Redistribution of extracellular water and sodium may contribute to saline tolerance in wild ducks.

The compartmentalization of body fluids was measured in three species of ducks that differ in saline tolerance. Half of the birds of each species drank freshwater, while the other half drank saline (300 mM NaCl). Among ducks that drank freshwater, total body water (TBW) was similar among all species, but Barrow's goldeneyes (Bucephala islandica), the most marine species, had larger extracellular fluid volume (ECFV) than freshwater mallards (Anas platyrhynchos) or estuarine canvasbacks (Aythya valisineria). When acclimated to saline, only goldeneyes shifted extracellular water and Na+ into the intracellular compartment. ECFV was correlated with plasma aldosterone concentration in goldeneyes, but not in canvasbacks (aldosterone was not measured in mallards). Data summarized from the literature showed that TBW does not differ among terrestrial, freshwater, or marine species, but marine species have a larger part of their TBW in the extracellular compartment. Saline induced movement of extracellular water and Na+ into the cells only in goldeneyes. ECFV and redistribution of extracellular water and Na+ into the cells may be important components in saline tolerance of marine birds.