A field study of incline use and preferred speeds for the locomotion of lizards

The maximal running speeds of lizards have commonly been measured using level racetracks in a laboratory, but the normal speeds of locomotion of lizards and most animals in their natural habitats are not well documented. Thus we quantified the locomotor activity of the lizard Uma scoparia in its natural sand-dune habitat to determine how commonly animals run near their maximal speed and how inclines in the environment might affect habitat use and the speed of locomotion. We used leaf blowers to erase footprints in the sand in three 40 m x 100 m plots differing in surface topography and vegetation and then measured attributes of the paths and tracks left by U. scoparia that were not disturbed (by observers) during their period of peak activity. Laboratory observations allowed us to estimate speed for each field measurement of stride length (n = 5993). The frequency distribution of stride speed had two distinct modes (mean values 0.25 and 2.00 m/s), for which only the lower one is likely to be aerobically sustainable. Bipedal locomotion was extremely rare (<0.5% of all strides), but much undisturbed locomotion was unexpectedly fast. The inclines of the surfaces upon which locomotion occurred conformed to random habitat use. In contrast, the inclines of paths made by lizards had lower than expected amounts of nearly horizontal locomotion and a propensity for traveling almost directly uphill, especially when surfaces were steep. Habitat structure varied significantly among plots and affected several aspects of locomotion. For example, path length, average speed per path, and average magnitudes of path and surface inclines per path varied significantly among the three plots. Multiple regression analysis also revealed that the average speed per 1-m interval was maximized for paths with relatively shallow downhill slopes (-6% grade) and decreased significantly both with increased amounts of vegetative cover and increased turning between successive intervals. Unexpectedly, during undisturbed and escape locomotion, lizards used inclines similarly.