June 10, 2009

Researchers Discover Secret Behind Snake’s Slither

Researchers have gained insight on how snakes slither with a simple physics experiment.

The research, which appears in the Proceedings of the National Academy of Sciences, dispels previous theories that snakes push off nearby obstacles while slithering.

Instead, snakes exploit the fact that their scales have different grip in different directions, and increase speed by lifting curved parts of their bodies.

The researchers from New York University and the Georgia Institute of Technology challenged the 70 year old "push off" theory by using an experiment straight from a physics textbook.

They wanted to find a snake's "coefficient of static friction," which is how much the snake's body grips a surface when at rest.

To do this, the researchers pointed anesthetized snakes in different directions along a flat board held at an angle, and lifted the board until the snakes would move.  By doing this, the scientists could determine the reptile's friction coefficient.

They discovered that snakes facing downhill on the board had the least amount of grip, snakes lying sideways had more grip, while snakes facing uphill on the board had the most grip.

According to the researchers, this data clearly points to a mechanism by which the snakes use scales on their bellies as a means to push off and move.

"What people thought was going on in the macroscopic scale with the sides of their bodies is actually going on in the microscopic scale, with their belly scales," said David Hu, of the Georgia Institute of Technology.

"If the friction were equal in all directions, a snake would just slither in place, as if it were on a treadmill," he told BBC News.

The overlapping scales help provide friction with the ground, giving snakes a preferred direction of motion, similar to the motion found in wheels.  Moving forward also takes less work for the snakes than moving sideways.

As the snakes came out of their anesthetic state, the researchers observed individual scales twitching, confirming the belief that scales can be independently controlled to maximize traction.

The team also looked at how snakes distribute their weight while moving.

"Most people think that when snakes slither they are completely pressed flat against the ground, but actually they lift their bodies," Dr Hu said.

"Sometimes it's perceptible and sometimes they just unload parts of their body so their weight distribution is only towards the center."

The researchers used polarized light and gelatin in a visualization scheme which changes properties when held under pressure.

The experiment showed that the snakes would concentrate their weight at points where their bodies were least curved.

According to the team, the weight distribution eliminates wasted effort because lifted parts cannot contribute to the forward motion.

"It's analogous to the way we walk or run," Dr Hu said.

"You shift your weight to the left or the right leg, but you don't drag the other leg. The snakes are only touching the ground where the friction force is going to help them move."


Image Caption: David Hu, an 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. (Photo: David Hu and Grace Pryor/ Georgia Institute Of Technology)


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