If you own a pet lizard, it could be a sandfish — a mild-mannered skink native to North Africa and the Middle East. The creature is so-called because it appears to "swim" under the sand — an ability that has fascinated biophysicists interested in animal propulsion. But because sand is opaque, the question is: does the sandfish really swim like a fish, or does it use its legs?

To find out, Daniel Goldman and colleagues at the Georgia Institute of Technology in the US allowed a sandfish to scurry (using its legs) into a container filled with deep sand. When the skink reached the sand it immediately burrowed into the material — where its motion was recorded by illuminating the sand with X-rays and capturing the images with a high-speed camera.

The experiment revealed an undulating motion that, Goldman says, is intermediate to how a snake moves across a solid surface and how a fish swims through water. A sandfish, in other words, seems a pretty accurate name.

Completely legless

The team was particularly interested to see if the sandfish uses its limbs to push its way through the sand, so markers were glued onto its legs. These revealed that the sandfish keeps its legs tucked against its body while swimming.

Goldman told physicsworld.com that this legless propulsion came as a bit of a surprise because a previous study (by others) using magnetic resonance imaging to follow a sandfish suggested that the skink was using its legs.

To gain a better understanding of the sandfish’s propulsion strategy, the team measured the thrust and drag forces on a skink-sized stainless-steel rod as it was pushed through sand. These data were used to predict the “wave efficiency” of the skink’s motion — the ratio of its velocity through the sand to the velocity of the wave that travels down its body.

Frictional fluid

By comparing their model and observations, the team was able to conclude that the sandfish was swimming through a “frictional fluid”. Drag arises in such a fluid because of friction between the skink’s body and sand grains, and between the sand grains themselves. It differs from water and other familiar viscous fluids because the drag forces are independent of velocity — whereas in viscous fluids the drag is proportional to velocity.

The team repeated its experiment several times using sands with different solid-to-air ratios. They found that the skink did not modify its swimming technique, even though the drag force in the densest sand was double that in the least dense sand.

Goldman said that the team’s next goal is to work out whether the sandfish expends more energy while swimming in denser sand.

This research appears in the latest edition of Science.