WALKING THE TIGHT ROPE - ARBOREAL SPRINT PERFORMANCE AMONG SCELOPORUS-OCCIDENTALIS LIZARD POPULATIONS

We document variation in the degree of arboreality among four populations of the western fence lizard (Sceloporus occidentalis), and investigate how this variation relates to locomotor performance capability in an arboreal context (e.g., artificial branches). We raced lizards on four different rod diameters, measured maximum sprint speed, and also scored agility on the rods. Lizards from more terrestrial populations ran faster on thick rods, but slower on thin rods than did lizards from more arboreal populations. The terrestrial lizards experienced a greater difference in sprint speed than did the more arboreal lizards over the range of rod sizes we used. Arboreal lizards were more adept at running on all surfaces in that they stumbled and fell less often, particularly from the thinnest rods. Thus, specialization for maximizing sprinting performance on arboreal surfaces of a particular diameter entails a performance trade-off that is reflected in both speed and "sure-footedness." We investigated whether the population differences in arboreal sprinting are correlated with the significant differences in hind-limb span found among populations. We found that lizards with relatively longer limbs sprinted faster. By using juveniles reared in a common laboratory environment, we demonstrated that these among-population differences in arboreal sprint performance are probably genetically based.