Yolk androgens as pleiotropic mediators of physiological processes: a mechanistic review

Avian egg yolks are made up of complex mixtures of physiologically relevant substances. Androgens, in particular, accumulate in yolks in variable amounts based on social and environmental conditions experienced by laying females, and using experimental elevations of yolk androgen content, researchers have unveiled potent physiological and behavioral effects on offspring. These patterns and effects are exciting in an adaptive context, as the transfer of endogenously-produced substances such as androgens to egg yolks may allow fine manipulation of offspring phenotype to maximize reproductive success. However, to gain an in-depth understanding of how yolk androgens function in an adaptive context, we must first understand the complex entanglement of physiological and endocrine interrelationships that change after exposure to yolk androgens. Here, we take a comparative approach towards a discussion of how yolk androgens can simultaneously affect multiple body systems within developing birds, ultimately resulting in the large-scale phenotypic endpoints that may represent adaptive consequences of exposure to elevated levels of yolk androgens.