Motion System Design

Snakes slither using friction, weight redistribution

Researchers at New York University and the Georgia Institute of Technology have discovered that snakes use both friction generated by their scales in addition to weight redistribution to slither along flat surfaces. Their findings, which appear in the latest issue of the journal Proceedings of the National Academy of Sciences, run counter to previous studies that have shown snakes move by pushing laterally against rocks and branches. David Hu, assistant professor in mechanical engineering at Georgia Tech, is able to visualize snakes slithering by watching them undulate on a mirrored surface, lifting the curves of their bodies.

“We found that snakes’ belly scales are oriented so that snakes resist sliding toward their tails and flanks,” says Hu. “These scales give the snakes a preferred direction of motion, which makes snake movement a lot like that of wheels, cross-country skis, or ice skates. In all these examples, sliding forward takes less work than does sliding sideways.”
The study centered on the frictional anisotropy — or resistance to sliding in certain directions — of a snake’s belly scales. While previous investigators had suggested that the frictional anisotropy of these scales might play a role in locomotion over flat surfaces, the details of this process had not been understood. To explore this idea, researchers developed a theoretical model of a snake’s movement. The model determined the speed of a snake’s center of mass as a function of the speed and size of its body waves, taking into account the laws of friction and the scales’ frictional anisotropy. The model suggested that a snake’s motion arises by the interaction of surface friction and its internal body forces.

To confirm movement as predicted by the model, researchers measured the sliding resistance of snake scales and monitored snake movement through experiments on flat and inclined surfaces. They used video and time-lapse photography to gauge the movements. Results indicated a close relationship between what the model predicted and the snakes’ actual movements.

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