Fig. 1. Setup and trajectory variables. (A) Observational setup. Snakes were launched from a horizontal branch (height=9.62 m) at the top of a scaffolding tower in the Singapore Zoological Gardens. Two digital videocameras recorded a stereo view of the trajectories from a position approximately 2.5 m above the branch (Position 1). In a few trials, the cameras were placed lower on the tower (at a height of approximately 5.8 m; Position 2) to record a closer view of the gliding phase of the trajectory (grey arrows). Markers were placed in the field in a rough 2 m grid to serve as reference points for the 3-D coordinate reconstruction; an average of eight were used per trial. An arbitrary reference system was set up with the Y axis projecting forward (perpendicular to the tower face), the X axis to the side (parallel to the tower face) and the Z axis in the vertical, with the origin (+) located at the ground directly beneath the distal tip of the branch. (B) Definition of glide angle and horizontal body angle. Sequence is a side view of a trajectory at an early stage of the shallowing glide. Points represent the head/body junction (triangles), body midpoint (circles) and vent (squares) from one snake during one trajectory, sampled at 30 Hz. Temporal sequence is from upper left to lower right. Instantaneous glide angle (inset, lower left) was calculated as the angle between the principal axis of variation of three consecutive midpoint coordinates and the horizontal plane. Anterior and posterior horizontal body angles (inset, upper right corner) were calculated as the angle between a line connecting the head to the midpoint and the horizontal plane (HBA_A) and as the angle between a line connecting the midpoint to the vent and the horizontal plane (HBA_P). HBA_P values were given a negative sign convention relative to HBA_A so that equal angles indicate equivalent body postures. Scale bar, 20 cm.