Scincus scincus's popular name – sandfish – really does say what it does on the tin. These reptiles literally swim through sand and they are perfectly happy to remain submerged for the majority of the day to avoid heat and predators. Daniel Goldman, from the Georgia Institute of Technology, USA, says, ‘There has been a lot of work looking at swimming in fluids, flying, and running on relatively flat rigid hard ground, but there has been much less work done on the movement of organisms on and within materials like sand that can behave as fluids and solids.’ Explaining that submerged sandfish wriggle through sand using a technique similar to that of C. elegans nematodes, Goldman adds that no matter how fast the lizards move through the material or how deep they travel, they always weave their bodies along the same characteristic wavy line, ‘Which was very surprising for us’, he says. Intrigued by the lizard's ability to penetrate more densely packed sand without altering their swimming style, Goldman and student Sarah Sharpe embarked on an ambitious series of experiments where they filmed the submerged animals while simultaneously measuring their muscle activity patterns to find out how the lizards control sand-swimming (p. 260).
Carefully inserting minute electrodes into muscles along the lizards' bodies – to record muscle electrical activity – and cautiously wrapping the wires into tight bundles to prevent the animals from tangling, Sharpe filmed the swimming sandfish moving through sand with high-speed mild X-rays while measuring their muscle activation. In addition, Sharpe wanted to find out how the lizards coped in tightly and loosely packed sand, so she blew air through the sand before each run, allowing it to resettle and produce densely or loosely packed pristine sandbeds for each run. However, even though the lizards were extremely cooperative – burying as soon as they were released – Sharpe had no control over their direction once submerged. ‘They don't always go in a straight direction. Many times they turned through 180 deg and swam backwards, so we had to depend on luck [to get straight swims]’, she remembers. ‘It took years to get everything working correctly’, recalls Goldman.
Analysing the painstakingly collected muscle activation patterns, Sharpe and Goldman could see that the muscles were activated more strongly as the sandfish penetrated deeper into the sand. ‘We found travelling waves of muscle activation moving down the body and we believe that the sandfish are producing more muscle force as they go deeper’, says Sharpe. So instead of altering their wavelike swimming style, the lizards pressed harder on the sand as it became denser.
However, when the duo investigated the muscle electrical activity patterns as the lizards speeded up, they found that they animals were able to move at much higher speeds without increasing their muscle activity and exerting more force on their surroundings. Goldman suspects that this is because sand is a granular fluid. He explains, ‘Friction is speed insensitive and the collection of speed-independent forces [as grains rub each other] is also speed insensitive. So, the animal experiences the same resistive forces from its environment, whether it is swimming at a high or low frequency [speed]’. In other words, the sandfish thrust sand out of the way with the same force regardless of their speed. And when Yang Ding calculated the mechanical cost of moving the swimming lizards through sand (the amount of energy needed to sweep sand out of the way) by mathematically exaggerating their movements, he found that the lizard's natural wriggling motion allows them to glide through the sand while expending the least energy, making sandfish remarkably effective burrowers.