Negative effects of early developmental stress on yolk testosterone levels in a passerine bird

Female birds incorporate in the yolks of their eggs significant concentrations of a number of different androgens. Yolk androgen has been shown to positively affect several fitness components at the embryo, nestling and juvenile stages. Previous experiments have shown that females lay eggs with higher androgen concentrations when they are paired with highly ornamented males. This pattern suggests that yolk androgens are costly to females. In this study, we experimentally manipulated adult female condition in zebra finches Taeniopygia guttata by modifying the level of developmental stress they suffered as nestlings. This was achieved by cross-fostering nestlings to broods of varying brood size. Subsequently, we measured the yolk testosterone contents of the female offspring that resulted from the experimental manipulation. As predicted, females deposited decreasing concentrations of testosterone with increasing brood sizes experienced as nestlings: testosterone concentration (mean +/- S.E.M.) of eggs laid by females from small broods, 20.66 +/- 2.08 pg mg(-1); medium broods, 15.32 +/- 1.94 pg mg(-1); and large broods, 14.51 +/- 1.66 pg mg(-1). Additionally, testosterone concentration decreased with laying order, and varied with clutch size in a complex way. Differences in egg testosterone between females exposed to different brood sizes are in line with previous findings in showing that early developmental stress can affect adult reproductive performance, although our study did not detect an effect in other breeding parameters, such as latency to breed or clutch size. Furthermore, the results are consistent with the hypothesis that there is a cost associated with yolk testosterone. However, it is still unclear what the nature of this cost may be, and whether it is paid by females, offspring, or both.