Effect of hematocrit on behavioral thermoregulation of the toad Bufo marinus

This study tested the hypothesis that the oxygen capacity of blood, indexed by hematocrit, affects the body temperature (Tb) selected by toads (Bufo marinus) in a thermal gradient. Anesthetized toads (Brevital sodium; 40 mg/kg im) were fitted with a cloacal thermistor and femoral artery cannula. After recovery, they were placed in a thermal gradient ranging from approximately 10 to 40 degrees C. Tb was measured for 24 h. Then 1-2 ml blood were withdrawn and replaced with an equal volume of either 0.66% saline or packed red cells from donor toads. Selected Tb was measured for the next 24 h. The mean Tb selected by B. marinus (n = 33) was 24.9 degrees C. Cross-sectional analysis showed no correlation between selected Tb and hematocrit. However, each toad rendered anemic showed a reduction of selected Tb (mean = 22.9 degrees C). The degree of Tb reduction was correlated with the hematocrit reduction, (r = 0.86; P = 0.013). Blood doping did not have a significant effect on selected Tb. The reduction of Tb could be adaptive by reducing oxygen consumption.