My aspiration for the book and this blog, beyond the hope that they would be read at all, has always been that readers would be surprised. This arose simply because of the extent to which I surprised myself - clearly, I had a good idea of the diversity of journeys and connections, and the fascinating questions, answered and unanswered, that the topic would lead to, but the more I looked, the more surprised I became. And it doesn't stop. I've lost track of all the ideas for posts that have simply gone on to the "to do" pile, shouldered aside by something immediate that comes up. I recently wrote about locomotion in sand, robotics, the intimate relationships small critters have with granular materials, and whether sand is a solid or a fluid or something else entirely. And just a couple of days ago, browsing through the Science Daily online news headlines, there was a report on research that intersects with all of these.
I've mentioned Scincus scincus, otherwise known as the sand skink, sandfish, or sand swimmer, before. This little desert lizard has a remarkable ability to disappear, and then "swim," beneath the surface of the Saharan sand, rather like its blind Australian relative, the itjaritjari, the local description of whose skill is the subtitle to this post. Now new research at the Georgia Institute of Technology has shed amazing light on how these lizards do it. The sandfish demonstrates distinctly interdisciplinary skills, and it took collaboration between the School of Physics and the Interdisciplinary Bioengineering Program to analyse them. Daniel Goldman and his colleagues built a small tank filled with glass beads (the sand) in which the density, the packing, could be controlled by pulses of air from below, a sort of fluidised bed. They then let a sandfish demonstrate its skills and used high-speed X-ray imaging to reveal its activities. It was already clear that the shape of the lizard's body and limbs was well-suited to sand locomotion, and there were some suggestions that the nano-scale character of its smooth scales also contributed to its skills. But does it use its limbs to "swim"? The Georgia Tech study suggests not - that, once below the surface, the sandfish tucks its legs alongside its body and propels itself with a snake-like wave motion, a single-period sinusoidal wave that travels from the head to the tail. The news release on the study discusses the kinematics in more detail, and contains links to several short videos of Scincus scincus in action; these make for compulsive viewing, and I've taken the liberty of using screenshots from them to illustrate this post.
It turned out that the density of the sand made little difference for the lizard - it simply changed the frequency of its undulation and maintained speeds of up to six inches a second; its methodology is unique, as Goldman describes:
The large amplitude waves over the entire body are unlike the kinematics of other undulatory swimming organisms that are the same size as the sandfish, like eels, which propagate waves that start with a small amplitude that gets larger toward the tail....The results demonstrate that burrowing and swimming in complex media like sand can have intricacy similar to that of movement in air or water, and that organisms can exploit the solid and fluid-like properties of these media to move effectively within them
The team then developed an empirical model of the drag forces faced by the lizard, and, indeed, any object moving through granular material:
Although viscous hydrodynamics can predict swimming speed in fluids such as water, an equivalent theory for granular drag is not available. To predict sandfish swimming speed, we developed an empirical model by measuring granular drag force on a small cylinder oriented at different angles relative to the displacement direction and summing these forces over the animal movement profile. The agreement between model and experiment implies that the noninertial swimming occurs in a frictional fluid.
And, as always with research of this kind, the implications and potential applications for handling of granular materials go far beyond the skills of a clever lizard:
In addition to having a biological impact, this study’s results also have ecological significance, according to Goldman. Understanding the mechanics of subsurface movement could reveal how the actions of small burrowing organisms like worms, scorpions, snakes and lizards can transform landscapes by their burrowing actions. This research may also help engineers build sandfish-like robots that can travel through complex environments.
“If something nasty was buried in unconsolidated material, such as rubble, debris or sand, and you wanted to find it, you would need a device that could scamper on the surface, but also swim underneath the surface,” Goldman said. “Since our work aims to fundamentally understand how the best animals in nature move in these complex unstructured environments, it could be very valuable information for this type of research.”
And, as always with our attempts to better understand the intricacies of the natural world, it's probably more complex. Research conducted recently in Germany and also reported in Science Daily last year, describes specifically how the sandfish uses its limbs to move "in a way very similar to the crawl stroke in swimming." Ah well, it just makes the little lizard all the more interesting - watch this space!
[The Georgia Tech study is described at https://www.sciencedaily.com/releases/2009/07/090716141140.htm and https://www.gatech.edu/newsroom/release.html?id=3161, and the abstract in Science is available at https://www.sciencemag.org/cgi/content/abstract/325/5938/314 - full version for subscribers.]