Sidewinders are snakes that move, as their name suggests, sideways. Instead of slithering in an S-shape with the head going first and the rest of the body following, these snakes lead with the middle portion of their body as they move in a direction that’s perpendicular to the head-to-tail axis of the body. Sidewinders appear to be throwing their bodies to the side and over the sand, landing periodically but never sliding on it.
Moving in this way has evolved multiple times in snakes. Sidewinder rattlesnakes in the Sonoran Desert, desert horned vipers in the Sahara Desert, and Peringuey’s desert adder in the Namib Desert independently evolved this form of locomotion, offering evidence that it’s a good solution to a common problem faced by desert dwellers. The typical slithering of snakes is likely to cause a sand avalanche and impede progress as the snakes attempt to move along the sand. Sidewinding is an effective way to move across loose and shifting sand, especially up and down inclines. It’s fast, too. Sidewinders are the fastest snakes in the world, reaching speeds of 18 miles per hour.
Another advantage to this form of movement is that it limits the amount of contact with the body on the dangerously hot sand. The whole body is not constantly touching the ground, and the parts that do land on the sand are only there briefly. No part of the body ever slides along the substrate but instead it lifts up again before setting down on the ground.
The skin of sidewinders helps them move in this unusual way. Snakes that slither have a specific type of textured skin on their bellies—spikes that point from the head to the tail. These spikes provide friction to help the snake push off to begin moving and to prevent the body from sliding from side to side on the surface. Mathematical models indicate that those spikes make forward undulation faster and more efficient, but get in the way of sidewinding.
Sidewinders have skin with a different form of texturing—tiny pits all along their belly. The texture of the skin evolved independently in different groups of snakes rather than reflecting a trait inherited from a common ancestor. The deserts in Africa have been around much longer, so those snakes have a longer evolutionary history in that habitat. The African sidewinders have no spikes at all—only pits—but the American sidewinder has a few small spikes in addition to all the pits. The American desert is newer, so the snake here has not had as long to evolve specialized locomotion for the sandy habitat, explaining why its adaptations are not as well developed.
The unusual locomotion of sidewinders has inspired people to build robots that move in a similar way. The value of these robots is their usefulness in search-and-rescue missions with limited space or on missions to other planets with terrains that make movement across them difficult. That makes it all too easy to claim that sidewinding is otherworldy.
Karen B. London, Ph.D. is a Certified Applied Animal Behaviorist, Certified Professional Dog Trainer, and an author of six books on canine training and behavior, including her most recent, Treat Everyone Like a Dog: How a Dog Trainer’s World View Can Improve Your Life.