Anyone who’s tried to run up a sand dune can tell you it’s not easy. The granular surface shifts with each clumsy step. Fortunately creatures such as the sidewinder rattlesnake that live full-time in sandy environs come well equipped for traversing such surfaces. The exact dynamics of how they slither from point A to B, particularly across sloping dunes, however, has been a mystery.
At least until a team of physicists, zoologists and engineers took a closer look at exactly what takes place when scales meet sand. Their discoveries explain a lot about legless mobility and the physics of granular surfaces. They could also advance sidewinding as a means of robotic locomotion on this planet and elsewhere.
Sidewinding is a seemingly complicated-looking gait but “really what it amounts to is basically a wave down the body along the horizontal plane and another wave down the body in the vertical plane,” says Daniel Goldman, associate professor at the Georgia Institute of Technology's School of Physics. Goldman and colleagues from Georgia Tech, Carnegie Mellon University, Oregon State University and Zoo Atlanta report their findings in the October 10 Science.
The snakes cross sandy slopes by increasing the amount of body area in contact with the granular surfaces they're climbing. They tackle changes in elevation by modulating their bodies’ waves. “If you change the amplitude of the vertical wave you can increase the contact length on the robot, helping the robot ascend the slope with minimal slip,” Goldman says. The researchers found that despite varying the amount of the body in contact with the sand the snakes kept the same basic sidewinding gait even as the hill angle increased.
The researchers learned this by observing venomous sidewinder snakes (Crotalus cerastes) in a special sand-filled enclosure built at Zoo Atlanta with help from Joe Mendelson, the zoo’s director of research. The enclosure could be raised to create different angles in the sand. Air blown into the chamber from below smoothed the sand after each snake was studied.
The three-year study also proved that not all snakes share the sidewinder’s command of movement across granular surfaces. When the researchers tested the zoo’s pit vipers in the same sandy enclave, those snakes did not use the sidewinding motion and most had little mobility even when the surface was flat, Goldman says. The team continues to study exactly how changes in the sidewinder’s wavy motion impact its maneuverability.
Beyond being simply another interesting factoid about how snakes go about their slithery business the study of sidewinder mobility is helping roboticists improve on existing devices that seek to mimic snakelike movement. Robot designers often take a biomimetic approach to their inventions based on the functions they seek. Boston Dynamics’s Legged Squad Support System robot under development for the U.S. Department of Defense, for example, aims to help troops transport gear across narrow, rocky terrain inhospitable to a wheeled design. Other robots have mimicked leaping lizards and wall-climbing geckos.
For the sidewinder study, researchers programmed a modular, 94-centimeter-long snake robot developed by Howie Choset, a Carnegie Mellon professor of robotics, to slither with the unique wave motion discovered in the sidewinders. Choset’s robo-snake already had the ability to move in a modified sidewinding motion but it was not very adept at ascending granular slopes. The team reprogrammed the mechanical snake—whose body is five centimeters in diameter and consists of 16 joints—using information collected from their sidewinder experiments and set it loose in the zoo enclosure. The new programming gave Choset’s wavy robot the ability to climb sandy slopes it had previously failed to scale.
Snake-inspired robots could serve a number of functions. In addition to wriggling through underground pipes in search of damage and through mine shafts in search of stranded workers, engineers hope that snakebots will someday be sent to explore other planets, following in the tracks of Mars wheeled rovers such as Curiosity, Opportunity and Spirit. The latter, after several years of good service, has been mired in a patch of soft Martian soil since 2009 and is no longer operational. Curiosity and Opportunity, however, roll on.

Video courtesy of the Georgia Institute of Technology