Variation in Locomotor Performance in Demographically Known Populations of the Lizard Sceloporus merriami

We examined correlates (sex, size, age) of variation in maximal burst speed (racetrack) and in stamina (time until exhaustion at 0.5 km . h-1) in twopopulations of the iguanid lizard Sceloporus merriami in Big Bend National Park, Texas. Because these populations were known demographically and were studied in several years, we were able to examine the effects of adult age on performance, the between-year repeatability of individualperformance, and environmental correlates ofyearly variation in performance. Variation in locomotor capacities was striking. Males were faster than females, and lizards from the Grapevine Hills were faster than those from Boquillas (lower elevation). However, sex andpopulation did not affect stamina. Young adults (1-yr-olds) were slightly faster but had slightly less stamina than did older adults (2-3-yr-olds). Adult size (mass, snout-vent length, hind limb length) had little efect on performance. Average performance variedsignificantly among years, but stamina varied more than did speed. Stamina was highest in a summerpreceded by relatively wetfall and winter. Despite the between-year variation in average performance, individual speed and stamina are both significantly repeatable over at least 1 yr. Neither tail loss nor toe loss affected sprint speed, but body temperature had a marked effect on speed.