  Terrestrial locomotion requires generating appropriate ground reaction forces
which depend on substrate geometry and physical properties. The richness of
positions and orientations of terrain features in the 3-D world gives limbless
animals like snakes that can bend their body versatility to generate forces
from different contact areas for propulsion. Despite many previous studies of
how snakes use lateral body bending for propulsion on relatively flat surfaces
with lateral contact points, little is known about whether and how much snakes
use vertical body bending in combination with lateral bending in 3-D terrain.
This lack had contributed to snake robots being inferior to animals in
stability, efficiency, and versatility when traversing complex 3-D
environments. Here, to begin to elucidate this, we studied how the generalist
corn snake traversed an uneven arena of blocks of random height variation 5
times its body height. The animal traversed the uneven terrain with perfect
stability by propagating 3-D bending down its body with little transverse
motion (11{\deg} slip angle). Although the animal preferred moving through
valleys with higher neighboring blocks, it did not prefer lateral bending.
Among body-terrain contact regions that potentially provide propulsion, 52%
were formed by vertical body bending and 48% by lateral bending. The
combination of vertical and lateral bending may dramatically expand the sources
of propulsive forces available to limbless locomotors by utilizing various
asperities available in 3-D terrain. Direct measurements of contact forces are
necessary to further understand how snakes coordinate 3-D bending along the
entire body via sensory feedback to propel through 3-D terrain. These studies
will open a path to new propulsive mechanisms for snake robots, potentially
increasing the performance and versatility in 3-D terrain.
