The sandfish lizard appears to “swim” like a fish through sand, but how exactly the animal does it has long puzzled biophysicists. Now, a study published in Science reveals that the four-legged creature really does swim through sand like it would in water by retracting its legs and undulating its body.
To examine the lizard’s movement, researchers had to peek underground. They did this using X-ray imaging, and found that once the lizard, or skink, has dived beneath the sand, it doesn’t paddle. “When started above the surface, the animals dive into the sand within half a 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 [LiveScience]. This movement was surprising because previous magnetic resonance imaging studies seemed to suggest that the lizards pushed themselves along using their legs.
To understand the physics of the sandfish’s swim, 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 [PhysicsWorld]. The frequency of the lizard’s undulations determined swimming speed; generally, the animals undulated about two to four times per second.
Interestingly, speed was unaffected by how compacted the sand was, even though compact sand requires much more force to travel through. The researchers determined that this was because both drag and thrust forces increase in compact sand, and thus the ratio of these forces is no different than it is in more loosely packed sand [The New York Times]. That’s where sand and water differ: whereas the drag that water imposes increases with an object’s speed, in sand, drag remains constant, regardless of the speed of the moving object.
The researchers hope next to study how much energy the lizard expends while swimming, and they say the results could aid the development of robots that can swim beneath granular materials like sand. “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 [LiveScience].
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Image: Ryan Maladen and Lionel London