and females during brood care. Significance statement Oxidative stress is one of the most important physiological costs of reproduction and thus a key modulator of life-history trade-offs. In this study, we manipulated reproductive effort in breeding pairs of barn swallows and found that females tending enlarged broods had increased levels of plasma oxidative damage. This effect, however, was not accompanied by either a depletion or upregulation in antioxidants that may explain the increase in oxidative damage. We found that female's oxidative damage and total glutathione levels measured at hatching decreased and correlated positively, respectively with annual return rate, suggesting that oxidative condition at hatching might be an important contributor to fitness. Brood size manipulation and the increased levels of plasma oxidative damage are not translated into decreased annual return rate; thus, our results support the hypothesis that reproductive effort has a transient effect on oxidative physiology.