Sex-specific patterns of yolk androgen allocation depend on maternal diet in the zebra finch

Females are predicted to adjust their reproductive investment in relation to resource quality. In zebra finches (Taeniopygia guttata), diet quality has been found to influence egg mass both between and within clutches. We tested the prediction that diet quality also affects the quantity of maternally allocated yolk testosterone and 5alpha-dihydrotestosterone (DHT) between and within clutches. We also investigated whether this pattern differed between male and female eggs. Females laid eggs on a high-quality (HQ) or a low-quality (LQ) diet. Eggs were removed at laying and artificially incubated for 72 h, after which time embryos were sexed and yolk androgens assayed. Diet treatments were then swapped and the experiment repeated. Because there was evidence of a carry-over effect between breeding rounds, we based our conclusions mainly on the results from the first breeding round. On the HQ diet, but not on the LQ diet, infertile eggs contained more testosterone than did fertile eggs in round one. Although there were no overall differences in within-clutch patterns of androgen deposition between the diets, this changed when embryo sex was taken into account. On the HQ diet, testosterone decreased with laying sequence for male eggs but increased with laying sequence for female eggs. On the LQ diet, mothers' male eggs contained more testosterone and DHT than did female eggs regardless of position in the laying sequence. Our data suggest that there are complex, context-dependent mechanisms of sex-specific androgen allocation in this species.