Locomotor Performance and Activity Energetics of Helodermatid Lizards

Abstract

Locomotor performance and activity energetics were analyzed in 19 individual helodermatid lizards ranging in mass from 24.5-1220 g. We measured maximum sprint speeds, endurance on a treadmill, cost of transport, and aerobic capacity (rO2max) at a mean body temperature of 30.8 C. The maximum sprint speed for both Heloderma species averaged 1.7 km/h, which is among the lowest recorded for lizards. A typical Heloderma had an endurance time of 16.0 min at 1.0 km/h, which is higher than expected for a lizard of this size at 31 C. At lower speeds, adult Heloderma could run for prolonged periods. The cost of transport for a 500 g Heloderma at 0.4 km/h was 0.46 ml 02, -' km-', which is comparable to the cost of transport in other lizards. The two Heloderma species showed no significant differences in VO2max, cost of locomotion, endurance, or sprint speeds. Neither cost of locomotion nor aerobic capacity differed between captive and wild Heloderma. At 31 C, helodermatids had a high rO2max, resulting in the highest factorial aerobic scope (30.4) of any lizard measured to date. The high aerobic capacity of Heloderma seems surprising for a sedentary lizard but may be advantageous during intensive male-male agonistic behaviors. Males had significantly higher VO2max than did females, suggesting that sexual selection may favor males with high aerobic capacities.