Generation of Propulsive Force via Vertical Undulations in Snakes

Lateral 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 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.