12-2 Thursday, Jan. 4 08:15 - 08:30 Snakes traversing large step obstacles: behavior, gait, and performance GART, SW*; MITCHEL, TW; LI, C; Johns Hopkins University email@example.com http://li.me.jhu.edu
Forest-, river valley-, and desert-dwelling snakes often traverse complex 3-D terrain like rocks and felled trees with variable surface height and frictional properties. Despite many studies on simple flat surfaces or branches, how snakes move in such complex 3-D terrain is less known. Here, we studied how the variable king snake (Lampropeltis mexicana) traversed a large step obstacle (up to 25% of snout-vent length) and tested how step height and surface friction affected traversal (3 animals, 101 trials). We discovered that, to traverse, the animal first lifted its head onto the step, and then climbed by simultaneously pulling with the anterior body and pushing with the posterior body. On high friction surface, the animal climbed using a tube-following gait, and slipped little (slip angle, β = 13 ± 8°). By contrast, on low friction surface, the animal slipped more (β = 39 ± 7°, P < 0.0001, repeated-measures ANOVA), climbed using a concertina gait that alternated between pushing and pulling, and moved more intermittently. In addition, when the step was higher and/or friction was lower, the animal moved closer to the step before initiating climbing. These behavioral and kinematic differences resulted in the animal’s slower traversal when step height increased and when friction decreased (e.g., traversal time t = 30 ± 16 s, traversal speed v = 1.9 ± 0.6 cm s−1 for the higher step with lower friction vs. t = 6 ± 1 s, v = 5.9 ± 1.8 cm s−1 for the lower step with higher friction, P < 0.0001 for time and speed, repeated-measures ANOVA). Finally, a principle component analysis showed that the animal can be approximated by superposition of planar modal shapes on flat ground that became non-planar during high step traversal. Our study demonstrated the importance of 3-D terrain geometry and mechanics in shaping locomotor behavior.