Birds Conserve Plasma Volume during Thermal and Flight-Incurred Dehydration

Dehydration tolerance in mammals is associated with the degree to which an animal is able to maintain plasma volume. During thermal dehydration, rock pigeons (Columba livia), quail (Coturnix coturnix japonica), and white-necked ravens (Corvus cryptoleucus) maintained nearly constant volumes despite mean body mass losses of 17%, 15%, and 13%, respectively. During flight-incurred dehydration in tippler pigeons (C. livia), no changes occurred in plasma volume although the birds lost 6%-10% of their preflight body mass. Thus, dehydrated birds were found to be excellent plasma volume conservers, like the most xericadapted mammal species. We suggest that plasma volume conservation in birds is an adaptation associated with high heat loads incurred during flight. In contrast, plasma conservation in mammals is associated with their ability to inhabit arid environments.