Abstract

In this paper, we investigate the structure and mechanical properties of the dense connective tissue in the tail flukes of dolphins. The SEM graphs show that the dense connective tissue has a laminated structure. The results of compression test show that the dense connective tissue had a higher modulus in the spanwise direction than in the other two directions, and the compression in the thickness direction and chordwise direction both bring high spanwise deformation, The lateral confined compression test show that the pressures in the thickness direction and chordwise direction can transfer to the spanwise direction.

Introduction

Bottlenose dolphins are capable of bursts of up to 9.7 m/s. Such an extraordinary speed has brought a famous paradox so-called "Gray's paradox" which was to figure out how dolphins can obtain such high speeds and accelerations with what appears to be a small muscle mass (Gray, 1936). Gray made an estimate of the power a dolphin could exert based on its physiology, and concluded the power was insufficient to overcome the drag forces in water. He hypothesized that Dolphin's skin must have special anti-drag properties. The paradox is not yet solved in spite of numerous investigations into the drag minimization of swimming dolphins from various viewpoints such as the streamlined body shape, viscous dampening, dermal ridges and boundary layer heating (Fish, 2006).

The dolphins generated forward thrust when water was pushed in the direction opposite to swimming by oscillating their flukes vertically (Fish, 1998). As the solo propulsive device of a dolphin, the tail fin has been considered to play an important role in their high speed swimming. Due to the absence of bony fin rays in the tail flukes of dolphins, dolphins cannot control the shape of their flukes like some fishes which can control tail fin shape by controlling each fin ray. The deflection of the tail flukes of a dolphin was driven by the hydrodynamic force imposed upon the flukes rather than being specified in advance (Sun, 2010). We thought that the mechanical properties of the tail flukes of dolphins have large effects on the deflection of the tail flukes, and therefore the mechanical properties of the tail flukes could impact on the thrust of dolphins. It was necessary to study the structures and mechanical properties of the tissues in the tail fin of dolphins. In the previous study, we have found that the tail flukes of dolphins were supported by two tissues, a dense connective tissue and two ligamentous layers with a sandwich structure. The dense connective tissue has nearly 90% weight of tail fin. The ligamentous layers are composed of collagen fiber bundles in a nearly parallel arrangement mode. When dolphins oscillated the tail fin to generate thrust, the ligamentous layer was stretched, the dense connective tissue was compressed (Sun, 2010).

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