Closer to the Heart: Cardiac Muscle Aerobic Capacity Correlates With Intraspecific Variation in Sprint Performance Rather Than Androgen Levels in the Neotropical Lizard <i>Tropidurus catalanensis</i>

Assessments of the interplay between physiology and whole-organism performance are fundamental to understand how individuals function in different ecological contexts. Here, we investigated the relationship between locomotor performance, androgen levels, and metabolic capacity of muscle tissues in the lizard Tropidurus catalanensis. We hypothesized that faster individuals would exhibit higher circulating androgen concentrations and greater metabolic capacity in skeletal and cardiac muscles, regardless of body size. We measured morphological variables, maximum sprint speed (v), plasma testosterone concentration, and the maximum activity of the enzymes lactate dehydrogenase (LDH) and citrate synthase (CS) in the gastrocnemius, iliofibularis, and cardiac muscles of adult males. We found that intraspecific variations in v were not explained by body size, plasma testosterone concentration, nor by the activity of LDH or CS in skeletal muscles. The absence of an effect of testosterone on locomotion suggests that androgen concentrations may change in response to other factors, such as environmental stressors or reproductive state. Our results indicated that the fastest lizards also had the highest CS activity in the heart. This relationship suggests that cardiac oxidative capacity plays an important role in clearing metabolites in the postexercise recovery phase. We also found a positive relationship between CS and LDH in all tissues, suggesting a functional complementarity between glycolytic and aerobic pathways that should be relevant in situations that require rapid alternation between bursts of speed and endurance, such as predator evasion or thermoregulation. Ultimately, our results highlight the importance of integrating performance and physiological traits to understand interactions between animals and their environment.