PINSHOW, Berry; Ben-Gurion University of the Negev: Plasma volume maintenance during long distance flight in birds vs. plasma volume maintenance in desert mammals: different origins for analogous traits

Both mammals and birds defend plasma volume under stressful, dehydrating environmental conditions. In mammals, plasma volume maintenance has been studied in a variety of desert species and it is apparent that adaptation to desiccating conditions has taken place in numerous taxa with varying use of the repertoire of behavioral and physiological mechanisms available, including evading stressful environmental conditions; increasing urine concentration, reducing volume and forming insoluble nitrogen end products; desiccating feces; recycling exhaled water vapor by temporal counter-current heat exchange; reducing transcutaneous evaporation; having labile body temperatures. Birds too use most of these mechanisms to save water, and like mammals, they closely defend plasma volume during dehydration by redistributing blood flow. However, maintenance of plasma volume is imperative to birds during flapping flight and data show that they do this as well as the best-adapted desert mammals, but not always by the same physiological mechanisms. I suggest that the selective pressures on birds that brought about their adaptations to withstanding dehydrating conditions evolved as requirements for long distance flight and are “exaptations” to living and functioning in other desiccating environments.