AERTS, P.*; VAN DAMME, J.; HERREL, A.: Intrinsic mechanics and control of fast cranio-cervical movements in aquatic feeding turtles

Aquatic feeding strikes on agile prey in snake-necked turtles involves fast neck extension, bucco-pharyngo-oesophageal expansion and head retraction. The ultimate, rectilinear acceleration of the head towards the prey requires complex vertebral rotations, largely differing from strike to strike. This poses serious control-demands to the numerous intrinsic neck-muscles which are the sole neck extensors. Mathematical modelling shows, however, that extensor activity might be redundant for this phase of the strike. The ultimate head acceleration always goes along with forceful bucco-pharyngeal expansion. The momentum of the thus induced suction flow suffices to pull the head (and the neck) straight towards the prey. The global control of this buccal expansion proceeds identically to what is observed in primary aquatic feeders : through a rostro-caudal expansion sequence characterized by an optimal timing of the functional components supporting the expansion wave. Yet, distinct structural solutions, both at the skeletal and muscular level, are involved. This points towards prominent hydrodynamic constraints. Extrinsic neck muscles retract the head and neck. Given the high number of degrees of freedom, this musculo-skeletal system is obviously under-determined which compromises control. Erroneous neck folding might be avoided through the presence of a subtle click system at the level of the joint between cervical 5 and 6.