Climbing without feet: Effect of surface-feature size on the physics of vertical snake locomotion

Arboreal environments present numerous physical constraints that are particularly challenging for animals like snakes which lack appendages that many animals use for gripping, propulsion, and stabilization. Tree trunks and branches of varying diameter, flexibility, length, and surface roughness provide these animals with varying “footholds” which they can use to support their weight. Here we created a simplified model of a large-diameter tree consisting of a flat, smooth, vertical surface with force-sensitive pegs acting as surface features. Using 3D kinematics acquired from IR-marker tracking combined with time-resolved force data, we analyze how the distribution of forces over the snakes’ bodies changes with varying peg length. Ordinarily, in corn snakes, strong vertical forces over the forward and midsections of the body support much of the snakes’ weight while strong lateral forces are applied over the body for stability during ascents. With shorter surface features, these lateral forces are reduced, and the snakes rely on comparatively greater vertical components of force over the forward and midsection. In descents, the rear third of the body anchors the animal by applying strong vertical forces, however, these forces are diminished as feature length decreases, inhibiting the snakes’ ability to move. Further work will investigate how species specialization and surface textures on the snake affect physical interactions and overall success of locomotion.