Sex differences in glucocorticoid responses to shipping stress in Pekin ducks

Some concerns have been raised recently about the assay of corticosterone vs. cortisol in poultry species. Thus, we tested the hypothesis that ducks secrete both glucocorticoids. First, we validated two commercially ELISA kits for the two glucocorticoids by first charcoal stripping duck serum in order to remove all steroid hormones. We ran serial dilutions of spiked, charcoal-stripped serum on kits of opposite glucocorticoid as well as a serial dilution using the respective ELISA buffer of the opposite assay kit. We found that the glucocorticoid standard curve in duck serum matched the respective curve in that kit's own buffer. However, when the opposite hormone was run in each kit in both duck serum or ELISA buffer, a near zero slope was obtained. Second, we further validated the presence of both glucocorticoids using mass spectrometry. Third, we tested the hypothesis that exogenous ACTH would stimulate the release of both corticosterone and cortisol. And, fourth, we tested the hypothesis that each glucocorticoid would have different serum levels in response to shipping stress. To test this hypothesis, we collected serum from 10 drakes and 10 hens from 2 flocks (N = 20 per time point per sex): 24 h prior to shipping, at shipping as ducks were walked off the truck, 24 h after shipping, and 1 wk after shipping. Data were analyzed by 2-way repeated measures ANOVA. Surprisingly, we also observed a sex difference in both glucocorticoid levels in that hens showed higher (P < 0.01) serum levels than did drakes at all-time points in response to either ACTH or transportation. Finally, no differences were observed in either glucocorticoid levels associated with shipping in either sex. The fact that both glucocorticoids are released in measurable amounts lends to the possibility that they may be differentially regulated, or at least there is a sex difference in the neural pathways associated with glucocorticoid release in ducks. Although corticosterone is the likely predominate glucocorticoid in ducks, serious attention should be given to the role of cortisol in poultry. Further consideration of sex, age, and timing of blood collection to stressor needs to be considered when assessing glucocorticoid levels in any avian species.