Although oxidative stress is a central topic in biochemical and medical research, the number of reports on its relevance in life-history studies of non-human animals is still low. Information about oxidative stress in wild birds may help describe functional interactions among the components of life-history traits. Currently available evidence suggests that oxidative stress may impart an important physiological cost on longevity, reproduction, immune response or intense physical activity. Given the gaps in our present knowledge, it is still premature to attempt to draw definitive conclusions and basic questions (e.g. how is oxidative stress generated and how do organisms cope with it?) have yet to be fully explored under natural conditions. Therefore, caution is needed in developing hypotheses or drawing general conclusions until additional data become available to perform more rigorous comparative analyses.

Prolonged high secretion of glucocorticoids normally reflects a state of chronic stress, which has been associated with an increase in disease susceptibility and reduction in Darwinian fitness. Here, we hypothesize that an increase in oxidative stress accounts for the detrimental effects of prolonged high secretion of glucocorticoids. We performed a meta-analysis on studies where physiological stress was induced by administration of glucocorticoids to evaluate the magnitude of their effects on oxidative stress. Glucocorticoids have a significant effect on oxidative stress (Pearson r = 0.552), although this effect depends on the duration of treatment, and is larger in long-term experiments. Importantly, there was a significant effect on tissue, with brain and heart being the most and the least susceptible to GC-induced oxidative stress, respectively. Furthermore, effect size was larger (1) in studies using both sexes compared to males only, (2) when corticosterone rather than dexamethasone was administered and (3) in juveniles than in adults. These effects were not confounded by species, biochemical biomarker, or whether wild or laboratory animals were studied. In conclusion, our meta-analysis suggests that GC-induced oxidative stress could be a further mechanism underlying increases in disease susceptibility and decreases in Darwinian fitness observed under chronic stress.

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In this review, it is our aim 1) to describe the high diversity in molecular and structural antioxidant defenses against oxidative stress in animals, 2) to extend the traditional concept of antioxidant to other structural and functional factors affecting the "whole" organism, 3) to incorporate, when supportable by evidence, mechanisms into models of life-history trade-offs and maternal/epigenetic inheritance, 4) to highlight the importance of studying the biochemical integration of redox systems, and 5) to discuss the link between maximum life span and antioxidant defenses. The traditional concept of antioxidant defenses emphasizes the importance of the chemical nature of molecules with antioxidant properties. Research in the past 20 years shows that animals have also evolved a high diversity in structural defenses that should be incorporated in research on antioxidant responses to reactive species. Although there is a high diversity in antioxidant defenses, many of them are evolutionary conserved across animal taxa. In particular, enzymatic defenses and heat shock response mediated by proteins show a low degree of variation. Importantly, activation of an antioxidant response may be also energetically and nutrient demanding. So knowledge of antioxidant mechanisms could allow us to identify and to quantify any underlying costs, which can help explain life-history trade-offs. Moreover, the study of inheritance mechanisms of antioxidant mechanisms has clear potential to evaluate the contribution of epigenetic mechanisms to stress response phenotype variation.

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