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Scales Are Key to Snake Locomotion, Study Finds

Snakes have bodies and methods of locomotion perfect for situations where limbs would be a disadvantage. They can glide in and out of rubble, penetrate crevices, and navigate situations that most other animals would find impassable.

This is what makes them so attractive to robot engineers, who envisage many applications for agile and stealthy “snake bots.”


National Park Service photo of a milk snake by Jonathan Mays

Robotic snakes could be created to help with bomb-disposal, search-and-rescue in dangerous situations like collapsed buildings, heart surgery (using microscopic snake bots slithering through blood vessels), and surveillance (imagine a spy bot popping its camera-carrying head out of a hole in the ground or a chink in the ceiling.)

Much of the quest to build a better snake bot has focused on trying to understand exactly how a snake moves.

Video of snake robot developed by Scandinavia’s SINTEF Group and the Norwegian University of Science and Technology:

Using a type of locomotion known as lateral undulation, snakes drive their flanks against rocks and branches along the ground, scientists have long believed. some snake bots have been designed to move accordingly.

Now, research published today reveals that, for certain types of locomotion, snakes use the friction created by their skin. Wide overlapping scales on their bellies snag rough surfaces and help the reptiles propel themselves. Speed and direction can be controlled by lifting their bodies and changing the weight and force applied by their scales on the surface they are on.

“The physical mechanisms that snakes use to slither have been the subject of much debate. Previous analyses have assumed that snakes push against rocks and trees to propel themselves forward,” a team of researchers led by David Hu said of their research published in PNAS Online Early Edition.


Hu, of the Applied Mathematics Laboratory, Courant Institute of Mathematical Sciences, New York University, and colleagues from the Georgia Institute of Technology, Atlanta, examined how snakes move over different surfaces.

NPS photo of milk snake by Jack O’Brien

Their experiments suggest that friction caused by the snake’s skin also plays a critical role in a snake’s ability to propel itself along a flat surface.

Hu’s team focused on lateral undulation, in which the snake presses its belly against an object to propel forward.

The researchers used 10 juvenile pueblan milk snakes in their experiments.

The snakes were given a general anesthetic and, when they were unconscious, were arranged in nine orientations on an inclined plane covered with two materials, a cloth of a roughness comparable with the thickness of the snake’s belly scales, and a smooth fiberboard, with a scale of roughness much less than that of the snake’s scales.

By comparing how the snakes slid along an inclined surface covered like this, the researchers showed that the belly scales perform a vital function: they snag on the irregularities on rough surfaces, which helps the snake slither laterally. There was no snagging of the scales on the smooth surface.

Conscious snakes were also observed and filmed slithering along a plank covered with either cloth or fiberboard, set at differing angles of inclination.


Photo courtesy David Hu

The scientists found that by lifting their bodies as they moved, snakes were able to adjust their weight distribution and direction that provides optimal friction.

Snakes are likely to also be able to dynamically change their frictional interactions with a surface by adjusting the attitude of their scales, the researchers said. But this theory could not be tested.

Snake Bots Could Cross Sand

The findings could lead to development of robotic snakelike organisms that can slither across flat surfaces such as sand, which lack obvious push points, the researchers said.

Snake robots designed on the basis of earlier research by others focused on the notion that snakes slither by driving their flanks laterally against neighboring rocks and branches found along the ground.

“This key assumption has informed numerous theoretical analyses and facilitated the design of snake robots for search-and-rescue operations … Snake robots have been generally built to slither over flat surfaces by using passive wheels fixed to the body that resist lateral motion,” Hu and colleagues said.

“We present a theory for how snakes slither, or how wheelless snake robots can be designed to slither, on relatively featureless terrain such as sand or bare rock, which do not provide obvious push points.”


Photo courtesy David Hu

Video of snake robots at Carnegie Mellon University’s Robotics Institute: