Kinematics of the escape head retraction in the common snake-necked turtle, Chelodina longicollis (Testudines: Pleurodira: Chelidae).

Cervical movements during the fast escape head retraction in the pleurodiran turtle Chelodina longicollis were studied by means of x-ray cinematography. Radio-opaque markers were inserted near the cervical joints to allow calculation of joint rotations between the successive vertebrae expressed as a function of time and head position. Head retraction as a percentage of the extended neck configuration and angular and linear velocities were also calculated. A combination of muscular organisation and kinematics shows that the neck is divided into two functional regions, anterior and posterior to the biconvex fifth cervical vertebra respectively. Head retraction proceeds in two phases. During the first phase the animal retracts the head very fast underneath the carapace but leaves the neck partially exposed. This phase (with exception for C3-2 and C5-4) shows no significant differences in the timing of the peak-velocities. During the second phase rotations occur mainly in the proximal joints leading to the maximally retracted configuration. Left and right head retractions are mirror images. Retractions never start from a completely extended neck configuration. Initial angles always occur in C6-5 (joint between vertebrae 5 and 6) and C8-7. These joints are also the major bending sites for full retraction of the neck. Peak-velocities of these joints strongly correlate with each ether but also with head retraction. When expressed as functions of head position, the rotation patterns of the proximal joints are particularly stereotyped. It is hypothesized that both initial angels and stereotypical retraction patterns are required to allow a fast (escape) retraction powered by a simple motor pattern.