No abstract

Summary1. 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 tradeoff 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.

Investment theory is founded on the premise that higher returns are generally associated with greater risk, and that portfolio diversification reduces risk. Here I examine parental investment decisions in birds from this perspective, using data from a model system, a 16‐year study of breeding red‐winged blackbirds Agelaius phoeniceus. Like many altricial birds, blackbirds structure their brood into core (first‐hatched) and marginal (later‐hatched) elements that differ in risk profile. I measured risk in two ways: as the coefficient of variation in growth and survival of core and marginal offspring from a given brood structure; and using financial beta derived from the capital asset pricing model of modern portfolio theory. Financial beta correlates changes in asset value with changes in the value of a broader market, defined here as individual reproductive success vs. population reproductive success. Both measures of risk increased with larger core (but not marginal) brood size; and variation in growth and survival was significantly greater during ecologically adverse conditions. Core offspring showed low beta values relative to marginal progeny. The most common brood structures in the population exhibited the highest beta values for both core and marginal offspring: many parent blackbirds embraced rather than avoided risk. But they did so prudently with an investment strategy that resembled a financial instrument, the call option. A call option is a contingent claim on the future value of the asset, and is exercised only if asset value increases beyond a point fixed in advance. Otherwise the option lapses and the investor loses only the initial option price. Parents created high risk marginal progeny that were forfeited during ecological adversity (the option lapses) but raised otherwise (the option called); at the same time parents maintained a constant investment and return in low risk core progeny that varied little with changes in brood size or ecological conditions.

No abstract

No abstract