Dose-dependent effects of dexamethasone on hatching sex ratio in Chukarpartridge (Alectoris chukar)

It is well documented that birds are able to manipulate offspring sex ratio in response to different environmental, social, and hormonal factors. However, the literature on the effects of environmental stress and corticosterone (the main stress related glucocorticoid in birds) on offspring sex ratio is inconsistent. These discrepancies may occur because of differences in hormone dosage and application.For example, in contrast to oral administration, implants and injections of corticosterone may result in supra-physiological circulating concentrations followed by strong negative feedback on these concentrations. In the current study, dexamethasone, a high potent synthetic glucocorticoid, was twice daily orally administrated for 10 days in different dosages to Chukar partridges (Alectoris chukar), to study its effect on sex ratio of the eggs, as well as on circulating levels of glucose, cholesterol calcium and aminotransferase, which have been directly or indirectly linked to sex ratio. Ninety-six female partridges were randomly assigned to four treatment groups: Intact, Vehicle, DEX-low (1.15 μg/mL), and DEX-high (20 μg/mL). DEX-low females produced more male hatchlings (72.1%) compared to vehicle(40.5%), and intact (50.0%) females (P < 0.05), but the percentage of sons in DEX-high birds (52.9%)was not significantly different from the other three groups (P > 0.05). No significant differences were found in egg production and hatchability suggesting that sex-specific mortality of embryos does not explain the effect on hatchling sex ratio. Group differences in glucose concentrations showed a similar patterns of statistical significance as that of sex ratio that in turn correlated with glucose and cholesterol.The results show that the glucocorticoid can increase the sex ratio in birds, indicate that such effects can be dose dependent, and suggest that glucose may be part of the underlying mechanism.