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How Snakes Fly (Hint: It's Not On A Plane)

Flying snakes like Chrysopelea paradisi, the paradise tree snake, normally live in the trees of South and Southeast Asia. There, they cruise along tree branches and, sometimes, to get to the ground or another tree, they'll launch themselves into the air and glide down at an angle.

They undulate their serpentine bodies as they glide through the air, and it turns out that these special movements are what let these limbless creatures make such remarkable flights.

That's according to some new research in the journal Nature Physics that involved putting motion-capture tags on seven snakes and then filming them with high-speed cameras as the snakes flew across a giant four-story-high theater.

How far they can go really depends on how high up they are when they jump, says Jake Socha at Virginia Tech, who has studied these snakes for almost a quarter-century. He recalls that one time he watched a snake start from about 30 feet up and then land nearly 70 feet away. "It was really a spectacular glide," Socha recalls.

Part of the way the snakes do this is by flattening out their bodies, he says. But the snakes' bodies also make wavelike movements. "The snake looks like it's swimming in the air," he says. "And when it's swimming, it's undulating."

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He's long wondered if these undulations served any purpose. Perhaps, he thought, this is just something the snakes do out of habit.

"All snakes undulate when they move. And so on the ground, on a tree, in the water, they are creating these side-to-side waves," Socha says. "It's not crazy to think that when the snake jumps into the air, the snake goes, 'Hey, I'm a snake. I undulate. That's what I should be doing.' "

The other possibility, however, is that the undulation actually helps the snakes fly. And a few years ago, Socha was talking about the snakes with someone he'd just met at Virginia Tech. "This guy said, 'Oh, well, you know, maybe you'd be interested in The Cube,' " Socha says. "And I was like, 'The Cube? What's The Cube?' "

The Cube, it turns out, is a vast theater space that looks like an empty black box— one that's equipped with high-tech, super-expensive motion capture cameras. "When I walked in into this black box theater," says Socha, "I just looked around and went, 'Oh, this is perfect. I so want to glide my snakes in this space.' "

After convincing decision-makers that the snakes wouldn't get hurt or escape from The Cube, Socha and his colleagues prepared the arena by putting foam padding down on the floor and sticking a fake tree in there to serve as a target for a flying creature.

Then they had to prepare the snakes. Normally, in motion capture done for Hollywood movies, actors wear little reflective tags that are placed all over their bodies. So, along the snakes' bodies, the researchers put around 11 to 17 bits of reflective tape.

"These reflectors were able to nicely indicate where different parts of the body were in 3D space," Socha says. "And our cameras were recording it at almost 180 frames per second. And so that is fast enough that we got lots of representations of the snake as it went through its full glide."

For Isaac Yeaton, a mechanical engineer at the Johns Hopkins University Applied Physics Laboratory, these experiments were the first time he ever got to see a flying snake really do its thing. He says the actual glide is quite short, only one or two seconds long.

"It happens really quickly. And it's hard to see all the detail by eye. So that's why we need high speed cameras and high speed motion capture," Yeaton says.

The snakes often bounced up off the foam padding on the floor, as nearby snake handlers waited to pick up the animal after it landed, he recalls. One time, he was standing at the perimeter of the room when a flying snake headed right toward him.

"I was able to put my hands out and catch it," Yeaton says. "So that was different!"

Over about two weeks, the researchers filmed dozens of glides. Yeaton says they found that the undulation has some distinct features in addition to the broad S-shaped horizontal movements that are easiest to see.

"There's a vertical wave. And this was really strange. We weren't expecting to see this," says Yeaton, who says the only other example of this he's aware of is in sidewinder snakes that move across the sand. The researchers also found that the back half of the flying snake's body makes a kind of up-and-down bending motion.

The researchers used all of the information they'd gathered about these movements to create a computer model: a virtual representation of a flying snake. Using this virtual snake, they could "turn off" different aspects of its motion to see what would happen to its flight.

What they found, says Yeaton, is that without undulation, the snakes' glides became unstable — the flying critters would plummet or tumble.

"Snakes usually undulate for propulsion — they're using it to push against the environment," Yeaton says. "But flying snakes are undulating for stability."

Jennifer Rieser, a physicist at Georgia Tech who has studied snake slithering and who was not part of the research team, says this is a "cool" finding. The snakes' undulation in the air, she says, "actually seems to have a pretty important consequence for their movement."

She thinks this would be interesting to explore in biologically inspired, flying robot snakes.

"As far as I know, there's not an aerial snake-like robot yet," says Perrin Schiebel, a researcher at Harvard University. She also found this to be an intriguing study, and says it suggests ways of potentially modifying existing snake robots to give them new abilities.

Some other "flying" animals, such as squirrels and lizards, just seem to hold a static pose, notes Schiebel. She thinks it's really striking that the same kinds of maneuvers that help snakes move in water and on sand can also help them when they're gliding through the air.

Copyright 2020 NPR. To see more, visit https://www.npr.org.

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