  Motivated by a possible convergence of terrestrial limbless locomotion
strategies ultimately determined by interfacial effects, we show how both 3D
gait alterations and locomotory adaptations to heterogeneous terrains can be
understood through the lens of local friction modulation. Via an
`everything-is-friction' modeling approach, compounded by 3D simulations, the
emergence and disappearance of a range of locomotory behaviors observed in
nature is systematically explained in relation to inhabited environments. Our
approach also simplifies the treatment of terrain heterogeneity, whereby even
solid obstacles may be seen as high friction regions, which we confirm against
experiments of snakes `diffracting' while traversing rows of posts [1], similar
to optical waves. We further this optic analogy by illustrating snake
refraction, reflection and lens focusing. We use these insights to engineer
surface friction patterns and demonstrate passive snake navigation in complex
topographies. Overall, our study outlines a unified view that connects active
and passive 3D mechanics with heterogeneous interfacial effects to explain a
broad set of biological observations, and potentially inspire engineering
design.
