SOCKMAN, KW*; WEBSTER, MS; TALBOT, V; SCHWABL, H; Univ. of North Carolina, Chapel Hill; Washington State Univ.; Washington State Univ.; Washington State Univ.: Sex-Specific Effects of Yolk Androgens in American Kestrels
Female birds deposit high concentrations of androgen hormones into the yolks of their eggs as they are formed on the ovarian follicle. In several species, yolk-androgen concentrations increase with laying order within a clutch. This raises the possibility that embryonic and post-hatching effects of differential yolk-androgen deposition may modify the developmental hierarchy commonly observed among sibling nest-mates as a result of hatching asynchrony. In a few species, experimental elevation of yolk-androgen concentrations enhances nestling growth-rates, suggesting that in some cases, elevated yolk-androgen concentrations in later-laid eggs of a clutch might buffer the effects of hatching late. In the American kestrel (Falco sparverius), experimental elevation of yolk-androgen concentrations reduces nestling growth-rates. Thus, in this case, the natural elevation of yolk-androgen concentrations with laying order seems to augment the effects of hatching asynchrony. It is not surprising that the effects of yolk androgens on nestling growth may vary with species or context, because whether or not the parent would benefit from a sibling developmental hierarchy is likely to depend on environmental conditions and factors intrinsic to laying order and individual eggs, such as their sex. We used PCR and morphological traits to sex nestlings and found that experimentally elevated yolk-androgen concentrations reduced growth rates in male but not female nestling kestrels compared to controls. From a proximate perspective, such sex-specific effects of yolk androgens may be related to differences between the sexes in the expression of androgen receptors. From an ultimate perspective, these sex-specific effects may tune parental reproductive effort to differences between male and female offspring in their reproductive value.
