LOCOMOTOR PERFORMANCE AND ACTIVITY ENERGETICS OF HELODERMATID LIZARDS

Locomotor performance and activity energetics were analyzed in 19 indi vidual helodermatid lizards ranging in mass from 24.5-1220 g. We measu red maximum sprint speeds, endurance on a treadmill, cost of transport , and aerobic capacity (VO(2)max) 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 e xpected for a lizard of this size at 31 C. At lower speeds, adult Helo derma could run for prolonged periods. The cost of transport for a 500 g Heloderma at 0.4 km/h was 0.46 ml O-2g(-1) km(-1), which is compara ble to the cost of transport in other lizards. The two Heloderma speci es showed no significant differences in VO(2)max, cost of locomotion, endurance, or sprint speeds. Neither cost of locomotion nor aerobic ca pacity differed between captive and wild Heloderma. At 31 C, heloderma tids had a high VO(2)max, resulting in the highest factorial aerobic s cope (30.4) of any lizard measured to date. The high aerobic capacity of Heloderma seems surprising for a sedentary lizard but may be advant ageous during intensive male-male agonistic behaviors. Males had signi ficantly higher VO(2)max than did females, suggesting that sexual sele ction may favor males with high aerobic capacities.