How Small, Arboreal Lizards Run on Water: Surface Tension, Morphology, Behavior, and Kinematics.

The vast majority of water-walking animals are tiny invertebrates. Most water running vertebrates are larger and specialized for this behavior. As size increases, the role of surface tension decreases and inertial mechanisms dominate. We report how Anolis sagrei, a small, quadrupedal, arboreal lizard with adhesive pads runs on water, using behavioral observations, high-speed video, and decreasing surface tension with a surfactant. We quantified how frequently anoles ran on water, and if this was affected by reduced surface tension. We also compared their running performance and kinematics in the two types of water. Finally, we tested whether certain morphological features affected performance, and what kinematic variables were used to modulate performance in water. We found that decreasing surface tension resulted in a decreased frequency of running on water, but no change in performance, likely due to kinematic compensation. Specifically, anoles ran successfully in 17% of trials and this decreased to 9% with surfactant. Performance, measured as average velocity, Froude number, and the proportion of the body out of the water were unaffected by reduced surface tension. Anoles used faster slaps and shorter undulatory wavelengths when surface tension was reduced. Relative mass and limb characteristics did not affect water running performance. Increased stride and undulatory frequencies were the primary strategies for increasing water running performance. The frequency of behavior may help reconcile past findings, where decreased surface tension decreased water running performance in house geckos but not basilisks.