Contribution of flippers and dorsal fins to the lateral/directional inherent stability during straight‐line swimming in small cetaceans

Abstract

AbstractFins are among the most important morphological appendages of aquatic animals. Although sall cetaceans have flippers and dorsal fins with a variety of morphologies and postures, the diversity and function of the fins have not attracted much attention thus far. In this study, we recorded the behaviour of five captive species of small cetaceans, including two species lacking dorsal fins, and examined the contribution of the dihedral angles of their flippers and dorsal fins in the lateral/directional inherent stability during straight‐line swimming. All the species were found to maintain their flippers at a specific negative dihedral angle during straight‐line swimming, and a significant difference was observed between species with and without dorsal fins: species without dorsal fins were found to swim by maintaining the dihedral angle of their flippers closer to 0° compared to species with dorsal fins. The rolling moment generated by the dihedral angle of the fins in all the species was a relatively small roll‐promoting moment that ensures manoeuvrability. In summary, small cetaceans are likely to have one of two characteristics, either left and right flippers with a near‐zero‐degree dihedral angle or a dorsal fin, to reduce the roll‐promoting moment. Our results highlight important aspects of swimming stability in dolphins, providing essential insights into the movement mechanisms of aquatic fish and mammals.