Snakes travel in the form of an undulating S-shape produced by a wave of muscle contractions propagating from their head to their tail. How do snakes preserve their motion when they encounter unexpected obstacles on their way? Perrin Schiebel and her colleagues at the Georgia Institute of Technology in Atlanta, USA, wanted to find out whether the snakes’ strategy to deal with unforeseen hurdles relies on sensory information to carefully guide their path or whether they exploit the unguided natural motion that is intrinsic to their undulating bodies to set them on the right course.

To answer their question, the team observed the desert-dwelling, shovel-nosed snake as it made its way across a 165 cm-long sandbox covered in carpet that the researchers had built. The snakes often travel at about 30–80 cm s−1 and usually encounter obstacles such as small plants and twigs in their natural environment, so the researchers mimicked these obstructions by placing six posts spanning the width of the box, mid-way along, spaced about 2 cm apart, which the snakes had to pass to get to the other end. Schiebel and her colleagues observed the angle of the path that the snakes took when they slid past or collided with the posts: 0 deg if the snake maintained its movement straight forward, negative angles if the snake veered to the left and positive angles if it veered to the right. To make sure that the snakes didn't pre-plan their encounters with the posts, the team temporarily blindfolded the animals by using face paint to cover the scales over their eyes.

When the team looked at the path deflections as the snakes passed between the posts, they found that they ranged from −57.2 to 56.1 deg, with three distinct peaks representing the angles that the snakes were most likely to take. The team noted that the pattern of deflections looked similar to the way that rays of light bend – diffract – when passing through a narrow slit. This remarkable similarity between the physical wave-motion of the snakes’ bodies and how light rays behave led the researchers to realise that the snakes were not responding to sensory signals that they had gathered about their surroundings. Were their movements being guided by a passive process instead?

To understand how the intrinsic springiness and structure of the snakes’ body might passively result in the deflection pattern, the team developed a series of equations that they used to calculate the snakes’ manoeuvres. By using the wave of muscle-activating nerve signals that pass along the length of the snake's body, the team determined the points where the animal would buckle, which could subsequently determine the direction in which it moved. Impressively, the deflection angles calculated by the equations replicated Schiebel's observations, which suggests that the deflections may arise purely from the intrinsic mechanical properties of the animal's body without any need for sensory inputs.

Schiebel and her colleagues have discovered a remarkable similarity between a snake’s route through an obstacle-strewn path and the way that rays of light diffract. Their results suggest that snakes use a simple strategy when they encounter obstacles that depends on the passive dynamics of their bodies without the need for sensory information to guide them. And they are optimistic that the snake's passive approach could inspire new strategies for helping snake-like robots overcome unexpected bumps in their way.

P. E.
J. M.
A. M.
D. Z.
D. I.
Mechanical diffraction reveals the role of passive dynamics in a slithering snake
Proc. Natl. Acad. Sci. USA