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Snake walk: The physics of slithering | Snake walk: The physics of slithering |
(about 4 hours later) | |
"This is the Mojave shovel-nosed snake," says Perrin Schiebel as she hands me a 40cm reptile. It is vibrantly patterned, apparently harmless, and quickly wraps itself around my fingers. | "This is the Mojave shovel-nosed snake," says Perrin Schiebel as she hands me a 40cm reptile. It is vibrantly patterned, apparently harmless, and quickly wraps itself around my fingers. |
"They're native to deserts of the American south-west. This is a full-grown adult." | "They're native to deserts of the American south-west. This is a full-grown adult." |
Ms Schiebel is studying for a PhD in physics at the Georgia Institute of Technology in Atlanta, US. She has spent many months putting 10 of these snakes through their slippery paces in a sand-filled aquarium. | Ms Schiebel is studying for a PhD in physics at the Georgia Institute of Technology in Atlanta, US. She has spent many months putting 10 of these snakes through their slippery paces in a sand-filled aquarium. |
Why is a team of physicists playing with snakes in a custom-built sand pit? Because, I am told, the way they move is a marvel. (The snakes, not the physicists.) | Why is a team of physicists playing with snakes in a custom-built sand pit? Because, I am told, the way they move is a marvel. (The snakes, not the physicists.) |
Innumerable critters have evolved superb ways to scuttle and slither - or even burrow and "swim" - across the most unhelpful of terrains: those that flow. | Innumerable critters have evolved superb ways to scuttle and slither - or even burrow and "swim" - across the most unhelpful of terrains: those that flow. |
If you've ever tried to walk up a sand dune, then you are familiar with the problem: unstable ground makes a mission out of locomotion. Now, imagine doing it on your belly. | If you've ever tried to walk up a sand dune, then you are familiar with the problem: unstable ground makes a mission out of locomotion. Now, imagine doing it on your belly. |
"One of the things that's really interesting about snakes is that their entire body is, in this type of locomotion, in sliding contact with the ground," Ms Schiebel explains. | "One of the things that's really interesting about snakes is that their entire body is, in this type of locomotion, in sliding contact with the ground," Ms Schiebel explains. |
"So they have to be able to push off things in their terrain effectively, to overcome the fact that they've got these frictional drag forces on their stomach all the time." | "So they have to be able to push off things in their terrain effectively, to overcome the fact that they've got these frictional drag forces on their stomach all the time." |
And these denizens of the desert - including my new shovel-nosed friend - make it look easy. | And these denizens of the desert - including my new shovel-nosed friend - make it look easy. |
In fact, as soon as this particular snake lands back in the glass tank, it vanishes. | In fact, as soon as this particular snake lands back in the glass tank, it vanishes. |
"They can travel fairly substantial distances completely submerged," Ms Schiebel says. "We think this body shape is an adaptation to that." | "They can travel fairly substantial distances completely submerged," Ms Schiebel says. "We think this body shape is an adaptation to that." |
Some of her colleagues have used X-rays to peer beneath the sand and study the snake's swimming in detail. But she is more interested in how they get around on the surface. | Some of her colleagues have used X-rays to peer beneath the sand and study the snake's swimming in detail. But she is more interested in how they get around on the surface. |
Ms Schiebel has "tens of thousands of frames" of high-speed video showing the shovel-nosed snake navigating the tank, both with and without obstacles. | Ms Schiebel has "tens of thousands of frames" of high-speed video showing the shovel-nosed snake navigating the tank, both with and without obstacles. |
"That's what I'm interested in - how they can use these this flowing sand and these obstacles, and push off them to travel." | "That's what I'm interested in - how they can use these this flowing sand and these obstacles, and push off them to travel." |
All in the wave | All in the wave |
This laboratory has sand-based experiments down to a fine art. For one thing, the snake is not slithering on any old sand. | This laboratory has sand-based experiments down to a fine art. For one thing, the snake is not slithering on any old sand. |
"These are 300-micron (0.3mm) glass beads," says Ms Schiebel. "So it's sort of a laboratory physics version of desert sand. That's very similar to the size and composition in their natural habitat." | "These are 300-micron (0.3mm) glass beads," says Ms Schiebel. "So it's sort of a laboratory physics version of desert sand. That's very similar to the size and composition in their natural habitat." |
Also in the name of precision, a remarkable routine unfolds in between trials. A pump blasts air through a mesh beneath the sand, to "fluidise" it; the artificial dune bursts into roiling movement. | Also in the name of precision, a remarkable routine unfolds in between trials. A pump blasts air through a mesh beneath the sand, to "fluidise" it; the artificial dune bursts into roiling movement. |
"If you want to do controlled, repeatable experiments… you need some way to control the initial state of the media. | "If you want to do controlled, repeatable experiments… you need some way to control the initial state of the media. |
"With a fluidised bed you're guaranteed that the entire bed will be in the exact same state. And it lets you control things like the compaction very precisely." | "With a fluidised bed you're guaranteed that the entire bed will be in the exact same state. And it lets you control things like the compaction very precisely." |
One thing these controlled experiments have revealed is a surprising simplicity in the snake's slither. It arranges its body into a very regular, flat "S" shape - no matter what. | One thing these controlled experiments have revealed is a surprising simplicity in the snake's slither. It arranges its body into a very regular, flat "S" shape - no matter what. |
"I've studied 10 of these snakes and they always make the same shape," Ms Schiebel says. "That was somewhat surprising. With their long body, they're very flexible - you can imagine they could make all sorts of shapes. | "I've studied 10 of these snakes and they always make the same shape," Ms Schiebel says. "That was somewhat surprising. With their long body, they're very flexible - you can imagine they could make all sorts of shapes. |
"But there's this very specific, stereotyped waveform that they all seem to use." | "But there's this very specific, stereotyped waveform that they all seem to use." |
This observation was in stark contrast to what the team expected - which was, in short, tortuous complexity. | This observation was in stark contrast to what the team expected - which was, in short, tortuous complexity. |
"What I've found... is that the snake is using a waveform that is beneficial for travelling quickly at the surface - and that all of these complex things, like the grains flowing away, or the tracks the snake makes, may not be important." | "What I've found... is that the snake is using a waveform that is beneficial for travelling quickly at the surface - and that all of these complex things, like the grains flowing away, or the tracks the snake makes, may not be important." |
It also marks out the shovel-nosed snake as rather different from other species, which the team has observed adopting much more irregular, complicated shapes in footage from a nearby zoo. | |
Wiggle and wind | Wiggle and wind |
One such species with a famously baffling gait is the sidewinder rattlesnake. Another member of the lab, Henry Astley, has been unravelling its secrets for some time. | |
"Sidewinding is famously confusing," he tells me as we look through some videos of real and simulated sidewinders in action. "There's actually a saying in herpetology, that if you want to go mad, watch sidewinders." | "Sidewinding is famously confusing," he tells me as we look through some videos of real and simulated sidewinders in action. "There's actually a saying in herpetology, that if you want to go mad, watch sidewinders." |
I can see his point. The hypnotic ripples of movement that propel these snakes - sideways - flow up-and-down along their bodies, as well as side-to-side. | I can see his point. The hypnotic ripples of movement that propel these snakes - sideways - flow up-and-down along their bodies, as well as side-to-side. |
"During sidewinding, portions of the snake are lifted and lowered cyclically, in order to allow them to lift themselves over the sand and place their body in a new location," Dr Astley explains. | "During sidewinding, portions of the snake are lifted and lowered cyclically, in order to allow them to lift themselves over the sand and place their body in a new location," Dr Astley explains. |
Working with a lab at Carnegie Mellon University in Pittsburgh, he and colleagues have already unpicked how the sidewinder climbs steep, sandy slopes by copying its movement with a robotic replica. | Working with a lab at Carnegie Mellon University in Pittsburgh, he and colleagues have already unpicked how the sidewinder climbs steep, sandy slopes by copying its movement with a robotic replica. |
"Sidewinders are fantastic at locomoting - they're the real deal, they've got sensory systems and brains and muscles. But with that come the consequences of behaviour; they do what they want to do. | "Sidewinders are fantastic at locomoting - they're the real deal, they've got sensory systems and brains and muscles. But with that come the consequences of behaviour; they do what they want to do. |
"With the robot, we can program it to do what we want it to do." | "With the robot, we can program it to do what we want it to do." |
Using a similar approach - combining video footage of real snakes at Zoo Atlanta, with the programmable Pittsburgh robot - Dr Astley recently reported, for the first time, how sidewinders turn corners. He discovered two contrasting techniques. | Using a similar approach - combining video footage of real snakes at Zoo Atlanta, with the programmable Pittsburgh robot - Dr Astley recently reported, for the first time, how sidewinders turn corners. He discovered two contrasting techniques. |
First, there is the lazy "differential" turn, which works much like a car. | First, there is the lazy "differential" turn, which works much like a car. |
"One side of snake - or one set of wheels on the car - moves further than the other, and as a result, the whole animal rotates and you get this large, gradual turn over many, many cycles," he says. | "One side of snake - or one set of wheels on the car - moves further than the other, and as a result, the whole animal rotates and you get this large, gradual turn over many, many cycles," he says. |
The second option is more of a handbrake turn: the snake instantly reverses its body wiggle and jack-knifes its overall progress by anywhere from 70 to 180 degrees. | The second option is more of a handbrake turn: the snake instantly reverses its body wiggle and jack-knifes its overall progress by anywhere from 70 to 180 degrees. |
"The key is that in differential turning, the sidewinder continues with its head on the left or the right, throughout the turn," Dr Astley says. | "The key is that in differential turning, the sidewinder continues with its head on the left or the right, throughout the turn," Dr Astley says. |
"However in reversals, they instantaneously switch from head-on-the-right to head-on-the-left, or vice versa, allowing them to rapidly change and take off in a whole new direction." | "However in reversals, they instantaneously switch from head-on-the-right to head-on-the-left, or vice versa, allowing them to rapidly change and take off in a whole new direction." |
Better bots? | Better bots? |
When the team applied these findings to the sidewinder robot, which is ultimately aimed at search-and-rescue missions in tricky terrains, they managed to navigate a small maze with a combination of these turns. | When the team applied these findings to the sidewinder robot, which is ultimately aimed at search-and-rescue missions in tricky terrains, they managed to navigate a small maze with a combination of these turns. |
But the research is not all about building better robots - though negotiating sandy landscapes will be crucial, for example, as humans continue to explore Mars. | But the research is not all about building better robots - though negotiating sandy landscapes will be crucial, for example, as humans continue to explore Mars. |
Dan Goldman, who runs the lab at Georgia Tech, says he is most powerfully driven by pure intellectual curiosity. | Dan Goldman, who runs the lab at Georgia Tech, says he is most powerfully driven by pure intellectual curiosity. |
"I'm interested in how nature works, in how living systems manage to do such beautiful and seemingly elegant behaviours in their natural environments." | "I'm interested in how nature works, in how living systems manage to do such beautiful and seemingly elegant behaviours in their natural environments." |
The robots fill a gap, Prof Goldman explains, between mathematical simulations and real-world animal actions. Both the animals themselves and natural terrain - from leaf litter to sand dunes - are difficult to model accurately with computers. | The robots fill a gap, Prof Goldman explains, between mathematical simulations and real-world animal actions. Both the animals themselves and natural terrain - from leaf litter to sand dunes - are difficult to model accurately with computers. |
"We don't have all the mathematics. There's a lot of parameters and it's also not clear how the biological subsystems are put together to effect interesting locomotor feats." | "We don't have all the mathematics. There's a lot of parameters and it's also not clear how the biological subsystems are put together to effect interesting locomotor feats." |
So a robot is a "physical model" that recreates certain elements of the interaction, in a programmable way. | So a robot is a "physical model" that recreates certain elements of the interaction, in a programmable way. |
"It's certainly not a faithful reproduction of an organism - but at least the leg of a robot, or the body of a snake robot, is interacting with a somewhat realistic material." | "It's certainly not a faithful reproduction of an organism - but at least the leg of a robot, or the body of a snake robot, is interacting with a somewhat realistic material." |
And therein, Prof Goldman concludes, lies a fruitful two-way street between his team of biology-focused physicists and the robotics engineers with whom they collaborate. | And therein, Prof Goldman concludes, lies a fruitful two-way street between his team of biology-focused physicists and the robotics engineers with whom they collaborate. |
"It just turns out that the animals we study, and the physics we study... turn out to have some relevance for making better robots." | "It just turns out that the animals we study, and the physics we study... turn out to have some relevance for making better robots." |
He and nearly 10,000 other physicists will be presenting their latest findings next week as the 2016 March Meeting of the American Physical Society. | He and nearly 10,000 other physicists will be presenting their latest findings next week as the 2016 March Meeting of the American Physical Society. |
You can hear an interview with Perrin Schiebel on Thursday's BBC Inside Science on Radio 4, or Science in Action on the BBC World Service. | You can hear an interview with Perrin Schiebel on Thursday's BBC Inside Science on Radio 4, or Science in Action on the BBC World Service. |
Follow Jonathan on Twitter | Follow Jonathan on Twitter |