Sex differences in serum glucocorticoid levels and heterophil:lymphocyte ratios in adult pekin ducks (Anas platyrhynchos domesticus)

•Sex differences in glucocorticoid release.•Sex differences in heterophil to lymphocyte ratios.•Disconnect between glucocorticoid release and change in heterophil to lymphocyte ratio.•Both cortisol and corticosterone released in response to ACTH. It is becoming more common for poultry scientists to utilize direct and indirect measures of stress hormones to monitor bird welfare. However, it has been clear that our understanding of the avian hypothalamic–pituitaryadrenal axis (HPA) is insufficient as evidenced by the many conflicting reports regarding stress responses, such as transportation stress, in poultry. It has long been assumed that the poultry HPA functions similarly to that of mammals, but now we know that there are considerable differences in the avian HPA compared to mammals. Synthesis and release of glucocorticoids (GC) are stimulated by adrenocorticotropin hormone (ACTH); GC are synthesized from a common pathway that begins with cholesterol and pregnenolone. The synthesis of one of the glucocorticoids does not depend upon the synthesis of the other. The purpose of our study was to test the hypothesis that ACTH will stimulate both corticosterone and cortisol release in ducks. To test this hypothesis, we injected artificial ACTH (cosyntropin; 0.0625 mg/kg, 0.031 mg/kg, or 0.016 mg/kg or saline as control) intramuscularly into adult drakes and hens (N = 10/sex/dose). Both glucocorticoids (GC) were assayed in serum using previously verified ELISAs. Blood smears were also assessed for heterophil to lymphocyte ratios (HLR). Data were analyzed by repeated measures 3-way ANOVA with Fishers PLSD as an ad hoc test. We observed that both GC were secreted in significantly (p = 0.0002) different patterns in a dose-dependent manner compared to controls, and that there was a significant (p = 0.0001) sex difference in both GC compared to saline controls. Further, we observed that all doses of ACTH elicited a significant (p = 0.004) sex difference in the HLR response compared to controls, but no dose-dependent effects were noted. Our data suggest that ducks, at least, may utilize more than just corticosterone to maintain physiological homeostasis in response to stress. Further, the time course of the stressor to release GC and subsequent HLR response may be dependent upon sex. More detailed analyses of the HPA are necessary in all avian species to better understand stress responses as we utilize biological bases for welfare assessments and stress responses.