Abstract
ABSTRACTLateral undulation is the most widespread mode of terrestrial vertebrate limbless locomotion, in which posteriorly propagating horizontal waves press against environmental asperities (e.g. grass, rocks) and generate propulsive reaction forces. We hypothesized that snakes can generate propulsion using a similar mechanism of posteriorly propagating vertical waves pressing against suitably oriented environmental asperities. Using an array of horizontally oriented cylinders, one of which was equipped with force sensors, and a motion capture system, we found snakes generated substantial propulsive force and propulsive impulse with minimal contribution from lateral undulation. Additional tests showed that snakes could propel themselves via vertical undulations from a single suitable contact point, and this mechanism was replicated in a robotic model. Vertical undulations can provide snakes with a valuable locomotor tool for taking advantage of vertical asperities in a variety of habitats, potentially in combination with lateral undulation, to fully exploit the 3D structure of the habitat.
Highlights
All animals achieve locomotion by applying force to the environment, thereby generating reaction forces which propel the animal (Dickinson et al, 2000)
Our hypothesis predicts that snakes should only be able to generate net propulsive forces from surfaces with a vertical slope beyond the angle of frictional slipping (Fig. 1B-D); we tested the snake in an experimental setup with only a single potential propulsive surface oriented at an angle predicted to be either sufficient or insufficient for propulsion via vertical undulation
Because the snake robot consisted of rectangular body segments connected by revolute joints, once a given segment achieved sufficient contact angle to generate propulsive force (Fig. 1D), the robot would slide forward until the subsequent segment collided with the dowel. This resulted in a discontinuous velocity which, in turn, precluded effective force measurements. These results confirm our hypothesis that snakes can generate propulsive force via posteriorly propagating vertical waves down the body (Fig. 1A-D), albeit in a highly constrained, artificial system
Summary
All animals achieve locomotion by applying force to the environment, thereby generating reaction forces which propel the animal (Dickinson et al, 2000). Lateral undulation uses posteriorly propagating horizontal waves of bending that contact and push against asperities in the environment (e.g. grass, rocks, sticks), generating reaction forces that propel the animal forward (Gans, 1962; Gray and Lissmann, 1950; Jayne, 1986). Snakes are capable of generating propagating vertical waves, observed during lateral undulation and sidewinding to reduce friction on certain body segments (Hu et al, 2009; Marvi et al, 2014) and during gliding for stabilization (Yeaton et al, 2020). We hypothesize that snakes can use vertical waves to generate propulsive forces via a similar mechanism to lateral undulation when in contact with vertical asperities in the substrate at suitable angles (Fig. 1A-D). We attempted to replicate propulsion via vertical undulation in a robotic model to show that observed propulsion in the snake is not attributable to unobserved mechanisms, and that the proposed mechanism is mechanically sound even in the absence of snake musculoskeletal anatomy and neural control
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