Over the life of this blog, I have written a number of times on the remarkable and innovative work carried out at Georgia Tech’s Crab Lab, investigations into how critters get around in loose sand and how this can be applied to robotics (see, for example, here, here, and here). Crab Lab is headed up by Dan Goldman, and I was recently delighted (and flattered) that he contacted me about the “Sand” book. I asked if he and his colleagues would be willing to contribute a guest post – and here it is. Courtesy of Henry Astley, a postdoc “who loves all things snakes” and collaborator Joe Mendelson: the state of the art on the physics of sidewinding.
In spite of their barren reputation, deserts around the world teem with life, including a wide array of animals from beetles to camels. While much has been written about their adaptations to deal with two of the most notable characteristics of deserts, extreme temperatures and scarce water, far less attention has been paid to their interactions with the other distinguishing characteristic of many deserts: sand.
The ability to move from place to place is crucial for animals to find mates and resources, regulate their body processes, avoid predators, and colonize new environments. But sand makes locomotion difficult, whether moving on it or through it. Particularly problematic is that sand will behave as a solid under certain loads, but will yield and flow like a fluid under others, and very small differences in foot placement and movement can be the difference between moving and becoming hopelessly stuck. Animals deal with this challenging substrate in a variety of ways, whether by anatomical changes or selecting the best movement patterns.
Perhaps the strangest movement pattern of desert animals is the famous “sidewinding” locomotion, seen in certain snakes of sandy deserts around the world. While prominent herpetologist Clifford H. Pope wrote in 1955 “A study of [sidewinding] is recommended to anyone who likes to be confused,”, the underlying motion is quite clear when examined in detail. The snake lifts its head and moves it forward, placing it on the ground, then repeats this motion in a propagating wave down the body – see Science paper supplementary movies.
This produces a characteristic trackway consisting of a series of parallel lines, with the imprint of each scale on the belly clearly visible, showing that the snake does not slip (which would smear the tracks and erase the fine scale imprints). Ultimately, this seemingly complex motion can be reduced to a pair of waves producing vertical and horizontal body undulation, +- 90 degrees out of phase, which propagate together down the body. This simple model may be a “neuromechanical template”, a simple model of a motion which captures all the essential features, potentially serving a simple “target” for the animals to control their locomotion (see Sidewinding with minimal slip: Snake and robot ascent of sandy slopes). This two-wave template of sidewinding also produces sidewinding locomotion when applied to a snake robot, allowing the robot to move on sand effectively.
Screen-grab from Science supplementary movies
This provides a great experimental tool, because the physics of sand have yet to be reduced to simple systems of equations (as has been the case in fluids for almost 200 years), making computer modeling of results difficult and time-consuming. However, the snake robot provides a “physical model” for movement in sand, allowing us to test hypothesized biological mechanisms. Further observations of biological snakes have revealed the modifications of the two-wave template responsible for effectively sidewinding up inclines and turning (see the recent PNAS paper), which in turn have further improved the effectiveness of the robot.
In spite of these insights, the serpents of the sand still hold many mysteries. Why do some snakes sidewind, while others don’t? Why can some species move effectively on sand, while others fail? How do sidewinders deal with obstacles? Can we reconstruct the evolution of this remarkable mode of locomotion from the tracks it has left, or has this history been lost in the sands of time?
[My sincere thanks to all at Crab Lab for their work and this post. Photographs by Henry Astley]