Passive cambering and flexible propulsors: cetacean flukes

The flukes are the primary locomotor structure in cetaceans, which produce hydrodynamic thrust as the caudal vertebrae are oscillated dorso-ventrally. Effective thrust generation is a function of the kinematics of the flukes, the angle of attack between the flukes and the incident water flow, and the shape of the flukes. We investigated the effect of bending within the caudal region of odontocete cetaceans to determine how changes in angular displacement between caudal vertebrae could effect passive shape change of the flukes. The internal and external changes of bent flukes were examined with computer tomography. Flukes and tailstock were removed from deceased Delphinus delphis, Lagenorhynchus acutus, Peponocephala electra, Phocoena phocoena and Tursiops truncatus, and bent on an adjustable support at 0, 45 and 90 degrees. At 0 degrees, cross-sections of the flukes displayed a symmetrical profile. Cross-sections of bent flukes (45 degrees, 90 degrees) were asymmetrical and showed a cambered profile. Maximum cambering occurred close to the tailstock and decreased toward the fluke tip. Maximum angular displacement occurred at the 'ball vertebra', which was located posterior of the anterior insertion of the flukes on the tailstock. Bending at the 'ball vertebra' passively cambers the flexible flukes. Cambering could increase hydrodynamic force production during swimming, particularly during direction reversal in the oscillatory cycle.