Undulation enhances stability, enabling gliding in flying snakes *

Aerial undulation is a unique form of undulation used by flying snakes as theyglide through the air. Ostensibly, the role of aerial undulation is the same as terrestrial and aquatic undulation: to generate propulsive thrust. However, flying snakes must contend with non-planar aerodynamic forces and rotational motion to glide successfully, which may not require undulation. Is aerial undulation a non-functional behavioral vestige, or is it a novel aerial control strategy? We used high-speed motion capture to fully quantify the aerial undulation waveform. The body motion consists of two waves of bending and a net out-of-plane motion of the body. The vertical wave has twice the spatial and temporal frequencies as the horizontal wave and is phase-shifted by 90°. Using these results, we tested the effects of undulation by developing an anatomically accurate mathematical model, which indicates that undulation is not a strict requirement for the simulated snake to glide. However, the addition of undulation, even in the absence of feedback control, drastically increases the glide performance by stabilizing the rotational motion. This work shows that aerial undulation serves a different purpose than other known uses of undulation in animals.