Functional consequences of microscopic skin features on snake locomotion

Interactions between limbless animals and their natural environments, which are essential for movement, are mediated solely through skin contact. We used atomic force microscopy to investigate how surface textures on shed skins vary across snakes and environments. While most snakes have microscopic spikes oriented from head to tail, a few distantly related snake species have convergently lost these features in favor of a more isotropic structure. We hypothesize that these microstructures affect the frictional interaction with the substrate and we use resistive force theory to model the effects of frictional anisotropy on snake locomotion. For lateral undulation, we predict that an anisotropic frictional interaction in movement along the body is favored over transverse movement, improving performance (measured in distance traveled per cycle), and that larger anisotropies produce larger displacements. In sidewinding locomotion, however, we predict the opposite trend: decreased frictional anisotropy improves performance. These predictions are consistent with our AFM measurements of small-scale features on snake skins and suggest a functional benefit for the convergent loss of structure shared by sidewinding vipers.