Vitamin E deficiency in last-laid eggs limits growth of yellow-legged gull chicks

Parental phenotype and environment have cascading effects on offspring performance. Such 'maternal' effects are often mediated by the size and quality of the eggs in terms of maternal endogenous substances or exogenous components. Maternal effects can function to maximize parental fitness via adaptive allocation of components among sibling eggs. In birds, the often documented variation in concentration of egg dietary antioxidants or other egg compounds with laying order may thus reflect adaptive parental favouritism or maternal physiological constraints. Vitamins are found in the yolk at high concentrations and accomplish important physiological functions. Tocopherols ('vitamin E') have major roles in antioxidant protection, but the consequence of variation in their concentration for chick phenotype has never been experimentally investigated under a natural selection regime. In the yellow-legged gull (Larus michahellis) free-ranging population we studied, vitamin E concentration and egg size decline from the first to the third (last) egg. Chicks from third eggs are smaller and have lower viability compared to those from sibling eggs. Here, we tested the effect of injecting a physiological dose of vitamin E in the egg yolk on chick body size at hatching by comparison with chicks from sham-injected eggs. As predicted, chicks from vitamin E injected third eggs were larger than chicks from control third eggs, whereas vitamin E had no effect on chicks from the other eggs. Thus, vitamin E injection disrupted the brood size hierarchy according to laying order normally found in this species. In species that adopt a brood reduction strategy, differential allocation of vitamin E to the eggs can contribute to establish a hierarchy of reproductive value among progeny members functioning to maximize parental fitness. The present results therefore have general implications for the study of the evolution of maternal effects mediated by transfer of potentially limiting maternal antioxidants to the eggs.