Individual variation in maximum aerobic capacity: cardiovascular and enzymatic correlates in Rana catesbeiana

We quantified natural variation in maximum aerobic capacity (V02max) exhibited by a free-living population of bullfrogs (Rana catesbeiana) and examined the degree to which such variation is associated with key parameters of the systemic oxygen transport apparatus and oxidative enzyme (citrate synthase) activity at the tissue level. Regression analysis of these data revealed that only ventricle mass and hemoglobin concentration accounted for significant fractions of the variation in V02max. Neither variation in maximum heart rate nor in citrate synthase activity were significantly correlated with individual variation in maximum aerobic capacity. These results support the contention that, in at least some taxa, maximum aerobic capacity is limited by the ability of the cardiovascular system to deliver oxygen to the tissues.