Maternal testosterone affects the primary sex ratio and offspring survival in zebra finches

Female birds have repeatedly been reported to adjust the primary sex ratio of their offspring to environmental, social and physiological cues. However, the mechanism behind sex adjustment remains unknown. It has been suggested that maternal hormones may constitute an important mediator in this mechanism, as androgen levels differ between eggs bearing male and female embryos. To evaluate whether the level of maternal androgens affects the offspring sex ratio, we injected female zebra finches, Taeniopygia guttata, with testosterone during egg laying. The sex ratio of eggs laid after testosterone administration became significantly male biased, compared to eggs laid by control females that received a vehicle injection. However, sons of testosterone-treated females suffered lower hatching success. In contrast, daughters seemed to benefit from elevated androgen level in terms of future survival prospects. The opposite effects on male and female offspring may constitute an important constraint on maternal androgen allocation to the eggs and reduce the benefits of biasing the sex ratio towards males by increasing the testosterone level.