With its “wedge-shaped snout and countersunk lower jaw,” the sandfish is built to move through its sandy environment. However, scientists at the Georgia Institute of Technology determine how the sandfish skink (Scincus scincus) uses its limbs and a wave motion to literally swim through its sandy environment. From Georgia Tech Research News (emphasis added):

A study published in the July 17 issue of the journal Science details how sandfish — small lizards with smooth scales — move rapidly underground through desert sand. In this first thorough examination of subsurface sandfish locomotion, researchers from the Georgia Institute of Technology found that the animals place their limbs against their sides and create a wave motion with their bodies to propel themselves through granular media.

“When started above the surface, the animals dive into the sand within a half second. Once below the surface, they no longer use their limbs for propulsion — instead, they move forward by propagating a traveling wave down their bodies like a snake,” said study leader Daniel Goldman, an assistant professor in Georgia Tech’s School of Physics.

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“Since loosely packed media is easier to push through and closely packed is harder to push through, we thought there should be some difference in the sandfish’s locomotion,” said Goldman. “But the results surprised us because the density of the granular media did not affect how the sandfish traveled through the sand; it was always the same undulatory wavelike pattern.”

For a given wave frequency, the swimming speed depended only on the frequency of the wave and not on the density. Unexpectedly though, the animals could swim a bit faster in closely packed material by using a higher frequency range. The team also varied the diameter of the glass beads, but still observed similar wavelike motion.

By tracking the sandfish in the X-ray images as it swam through the glass beads, Goldman was able to characterize the sandfish’s motion — called its kinematics — as the form of a single-period sinusoidal wave that traveled from the head to the tail.

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“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,” noted Goldman.

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.

Videos from the research illustrate how the sandfish skink swims through sand

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Image: Scincus mitranus. Photo source for attribution. The author or licensor of this image does not endorse my work or me and their image is protected under an attribution license.

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