Maternally derived androgens and antioxidants in bird eggs: complementary but opposing effects?

Maternally derived traits, such as within-clutch variation in the amount of testosterone deposited in egg yolks, may have profound effects on offspring fitness. Offspring with elevated levels of testosterone may benefit from increased competitive ability through effects on aggression and growth rate. However, elevated levels of testosterone are also associated with costs of increased peroxidative damage from free radicals and consequent oxidative stress. Diet-derived antioxidants, such as vitamin E and various carotenoids, provide protection against the deleterious effects of oxidative stress. Here we show that within-clutch variation in yolk testosterone is the opposite to that of yolk antioxidant concentration in the lesser black-backed gull Larus fuscus. We provide evidence that suggests that these two direct maternal effects are, in fact, complementary and, in conjunction with an indirect maternal effect (the onset of incubation), may provide an adaptive mechanism for parental favoritism in response to environmental variability. The potential implications of these findings with respect to previous investigations on variation in yolk testosterone concentrations and on the understanding of intrafamilial dynamics are discussed.