Does oxidative stress mediate the trade-off between growth and self-maintenance in structured families

1. Life-history theory predicts a trade-off between growth and self-maintenance. However, little is known about the underlying mechanisms. Oxidative stress, the imbalance between the production of reactive oxygen species (ROS) during normal metabolic processes and the level of antioxidant defences, is a potential mechanism. 2. In many altricial birds asynchronous hatching creates different castes of progeny as a hedge against developmental and environmental uncertainty (a 'structured' family). Older, 'core' offspring and later hatched 'marginal' offspring consequently experience contrasting levels of resource availability during development. To be adaptive the phenotypic handicap incurred by marginal offspring must be reversible when conditions allow. 3. Dietary-derived antioxidants are an important component of antioxidant defences, which modulate oxidative damage arising from normal metabolic processes such as growth. Dietary antioxidants could therefore be important determinants of trade-offs during growth, with the balance of these trade-offs differing between core and marginal offspring. 4. To investigate this, we manipulated brood size and structure as well as antioxidant availability (by vitamin and mineral supplementation), then measured growth and oxidative damage in nestling red-winged blackbirds Agelaius phoeniceus. 5. We found that individuals supplemented with dietary antioxidants allocated these extra resources to increasing growth rate, rather than reducing oxidative damage. Irrespective of hatch rank, nestlings that received supplementary antioxidants grew faster than controls, despite no difference in their begging behaviour or parental provisioning rates. There was no overall difference in the level of lipid peroxidation (malondialdehyde; MDA) in the blood plasma of supplemented and control chicks, but among marginal offspring those that received an antioxidant supplement had lower plasma MDA. Antioxidant supplementation did not affect nestling survival to fledging age. 6. These novel results support the idea that oxidative stress may play a central role in the trade-off between growth and self-maintenance. Moreover, they further highlight the importance of physiological costs in the evolution of life-histories and propose a role for oxidative stress in the adaptive basis of the structured family.