Prenatal manipulation of yolk androgen levels affects egg sizebut not egg colour in a songbird

Abstract

Maternal allocation of androgens to the egg yolk allows mothers to adaptively manipulate offspring phenotype. Increases of eggandrogen levels have often been shown to induce sex-specific effects. Some previous studies suggest that females specificallymay suffer a fitness reduction after early exposure to high androgen levels. In this study, we explored whether female birds thatdeveloped exposed to high yolk androgen concentrations would reduce their maternal investment later in life. We explored twodifferent aspects of female reproduction in spotless starlings (Sturnus unicolor): egg coloration (a post mating sexual signal in thisspecies) and egg size. We manipulated androgen yolk levels in a large sample of clutches of this species. We monitored thereproduction of the females that hatched from these eggs, measuring the size and colour of the eggs they laid throughout their life(from 1 to 7 years of age). We found no overall difference in egg colour in relation to treatment, clutch number or age. However,females exposed to high egg androgen levels showed a steeper decrease in egg size along the laying order than controls. Thispattern likely results in a more unbalanced distribution of resources within the clutch, possibly favouring brood size reductions inexperimental females. In addition, control and experimental females differed in how egg volume changed with age. These resultsshow that some egg characteristics may be affected by the female’s exposure to yolk androgens during her own embryonicdevelopment. Our research calls for further long-term research on the influence of prenatal androgens on the fitness mechanismsregulating reproductive investment and its potential signalling role in a perspective of sexual selection.