Decreased brain serotonin activity is consistent with transportation stress in both sexes at different ages while circulating corticosterone levels have both age- and sex-related differences in Pekin ducks.

Previous work from our lab has demonstrated age and sex-dependent differences in ducks' neurochemical responses to transportation stress. To determine whether sex differences observed in adults are present prior to sexual maturation, we evaluated 5-day-old ducklings (n = 10/treatment), 23-week-old, and 35-week-old ducks (n = 6/sex/treatment). For all ages, the ducks were randomly assigned to one of three treatment groups: control (CON), immediately euthanized in their home pens; crate (CRA), caught, crated, and kept in pen for 90 minutes; or transport (TRA), caught, crated, and transported for 90 minutes. Brains were removed and microdissected into the caudal mesencephalon (CM), rostral mesencephalon (RM), and diencephalon (DI), medial telencephalon (MT) and lateral telencephalon (LT). Serotonin (5-HT), dopamine (DA), and their metabolites were quantified via mass spectrometry, and monoamine turnover ratios were calculated. Data were analyzed using ANOVA. For the 5-day-olds, there was a significant increase in 5-HT turnover from CRA to TRA (P = 0.0030) compared to CON but no significant differences in corticosterone levels among treatments. For the 23-week-olds, an increase in serum corticosterone was observed for CRA hens compared to CON hens (P = 0.0017) and TRA hens compared to the CON hens (P = 0.0017). The TRA drakes had higher serum corticosterone levels compared to the CON drakes (P = 0.0439). The TRA hens had a significant increase compared to the TRA drakes (P = 0.0131). The hens exhibited an increase in 5-HT turnover from CON to CRA (P = 0.0123) and from CRA to TRA (P = 0.0017). For the 35-week-old, drakes showed an increase in corticosterone levels compared to hens (P = 0.0001) and in the CM, drakes showed an increase in 5-HT turnover compared to the hens (P = 0.001). Collectively, transportation stress altered serotonergic activity at all ages examined, but the presence and direction of sex differences were developmentally dependent. These findings strongly support a developmental role for gonadal steroid influence and underscore the importance of considering both age and sex when evaluating neurobiological stress responses and welfare outcomes in poultry species.