Intrinsic Mechanics and Control of Fast Cranio-Cervical Movements in Aquatic Feeding Turtles

Aquatic feeding strikes on agile prey in snake-necked turtlesinvolve fast neck extension, bucco-pharyngo-oesophageal expansion,and head retraction. The ultimate, rectilinear accelerationof the head towards the prey requires complex vertebral rotations,that vary widely from strike to strike. This poses complex motorcontrol issues for the numerous intrinsic neck-muscles, whichare the sole neck extensors. Mathematical modelling revealsthat extensor activity might be superfluous for this phase ofthe strike. The ultimate acceleration of the head at the endof the strike always coincides with forceful oropharyngeal expansion.The momentum of the induced flow of water is sufficient to pullthe head (and the neck) straight towards the prey. This buccalexpansion proceeds identically to that observed in primary aquaticfeeders: a rostro-caudal expansion sequence characterized byan optimal timing of the functional components supporting theexpansion wave. Yet distinct structural solutions, both at theskeletal, and muscular level, are involved. This points towardsprominent hydrodynamic constraints. Head and neck are retractedby extrinsic neck muscles. Given the high number of degreesof freedom, this musculo-skeletal system is obviously under-determined,which compromises control. We propose that erroneous foldingof the neck (i.e., diverging from the highly persistent retractedconfiguration) might be avoided through the presence of a subtleclick system at the level of the joint between cervical vertebrae5 and 6.