Thyroid and adrenal responses of ducks (Anas platyrhynchos) during saline adaptation

Fresh water-reared Aylesbury ducklings were given 0.1 or 0.2 M NaCl drinking water and variations in the plasma concentrations of thyroxine (T4), triiodothyronine (T3), corticosterone, and aldosterone were determined. Within 24 hr of being transferred to 0.2 M NaCl the corticosterone concentration was increased, whereas the levels of plasma T3 and aldosterone were reduced. The increase in corticosterone secretion persisted for at least 7 days, but the T3 and aldosterone concentrations had returned to control levels within 5 days. The concentrations of plasma aldosterone and T3 were similarly affected in birds transferred to 0.1 M NaCl, although corticosterone levels increased only after the birds were maintained on 0.1 M NaCl for 7 days. Plasma concentrations of T4 were increased in 1- to 2-week-old (but not 7-week-old) birds 7 days after their transfer to 0.1 and 0.2 M NaCl. In birds maintained on saline for 6 weeks the basal concentration of each hormone was not significantly different from that in fresh water-reared controls. When saline-reared birds were returned to freshwater conditions the plasma aldosterone and T3 concentrations were increased over a 5-day period. Transfer to saline consistently reduced body weight and food intake, whereas the transfer of saline-reared birds to fresh water stimulated body weight gain. Consequently, although these endocrine responses during saltwater adaptation might be expected to minimise the loss of body reserves and to facilitate the extra renal excretion of sodium, these alterations in endocrine function may be partly due to salt water-induced changes in food intake.