To understand the effect of the ridges on the drag and lift forces and flow near the body, a carapace model without the ridges but with same frontal ( A f) and planform ( A p) areas was also constructed ( Fig. A carapace model for wind tunnel experiments was constructed based on a geometric information of a stuffed leatherback turtle at National Science Museum, Daejeon, Korea. In the present study, we constructed carapace models of a leatherback turtle with and without the longitudinal ridges, and conducted force and velocity measurements to investigate the hydrodynamic roles of the ridges in different modes of swimming of the leatherback turtles. Therefore, the hydrodynamic characteristics during the vigorous swimming (swimming at large negative angles of attack) should also be considered to fully understand the roles of the longitudinal ridges. The vigorous swimming is important for hatchlings as it provides a means to overcome positive buoyancy and escape from predators 26, 27. Their swimming patterns are divided into a routine swimming (slow) near the water surface and a vigorous swimming (fast and large energy consuming) underwater 26. Therefore, hatchlings swim mainly in shallow water 25. On the other hand, hatchling or juvenile leatherback turtles cannot dive as deep as adults since they cannot hold their breath for a long time due to the low tissue volume for oxygen storage and high mass-specific metabolic rates 24, 25. Therefore, the hydrodynamic performance in ascending swimming conditions at positive angles of attack can be energetically important for the leatherback turtles. In contrast, they have to actively swim up at high pitch angles (or high angles of attack) during the ascending period to overcome the negative buoyancy 5. This negative buoyancy enables diving, so their descending swim can be largely energy-efficient 5, 23. Breath-hold divers such as leatherback turtles have no buoyancy-control organ and thus experience negative buoyancy in deep water due to the compression of pulmonary air by water pressure 5, 23. The V-shaped diving, used for foraging and transit, is a typical diving pattern of adult leatherback turtles 3, 5, 21, 22. The diving patterns of leatherback turtles are divided into the V-shaped diving, U-shaped diving, and sub-surface swimming according to the shapes of the diving profile 3, 5, 21, 22 (see, for example, Supplementary Fig. Therefore, in the present study, we investigate their roles in the conditions that represent the swimming modes of hatchling and adult leatherback turtles. However, no study has been made for their hydrodynamic roles yet. In this respect, the hydrodynamic roles of ridges in leatherback turtle’s swimming should be interesting to investigate. ![]() Unlike these morphological features, the longitudinal ridges of a leatherback sea turtle are located along the entire body. All of these morphological features are located on the leading edges of the wing and flipper, on the frontal part of the body, and on the trailing edge of the wing. ![]() ![]() For example, dorsal and ventral keels of a boxfish generate streamwise vortices, and these vortices are considered to increase the hydrodynamic stability 11, 12, 13 tubercles on the leading edge of a humpback whale’s flipper increase the lift by generating streamwise vortices and delaying separation 14, 15, 16 an alula on the leading edge of a bird’s wing produces a streamwise vortex and increases the lift 17 a serrated leading edge of an owl’s feather also produces streamwise vortices to fly silently 18, 19 spade-like protrusions on the trailing edge of a dragonfly wing provide an idea for reducing drag on an airfoil with a gurney flap 20. Some of nature’s morphological features have been shown to provide better aero- and hydrodynamic performances. Some conjectured that these ridges represent an evolutionary adaptation for keeping the flow around the body laminar 9, 10. Among these, five longitudinal ridges on their carapace are a notable feature. On the body of leatherback turtles, there are a few remarkable morphological features such as soft carapace, big flippers, and longitudinal carapace ridges that distinguish them from other marine turtles 8. They are also known for long-distance migration and considered to be highly adapted to pelagic swimming 2, 4, 5, 6, 7. Leatherback sea turtles ( Dermochelys coriacea), the largest and the deepest diver among marine turtles, are known to have superior diving ability 1, 2, 3.
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