Mechanics of snake (limbless) locomotion

Settled in 2009 by David L. Hu, Jasmine Nirody, Terri Scott, Michael J. Shelley (NYU/Georgia Tech).

How a limbless animal propels itself forward using only body waves puzzled naturalists from Aristotle onward, since simple lateral undulation on a uniform frictionless surface produces no net displacement in idealized models. Hu, Nirody, Scott and Shelley combined snake-motion kinematics with direct measurements of the friction coefficients of real snakeskin (which differ along versus across the scales) and built a theoretical model showing this frictional anisotropy alone is sufficient to convert undulation into forward thrust, even on flat, uniform ground. Their 2009 PNAS paper matched predictions to observed snake trajectories and speeds, resolving what forces actually drive slithering. The work also inspired later snake-like and limbless robots exploiting the same anisotropic-friction principle.

A snake's ability to glide forward with no legs and nothing to push against turns out to come from nothing more exotic than the direction its own scales point.

Explanations that circulated

• The long-standing assumption that snakes require heterogeneous ground friction (rough patches, pegs, or pushing off external objects) to move forward — the PNAS study showed snakes can generate net thrust on a uniform flat surface using only the frictional anisotropy of their own scales

References

• The mechanics of slithering locomotion, David L. Hu, Jasmine Nirody, Terri Scott, Michael J. Shelley, Proceedings of the National Academy of Sciences, 2009