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 pleur odiran turtle Chelodina longicollis were studied by means of x-ray cin ematography. Radio-opaque markers were inserted near the cervical join ts to allow calculation of joint rotations between the successive vert ebrae expressed as a function of time and head position. Head retracti on as a percentage of the extended neck configuration and angular and linear velocities were also calculated. A combination of muscular orga nisation and kinematics shows that the neck is divided into two functi onal regions, anterior and posterior to the biconvex fifth cervical ve rtebra respectively. Head retraction proceeds in two phases. During th e first phase the animal retracts the head very fast underneath the ca rapace but leaves the neck partially exposed. This phase (with excepti on for C3-2 and C5-4) shows no significant differences in the timing o f the peak-velocities. During the second phase rotations occur mainly in the proximal joints leading to the maximally retracted configuratio n. Left and right head retractions are mirror images. Retractions neve r 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 ne ck. Peak-velocities of these joints strongly correlate with each ether but also with head retraction. When expressed as functions of head po sition, the rotation patterns of the proximal joints are particularly stereotyped. It is hypothesized that both initial angels and stereotyp ical retraction patterns are required to allow a fast (escape) retract ion powered by a simple motor pattern.