The dolphin's flipper plays a role as a rudder, but the specific mechanism has not yet been elucidated in detail. To clarify what kind of flipper movement is related to its steering function, a swimming experiment was performed using a wooden dolphin model. The model's flippers were made of polyvinyl chloride and tilted ventrally and swung backwards by approximately 45°. This was defined as the neutral position of the flipper. When the base of the flipper, which corresponds with the common tubercle of the humerus and presumably formed by fusion of the greater and lesser tubercles, was laterally rotated, the flipper was drawn forward (forward swing). Conversely, when it was medially rotated, the flipper was drawn backwards (backward swing). The forward swing of the flipper was accompanied by lifting of the leading edge (radial edge) of the flipper, whereas the backward swing was accompanied by lowering of the leading edge. The experiment was performed using nine combinations of neutral positions and forward and backward swings. When the right and left flippers had the same swing, the model body swam straight, submerged, or surfaced. The model body turned to the forward swinging side when the swing movements of the right and left flippers diverged. When only one side was involved in the swing movement, the model body turned to the swing side. Stated differently, the flipper swung in response to the rotation of the flipper base, and the combination of right and left flippers swinging movements dictated the direction in which the model swam. Therefore, the results of the swimming experiment suggest that the rotation of the flipper base and the ensuing swinging motion are related to dolphin model’s steering function.
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