Maternal hormones as a tool to adjust offspring phenotype in avian species

Avian eggs contain substantial amounts of maternal hormones and so provide an excellent model to study hormone-mediated maternal effects. We review this new and rapidly evolving field, taking an ecological and evolutionary approach and focusing on effects and function of maternal androgens in offspring development. Manipulation of yolk levels of androgens within the physiological range indicates that maternal androgens affect behaviour, growth, morphology, immune function and survival of the offspring, in some cases even long after fledging. Descriptive and experimental studies show systematic variation in maternal androgen deposition both within and among clutches, as well as in relation to the sex of the embryo. We discuss the potential adaptive value of maternal androgen transfer at all these three levels. We conclude that maternal androgen deposition in avian eggs provides a flexible mechanism of non-genetic inheritance, by which the mother can favour some offspring over others, and adjust their developmental trajectories to prevailing environmental conditions, producing different phenotypes. However, the literature is less consistent than often assumed and at all three levels, the functional explanations need further experimental testing. The field would greatly benefit from an analysis of the underlying physiological mechanisms.