Slither. A snake's overlapping belly scales snag on tiny irregularities as it slides over a flat surface. At top right, a corn snake.

Grace Pryor and David Hu

Snakes on a Pane

Snakes just wouldn't be snakes without their characteristic slither. Known scientifically as "lateral undulation," the creatures wiggle their bodies like a sideways wave and leave a perfect S-shaped track as they go. But just how does slithering get snakes from point A to point B? A new mathematical model may have the answer.

Just like a horse can walk, canter, or gallop depending on its travel needs, snakes have a variety of gaits. In sidewinding, the snake turns its body into a helix and throws itself to the side. In the concertina gait, the snake moves forward by folding itself up like an accordion. And in lateral undulation, the snake's body moves in waves. Researchers had found that snakes propel themselves forward by pushing off from obstructions on the ground, such as rocks and twigs.

But snakes obviously slither along twig-free surfaces, too. For example, the snakes used in this study are champs at escaping across office carpet, says David Hu, a mechanical engineer at the Georgia Institute of Technology in Atlanta and lead author of the paper published today in the Proceedings of the National Academy of Sciences. "One snake escaped, and we didn't know where it was until we got a printer jam," he says. (The snake was fine.)

To come up with their mathematical model, Hu and his colleagues measured aspects of snake travel. They dragged unconscious Pueblan milk snakes, a small, docile red-white-and-black species, across a smooth table and a cloth-covered table to measure the friction generated by the snake's scales. A snake's belly scales are wide and overlapping like a spread-out deck of cards, explains Hu. "You can feel this with your fingertips. If you run them one way along the snake, it feels rougher than the other way." The tiny difference in friction in the different directions is enough to move the snake forward when it contracts itself in a wave, the team reports.

The snakes actually moved somewhat faster than predicted by Hu's model; he thinks this is because they can lift parts of their bodies off the ground, even when slithering. Hu calculates that shifting weight off the body's outer curves helps snakes move faster, by putting more weight on the middle bits of the body, which are pointing forward. "The Bible says they're lowly creatures that are totally flat on the ground," says Hu. "There's a lot of vertical undulation, actually."

Functional morphologist Bruce Jayne of the University of Cincinnati in Ohio says it makes sense that skin texture is important for slithering. "One of the things about being limbless is that a large chunk of skin is interacting with the environment, so it shouldn't be too surprising that the skin is important," he says. "But it is really nice to see exactly how important."

One possible application for any new information about snake movement is in the building of snakelike robots, such as for search and rescue or even minimally invasive surgery. But robotics scientist Howard Choset of Carnegie Mellon University in Pittsburgh, Pennsylvania, says he doesn't plan to use this particular mode of movement with his mechanical slitherers. Choset's robots usually move across flat ground by sidewinding, scrunching up like an inchworm, or rolling like a sausage.

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