Physics of burrowing sandfish revealed

Simulations show how lizard wriggles a fine line to maximize thrust, extension

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BUMP AND GRIND Computer simulations of lizards swimming through buckets of small glass beads can give researchers a look at how every single bead responds as the reptile passes. Yang Ding

The sandfish lizard wriggles through desert sands like a sci-fi monster. Now, using computer simulations and bendy robots, researchers at Georgia Tech in Atlanta have taken the most complete look yet at the everyday physics of burrowing animals. And, boy, does this reptile wriggle, the team reports online February 23 in the Journal of the Royal Society Interface. “This particular behavior is built for speed,” says physicist Daniel Goldman, one of the study coauthors.

Like the deadly sandworms in the Dune science fiction series, a host of animals from scorpions to snakes haunt subterranean deserts across the planet. It’s not easy to study how these creatures careen through their environments, Goldman says. Scientists have a good idea how water behaves in the wake of an undulating eel or how air flows over a bird wing. But shuffling sand grains ping off each other like a wickedly complicated game of pool.

X-ray studies have shown that sandfish lizards (Scincus scincus) navigate such chaos with a wormlike wriggle, Goldman says, tucking in their legs and curling from side to side in S-shaped waves. A fast sandfish lizard dive covers two body lengths per second — and the creatures can grow to 4 inches long, he adds. But just how the lizards achieve such speed in a complex sandy environment wasn’t clear. For that, Goldman’s team turned to a new set of tools.

First, researchers simulated sandfish lizards swimming through a field of 3-millimeter-wide glass beads on a computer. The program — which ate up 20 to 30 desktop PCs and still took days to run — illustrated how every bead bumped and thudded as the virtual lizard passed by. The real fun came next. The team built a spandex-covered robo-reptile that could wriggle much like the real thing. “The beauty of robotics compared to the simulation and theory: It’s all in the real world,” Goldman says. If the team wanted the robot to bend more or less, the researchers just asked it to bend more or less.

On-screen or clad in spandex, the tests agreed. If virtual lizards curl too much, they don’t move far enough forward with each wriggle. If they bend too little, the lizards can’t give enough push. Real-life sandfish lizards walk, or wriggle, this fine line nearly perfectly. “They dive into the sand as fast as they can,” Goldman says.

Such finely tuned diving isn’t useful just for lizards, says Robin Murphy, director of the Center for Robot-Assisted Search and Rescue at Texas A&M University in College Station. She designs robots to help in the aftermath of disasters like earthquakes or mudslides. But when it comes to machines that can dig like earthworms and slip through rubble, nothing like that exists, she says. “There’s a lack of any technology short of a shovel.” Burrowing animals could inspire new machines, but so far, few studies have been able to capture the constraints robots would face in dirt-filled or muddy environments. “This is the first I’ve seen that I said, ‘Okay, we’ve got it,’” she says.

Robots inspired by animals are neat, admits Eric Tytell, a researcher at Johns Hopkins University who studies how fish swim in water. But the Georgia team flipped that inspiration around, too. Goldman and his colleagues used robots to get a better grasp of biology. And that’s really clever, Tytell says.

Goldman says his studies have convinced him that sandfish lizards dive for one reason — to escape. In the desert, there’s nowhere else to hide. “You just want to get the hell out of there as fast as you can,” he says.

Quick getaway from Science News on Vimeo.

Sandfish lizards native to the Saharan desert may dive deep to get away from predators.
Credit: Ryan Maladen, Yang Ding, Paul Umbanhower, Adam Kamor, Daniel Goldman/Georgia Institute of Technology.

A desert dive from Science News on Vimeo.

Sandfish lizards swim through sand by tucking in their legs and wriggling their bodies, not unlike some worms.
Credit: Ryan Maladen, Yang Ding, Paul Umbanhower, Adam Kamor, Daniel Goldman/Georgia Institute of Technology

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