In the face of the physical and physiological challenges of performing breath-hold deep dives, marine vertebrates have evolved different strategies. Although behavioural strategies in marine mammals and seabirds have been investigated in detail, little is known about the deepest-diving reptile – the leatherback turtle (Dermochelys coriacea). Here, we deployed tri-axial accelerometers on female leatherbacks nesting on St Croix, US Virgin Islands, to explore their diving strategy. Our results show a consistent behavioural pattern within dives among individuals, with an initial period of active swimming at relatively steep descent angles (∼–40 deg), with a stroke frequency of 0.32 Hz, followed by a gliding phase. The depth at which the gliding phase began increased with the maximum depth of the dives. In addition, descent body angles and vertical velocities were higher during deeper dives. Leatherbacks might thus regulate their inspired air-volume according to the intended dive depth, similar to hard-shelled turtles and penguins. During the ascent, turtles actively swam with a stroke frequency of 0.30 Hz but with a low vertical velocity (∼0.40 ms–1) and a low pitch angle (∼+26 deg). Turtles might avoid succumbing to decompression sickness (‘the bends’) by ascending slowly to the surface. In addition, we suggest that the low body temperature of this marine ectotherm compared with that of endotherms might help reduce the risk of bubble formation by increasing the solubility of nitrogen in the blood. This physiological advantage, coupled with several behavioural and physical adaptations, might explain the particular ecological niche the leatherback turtle occupies among marine reptiles.
This study was supported by a National Oceanic and Atmospheric Administration grant (#383 NA04NMF4550391) to M.E.L. S.F. was supported by an AXA ‘Young Talents’ Postdoctoral fellowship. A.C.G. was supported by a Wingate Foundation Scholarship and a Swansea University Research Fees Scholarship. We also thank James Casey, Kara Dodge, Emily Karmen, Amanda Southwood, Matthew Witt and all of the WIMARCS representatives and volunteers for their assistance in the field. We are also very grateful to Anders Rhodin, who kindly supplied us with both equipment and sound advice regarding the attachment of the devices. This study adhered to the legal requirements of the countries in which the work was carried out and to all institutional guidelines.
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