Meeting Abstract
The flukes are the primary locomotor structure in cetaceans; they produce hydrodynamic thrust as the tail oscillates dorso-ventrally. As opposed to conventional propellers, the flukes of dolphins were shown to maintain a high propulsive efficiency over an extended range of speeds. It was hypothesized that the flexibility of the flukes was responsible for the high efficiency. The flukes are largely composed of a dense collection of collagen fibers, permitting both spanwise and chordwise bending. We investigated the effect of bending within the caudal region of odontocete cetaceans over a range of swimming speeds of 0.4 to 6.9 m/s. Lateral views from video recordings (60 frames/s) were analyzed for Tursiops truncatus and Orcinus orca as they swam around a large pool freely or under trainer control. Spanwise bending was restricted to the fluke tips. To measure the degree of chordwise bending of the flukes throughout the stroke cycle, a Flex Index was computed as ratio of the chord length to the camber line length of the longitudinal vertebral axis in the flukes, where a value of unity indicted no flexing. Fluke chord was maximally bent as the fluke reversed vertical direction during the oscillatory cycle with a chord reduction of 2-16%. No difference in Flex Index was found between transitions of upstroke and downstroke, but the Flex Index was greater at the transition points than at mid-stroke. Despite the 12.6-fold size difference between the two species, fluke bending was similar. At speeds greater than 2 m/s, the Flex Index increased directly with increased swimming speed for both species. The flexibility exhibited in the flukes changed with increasing propulsive load and could aid in enhanced propulsive efficiency.
