Plasma volume conservation in pigeons: effects of air temperature during dehydration.

We assessed the effects of the ambient air temperature prevailing during dehydration, which included complete water and food deprivation, on plasma volume (PV) conservation in pigeons (Columba livia) exposed to air temperatures of 25 degrees C (n = 7), 36 degrees C (n = 12), and 40 degrees C (n = 24). The pigeons were dehydrated for 65.4, 32.3, and 27.7 h, on average, at 25, 36, and 40 degrees C, respectively. Dehydration rates averaged 5, 10, and 15.2% of initial total body water (TBW) per day at the three air temperatures, in ascending order. Birds exposed to 25 and 36 degrees C maintained PV despite the fact that they lost, on average, 14 and 17.3% of their TBW, respectively. In contrast, PV in pigeons exposed to 40 degrees C decreased by 8.9%, even though the reduction in TBW (17.7%) was similar to that incurred at 36 degrees C. Mean plasma osmolalities reached 347.7, 345.6, and 374.8 mosmol/kgH2O at 25, 36, and 40 degrees C, respectively. No significant changes occurred in hematocrit (Hct) and hemoglobin concentrations (Hb) during dehydration at 25 and 36 degrees C. However, at 40 degrees C Hb increased by 8.2% with no change in Hct, indicating a decrease in red blood cell volume (RBCV). The results of this study suggest that maintenance of PV and RBCV is affected not only by the dehydration level attained but also by the ambient conditions under which dehydration is induced and the rate at which dehydration takes place.