Friction gives snakes a smooth slither

View video of a snake’s gait as it slithers.

A slithering snake gets a leg up from its scales, which provide much needed friction when moving forward, scientists report online June 8 in the Proceedings of the National Academy of Sciences. On a flat surface, the grip provided by scales goes hand in hand with a precise redistribution of body weight, suggesting snakes tailor their gait to their environment.

“You would think that if you were basically a living string, your life would be very simple,” says lead author David Hu, a mechanical engineer at Georgia Institute of Technology in Atlanta. “But there are all different ways of moving—there may be real physical reasons underlying when snakes change gaits.”

The new work is the first to analyze snake slithering in detail with a combination of experiments and theoretical models. Researchers examined the gait used when snakes move forward on a flat surface with little to push against. This terrestrial lateral undulation is one of three standard snake gaits.

The findings reveal that snakes use a combination of pushing off the surface and gripping it with friction to propel themselves forward. This fits with the notion that snakes have a “remarkable capacity to manipulate their belly scales,” says Bruce C. Jayne of the University of Cincinnati. Many snakes have specialized muscles that go from scale to scale or from scales to the ribs. These muscles may change the orientation of the scales just a bit, Jayne says.

Experiments with anesthetized snakes found that it was harder to slide a snake sideways—at a right angle to the direction of movement, than to slide it backwards or forwards. Because the layers are arranged like shingles on a roof, the greatest friction would have been expected when sliding backward against the scales’ edges. So this suggested that the relationship between scales and slithering depends on more than just the scales’ arrangement.

Hu and collaborators used video and mathematical models to analyze the distribution of forces for a milk snake moving forward on a flat surface. The researchers also placed moving snakes on a photoelastic gelatin that lights up when force is applied. This revealed areas where the snake uses more force to lift its body slightly and push forward, much like a foot pressing off the sidewalk to move.  

While friction accounts for about 65 percent of the forward movement, this push accounts for the other 35 percent, the researchers’ models showed.

The work may have implications for designing better snakelike robots, which also depend heavily on friction with the ground, says Alessandro Crespi of the Swiss Federal Institute of Technology in Lausanne.

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Credit : David Hu, Georgia Institute of Technology