Many animals from fish to snakes move using undulations, or rhythmic back-and-forth motions of their bodies, for propulsion. As snakes have no limbs for additional propulsion, they rely entirely on the undulations of their bodies to get around. Some snakes in Southeast Asia have even taken to the skies, flattening their bodies to resemble an airplane wing to create lift as they glide from trees. All these snakes undulate when gliding, but it was not known whether serpentining helps them fly or whether it is just a leftover behavior from their ground-dwelling ways. Do undulations give flying snakes more air time?

Isaac Yeaton and colleagues at Virginia Tech, USA, sought to answer this question by recording high-speed 3D videos of paradise flying snakes (Chrysopelea paradisi) making gliding descents between an oak branch on an artificial tree and the ground in a theatre on the Virginia Tech campus. The team then captured the motion of small reflective markers placed along the length of the snake’s body with 23 cameras arranged around the test area to accurately measure how the snakes moved and changed their body shape during the descent. Then they used the measurements to reconstruct the snake’s movements and create simulations of snake glides without using undulations. Finally, they compared the gliding performance of these simulations with simulations of regular undulating snakes to determine whether the maneuver helps the reptiles fly.

While ground-dwelling snakes oscillate their bodies from side to side when moving on land, Yeaton and colleagues’ new measurements showed that flying snakes use not only lateral undulations but also vertical undulations. However, these up and down body ripples are smaller and twice the frequency of the side to side waves, and the peaks of the vertical bends coincide with the locations that are bent sideways least at that instant. In simulations, the snakes that used horizontal undulations when descending from a height of 75 m were able to glide an average of 7 m farther than snakes with no undulations by improving their ability to stabilize rotations of the body. In contrast, vertical waves had a smaller effect on performance.

Another behavior that the snakes used to improve their flight stability was elevating their tail slightly above their head during glides. The team found that simulated snakes with the tail well below their head began pitching downward and fell, while snakes with tails high above their head pitched upward and glided over shorter distances than snakes that held their tail either level with, or slightly above, their heads. Simulated snakes that held their tails slightly higher than their head flew the furthest. Overall, the snakes used undulations and the slope of their bodies from head to tail relative to the ground to achieve longer air times by preventing falls out of the sky due to rotational instability.

It turns out that flying snakes’ undulations are not just a vestigial leftover from their grounded ancestors, but actually help them to glide farther and with better stability. These reptiles have co-opted a movement pattern already in their repertoire to take to the skies. Understanding how the undulations of flying snakes help them to remain airborne will hopefully enable the development of robotic models to test how the nervous system and muscles of these animals control their flights and may also inspire novel mechanisms to stabilize flying robots.

References

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Undulation enables gliding in flying snakes
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Nat. Phys.