Timing matters: corticosterone injections 4 h before ovulation bias sex ratios towards females in chickens.

Birds have the ability to influence offspring sex prior to egg laying and may use hormones to mediate these skews. Corticosterone is of particular interest as a mediator of offspring sex because, as the primary stress hormone in birds, it regulates responses to environmental and social stimuli that trigger sex ratio biases. In previous studies in birds, chronic elevations of corticosterone stimulated female-biased sex ratios while acute pharmacological elevations that were provided 5 h prior to the expected time of ovulation stimulated male-biased sex ratios. Here, we aimed to determine the magnitude of corticosterone necessary to influence offspring sex and to further pinpoint the timing of the hormonal influence. Because high-dose injections of corticosterone stimulated male-biased sex ratios in hens, we hypothesized that females receiving acute pharmacological elevations of corticosterone would produce more male offspring while females with acute physiological elevations would produce an intermediate proportion of males compared to controls. We tested our hypotheses in laying hens by elevating corticosterone in the physiological or pharmacological range through injections of corticosterone administered 4 or 5 h prior to the expected time of ovulation. Contrary to our hypothesis, a physiological dose of corticosterone provided 5 h prior to the expected time of ovulation did not bias offspring sex ratios when compared to controls. Further, when corticosterone injections were given at 4 h prior to the expected time of ovulation, sex ratios were instead biased towards females. These results suggest that the timing and magnitude of the corticosterone elevation are both critical not only to whether a sex ratio bias occurs, but also the direction of the bias.