Maternal androgens in egg yolks: relation with sex, incubation time and embryonic growth

Hormones of maternal origin are known to be transferred to the egg yolks of oviparous species. Several studies have shown that within and between clutch variation of maternal androgens may be adaptively tuned. Moreover, it has recently been hypothesized that sex steroids of maternal origin may play a role in adaptive sex ratio manipulation. For sex determination the eggs have to be incubated to allow the germinal disc to grow and thus extract sufficient DNA. This means that yolk hormone levels are determined after a number of days of incubation and this may hamper interpretation of the data. If yolk utilization or embryonic hormone production are influenced by the sex of the embryo, differences in hormone content at a certain stage of incubation do not reflect the mother's initial allocation. In this experiment we show that testosterone levels in chicken eggs do not change with incubation period. A4 levels decrease between 3 and 5 days of incubation, which we cannot explain. Male eggs did not contain higher levels of testosterone or androstenedione than female eggs, in contrast to the data reported for another galliform species, the peacock. We conclude that it is unlikely that maternal androgens are a key factor in the avian sex determination mechanism.