Odontocete peduncle tendons for possible control of fluke orientation and flexibility

Dorso-ventral oscillations of cetacean caudal flukes generate lift-based thrust for swimming. Movements of the flukes are actuated by epaxial and hypaxial muscles through caudal tendons inserting onto vertebrate in the peduncle. To determine if the caudal tendons in the peduncle affect the flexibility of the flukes, we must understand how the tendons from axial muscles insert onto the caudal vertebrate. The purpose of this study was to provide a detailed description of the various tendons within the cetacean peduncle with regard to their role in swimming and flexibility. Dissection of the peduncle and flukes of multiple odontocete species showed that there were two distinct epaxial tendon sets within the peduncle: (1) extensor caudae medialis tendon (ECM) and (2) extensor caudae lateralis tendon (ECL). There is one distinct hypaxial tendon set, the medial tendon of the hypaxialis lumborum (MHL). The ECM and MHL tendons inserted serially onto caudal vertebrae while the ECL inserted exclusively onto the terminal vertebrae posterior to the fluke insertion. It is typical that tendons insert onto bone, however, the connection to the core fibrous layer of the flukes suggests an element of active control of the flexibility of the flukes via the axial muscles. Tension from muscular contraction transmitted through the tendons could affect both spanwise and chordwise flexibility. Changing flexibility could modulate thrust and efficiency over an extended operation range of swimming speeds in cetaceans.