Evolutionary morphology of the neck system in Ratites, Fowl and Waterfowl

The avian neck is a complex, kinematically redundant system that controls t he position and orientation of the head. The kinematic redundancy is resolv ed by movement principles, which result in characteristic movement patterns . General neck movement patterns are compared between Ratites, Fowl and Wat erfowl in order to find a relationship with anatomical differences and to i dentify the biological role(s) to which neck movement is adapted. Kinematic analyses show that Fowl move their vertebrae according to a minimal rotati on principle that maximizes rotation efficiency. The resulting movement pat tern shows rotations in some joints, while keeping the other vertebrae as s traight bars. Waterfowl show a pattern of successive, rather than simultane ous rotations of vertebrae, limited to the rostral part of the neck. A thir d movement pattern is found in Ratites, which show successive rotations of the vertebrae in the middle region of the neck. The ratite-pattern is relat ed to large vertical head trajectories, and is occasionaly also found in th e Chicken. However, due to large body movements in the Chicken, head trajec tories are relatively much shorter than in the Rhea. A kinematic neck model based on the minimal rotation principle only produces the Waterfowl patter n if a constraint on the movement of the caudal vertebrae is introduced. We conclude that a fundamental change occurred in the movement pattern of the Waterfowl neck, which is energetically advantageous and an adaptation to a quatic feeding.