Haematological adjustments with diurnal changes in body temperature in a lizard and a mouse.

l. Hypothermic lizards (Egernia ctmninghami) showed significant reductions in plasma volume, haematocrit and haemoglobin concentration. The changes in the distribution of red cells were acutely reversible when body temperature was increased. 2. Consequently, there were no significant alterations in blood viscosity (when measured in z&o in a capillary viscometer) between body temperatures of 32°C (preferred body temperature) and 20°C. 3. In contrast, mice (Peromyscus leucopus) showed no significant changes in haematocrit, haemoglobin concentration or red cell count, associated with diurnal torpor. 4. These results are discussed in relation to haemofluidity and optimization of oxygen transport.