A 3-D kinematic analysis of gliding in a flying snake, Chrysopelea paradisi

Abstract

Flying snake species (Chrysopelea) locomote through the air despite a lack of appendages or any obvious external morphological specialization for flight. Here photogrammetric techniques were used to investigate C. paradisi's aerial trajectory in three dimensions. Two videocameras arranged in stereo were used to record head, midpoint and vent landmarks on snakes that jumped from a horizontal branch at a height of 9.62 m and landed in an open field. The coordinates of these landmarks were reconstructed in three dimensions and used to analyze patterns of position, glide angle and speed concurrently with changes in body posture in 14 glide sequences from different individuals. C. paradisi's trajectory was composed of a ballistic dive followed by a shallowing phase in which the path became more horizontal; for most glide trials, no equilibrium phase was observed. In the ballistic dive, the snake changed posture from generally straight to a wide 'S' shape in planview and began aerial undulation. Shortly after the ballistic dive, the snake's speed transitioned from an initial acceleration to stable or to a different rate of increase or decrease. Aerial undulation, in which high-amplitude traveling waves pass posteriorly down the body, was a prominent locomotor behavior. In mid-glide, this undulation occurred with the anterior body oriented approximately parallel with the ground and the posterior body cycling up and down in the vertical plane. The body angle of attack for the anterior body for one trial was 20-40°. Snakes traveled a horizontal distance of 10.14±2.69m (mean ± S.D.) while reaching an airspeed of 10.0±0.9 m s-1, sinking speed of 6.4±0.8 m s-1 and horizontal speed of 8.1±0.9 m s-1. The glide path shallowed at a rate of 20±6° s-1 and reached a minimum glide angle of 28±10°, with a minimum recorded glide angle of 13°. C. paradisi are surprisingly good gliders given their unconventional locomotor style, with performance characteristics that rival or surpass more familiar gliding taxa such as flying squirrels. As in other gliders, C. paradisi is potentially capable of using aerial locomotion to move effectively between trees, chase aerial prey, or avoid predators.