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Moss Arts Center, Virginia Tech. (Photo: Wikimedia Commons)
Moss Arts Center, Virginia Tech. (Photo: Wikimedia Commons)

Opinion | Flying snakes, those guides from beyond

For these type of snakes, gliding without undulation tends to become unstable

At the Virginia Polytechnic Institute and State University—known as Virginia Tech—its Moss Arts Center has a building called, simply, “The Cube". Because it is shaped like one. Just a vast empty room four storeys high, this is a hi-tech theatre and laboratory. It’s used for music, dance, films and immersive art installations—as also for scientific experiments and research of all kinds. And in May and June of 2015, it was used by snakes.

Not just any snakes, but the genus Chrysopelea: flying snakes from South-East Asia, India, Sri Lanka and parts of China and the Philippines. They are the only limbless vertebrate animals we know of that are able to fly. Now this is not like birds fly. The snakes just glide through the air. But they use their bodies to make them better gliders than, for example, flying squirrels: they can fly horizontally, slow their descent and turn in the air. The best Chrysopelea flyer is the paradise tree snake (Chrysopelea paradisi); only about a metre in length, it has been known to fly as far as 100 metres.

So why were these flying snakes infesting The Cube? Involuntarily, of course. The Cube is equipped with sophisticated motion-capture cameras, capable of taking nearly 200 frames per second. That made it a perfect space to study the motion of these little flying creatures.

Jake Socha is a professor in Virginia Tech’s Department of Biomedical Engineering and Mechanics and has studied flying snakes for years. He has several clips online of their flights. Socha launches Chrysopelea specimens from a height and they undulate gracefully through the air, almost as if swimming, to the ground—where assistants grab them before they slither into the unknown. Unmistakable is the muscular, sinuous leap that snakes make to start their flight. They also flatten their bodies in the air to almost twice their usual width—resembling, in cross-section, a frisbee, the popular flying disc. For the same reason that frisbees glide so well, Chrysopelea glide well too.

But “undulate"? We know that’s how snakes move on the ground. That undulation propels them forward. But why undulate in the air? Is it just a biological memory, an instinct that snakes have? Is it just that motion, to them, means undulation, so they undulate while moving through the air just like on the ground? Does undulation actually help them fly, and if so, how? In scientific language, “it is unclear if undulation is a functional requirement of gliding, or simply a behavioural remnant of snake locomotion, as all snakes are capable of lateral undulation, an evolutionary ancient motor pattern produced by waves of muscle contraction propagating down the body."

Socha puzzled over this dilemma for years. On the face of it, the “memory" explanation should not, well, fly. After all, evolution’s remorseless logic argues against features and characteristics that have no utility. For that reason alone, undulating in the air must have some purpose. Still, reasoning alone is not good enough. “To determine if aerial undulation is a flight control strategy or a non-functional behavioural vestige of lateral undulation" in these snakes, Socha and his fellow-researchers took Chrysopelea to The Cube.

They placed an oak tree branch about 9 metres up—the snakes’ launch platform—and padded the floor so the snakes would not be hurt on landing. They erected a make-believe tree on the floor because, in the wild, the snakes don’t always fly down—sometimes they fly from one tree to another. They set up the Cube’s cameras to take high-speed images of the snake in flight. And they also worked on the snakes. Given how short most of these flights actually are—just a few seconds—it’s difficult to comprehend exactly what the snake is doing in the air. To better see and understand their motion, the team stuck reflective tape along their bodies: 11 to 17 pieces on each animal. This allowed them to trace how different parts of the body move.

There’s a fascinating collage of images in their paper, though it took me a while to understand their full import. These are different views of a single flight, from the top, the side and the rear. Each has several squiggly lines of different colours. These are, of course, the paths traced through the air by different bits of tape. And of course again, the squiggly lines are far from synchronized. Seen from the side, they are relatively spread out at the start, then bunch together, then spread out again. This suggests that the snake is more or less stretched to its full length as it begins its flight, then draws its tail in, then stretches out again. Seen from the top, the asynchrony is much more dramatic. All the paths move from side to side—much like individual snakes themselves—but the extent of the side-to-side movement (the “amplitude") increases markedly as you go from the head of the snake to its tail. The head, then, is relatively static compared to the rest of the body. And all this, in a flight that lasts just under a second-and-a-half.

The researchers combined the data from several such flights by seven different snakes to create an “anatomically accurate" 3-D image of a Chrysopelea snake in flight. This allowed them to break down the undulation mathematically into two separate “orthogonal" motions. That is, there are in effect two waves that travel along the snake’s body—one horizontal and one vertical—and taken together, they result in the undulation that is observed. Both start at the head as the snake leaps from the platform, and travel along the body as it glides. The horizontal wave has a larger amplitude, but the vertical wave has twice its frequency.

From here on out, the mathematics the researchers use gets too esoteric for this column. But it allowed them to simulate snake flights and compare these simulations with the actual snake flights. They found that “the measured glides show slightly better performance than simulated glides, but they show qualitatively similar behaviour." So because the simulations were broadly similar to the observed flights, they could now use the mathematics to vary the nature of the horizontal and vertical waves, and thus the nature of the undulation itself. What effect would that have on the flight paths? As you would expect, they found that undulation does generally improve the flight performance. Launching from 10m above ground, 94% of the flights that used undulation were “stable"—meaning they didn’t degenerate into free fall. Whereas without undulation, just 50% were stable. Undulation also increased the horizontal distance the snake travelled. In fact, when the launch was 75m up—not unusual for Chrysopelea in their natural habitat—resulting in a longer flight, undulation had an even more pronounced effect on the horizontal distance travelled.

The simulations also showed that gliding without undulation tended to fail because it resulted in “roll and pitch instabilities". That is, the (simulated) non-undulating snake would go into a nosedive, or a nose-up stall, or its body would roll over uncontrollably, and any of these would destroy the flight—just as they might do to a plane. The inference is that in-air undulation acts to prevent these destructive motions and, in so doing, “markedly increase[s] glide performance." Very different from what undulation does for a snake on the ground. Besides, there are implications here for any attempt to build robots that fly like snakes. This is a worthwhile goal because the mechanics of a snake’s flight seems significantly simpler than, say, a crow’s. But we’d have to build into it the ability to undulate. Only then will the robot fly as Chrysopelea paradisi does: gracefully and elegantly, but also reliably.

Elsewhere in the Moss Arts Center, you will find a staircase with The Guest House, a poem by Jalal al-Din Rumi, engraved line by line into its steps. I wonder if, while snakes were gliding about The Cube, anyone recited these lines from it:

Still treat each guest honorably.

He may be clearing you out

for some new delight…

Be grateful for whoever comes,

because each has been sent

as a guide from beyond.

All quotes from “Undulation enables gliding in flying snakes", Jake Socha et al. Nature Physics.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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