Abstract
Recently, evidence is mounting that females can adaptively engineer the quality of their offspring via the deposition of yolk compounds, including carotenoids and androgens. In this study, we simultaneously consider how both carotenoids and androgens in egg yolk relate to parental quality in barn swallows (Hirundo rustica erythrogaster). First, we found no relationship between concentrations or amounts of yolk androgens and carotenoids. Yolk carotenoids decreased with laying order, whereas we found no relationship between yolk androgens and laying order. Second, we tested the Investment Hypothesis, which predicts that high-quality females or females paired to high quality mates, allocate differentially more of these yolk compounds to their offspring. For carotenoids, we mostly found evidence to counter predictions of the Investment hypothesis: (1) Carotenoid concentrations varied among females, (2) heavier eggs contained lower carotenoid concentrations, although heavier yolks contained greater amounts of carotenoids, (3) eggs of earlier-laying females had lower concentrations in their eggs, and (4) yolk carotenoids were not correlated with clutch size or male plumage ornamentation. For androgens, we found weak support for the Investment Hypothesis: (1) Yolk androgens varied among females, (2) heavier eggs and yolks contained greater amounts, although not concentrations of androgens, (3) females paired to more colorful males laid eggs with greater concentrations of androgens, and (4) no effects of laying date or morphological correlates of female quality on androgen concentrations in egg yolks. Overall, these findings suggest that each yolk compound may have different functions and therefore may be regulated by different mechanisms.
