Maternal allocation of androgens and antagonistic effects of yolk androgens on sons and daughters

Mothers can influence the phenotype of their offspring by adjusting the quality of their eggs in relation to sex and reproductive value of the progeny. Maternal androgens in the eggs of vertebrates may mediate such adaptive early maternal effects. However, the evolution of early maternal effects mediated by egg androgens may be constrained by the inability of mothers to differentially allocate androgens to eggs with a male or a female if androgens have different effects on sons and daughters. In this study, we increased the concentration of androgens in the eggs of barn swallows (Hirundo rustica) within the physiological range of variation and analyzed the effect on nestling growth and begging behavior. Egg androgens increased body size and mass of sons but reduced these characters in daughters when compared to two control groups in a repeated-measures analysis of variance of data collected at different ages. However, the differential effect of androgen on the two sexes was no longer significant when the analysis was restricted to the age of 12 days, when final body size is attained. In a second experiment, we tested whether mothers differentially allocated androgens to eggs with sons rather than daughters while manipulating a paternal secondary sexual character. Androgen concentration did not vary in relation to paternal ornamentation or embryo sex. Hence, antagonistic effects of egg androgens on sons and daughters may exist in the very early posthatching life and may constrain the evolution of adaptive maternal effects because mothers do not differentially allocate androgens in relation to embryo sex. Copyright 2006.