The Scincus that swims through sand like a snake

In old Disney comics, Scrooge McDuck would often be shown swimming in his money bin, diving through the coins like an exuberant dolphin. Leading many young minds to wonder, “How does he do that?” Coins don’t move like water; they’re arguably closer to something like dry sand.

A new paper shows that one lizard may not be able to get through McDuck’s nine cubic acres of money, but it comes a lot closer than anything else we know about so far. Scincus scincus is a cute little lizard a few inches long, whose common name, “sandfish,” tells you a lot about its behaviour.

These lizards dive through sand like Unca Scrooge dives through silver dollars. Previously, people had suspected they paddled though the sand using their legs, much like some fish might use their pectoral fins in addition to their trunk. The problem with testing an hypothesis like this is that sand has this irritating property of being opaque – a problem I had significant personal experience with, I might add. I solved it using wires and recording from muscles.

Maladen and colleagues went to high speed X-ray videography. I suspect that they probably spent a long time trying to find a combination of materials with the right combination of transparencies to X-rays, but they did it. And they found that the sandfish might better be described as a sand eel. The swimming that these lizards did (rather fast, about 10 cm per second) was entirely driven by the trunk. The legs were simply held in position and didn’t play a part after the animal got under the sand. There are some fantastic movies of this in the supplemental material.

From here, the paper looks into the physics of the situation. To be entirely honest, it’s fairly difficult stuff for me. When they write:

It is remarkable that η does not change significantly
for different φ...

I have to take their word for the remarkable nature of those Greek letters. I am rather hoping that some physics blogger out there can walk through the granular materials math in this paper.

Maladen and company end by noting that they have helped to show how organisms can exploit the alternately solid-like and fluid-like properties of sand to move through it. And this is indeed a substantial achievement, but what if you turn that around? If animals can do this, why haven’t more done so? To the best of my knowledge, no animals besides other lizards swim through sand like sandfish do. And I doubt that this is due to visibility problems; I think it is just that digging organisms are relatively rare.

There is so much nice stuff in this paper that I might forgive them for citing a sand crab digging paper from the 1970s instead of more recent and more detailed articles.

Reference

Ryan D. Maladen, Yang Ding, Chen Li, & Daniel I. Goldman (2009). Undulatory Swimming in Sand: Subsurface Locomotion of the Sandfish Lizard Science, 325 (5938), 314-318 DOI: 10.1126/science.1172490

Sandfish photo by user thew...g's on Flickr. Used under Creative Commons license.