Shape and material characteristics of the trachea in the leatherback sea turtle promote progressive collapse and reinflation during dives.

The leatherback turtle regularly undertakes deep dives and has been recorded attaining depths in excess of 1200 m. Its trachea is an almost solid, elliptical-section tube of uncalcified hyaline cartilage with minimal connective tissue between successive rings. The structure appears to be advantageous for diving and perfectly designed for withstanding repeated collapse and reinflation. This study applies Boyle's law to the respiratory system (lungs, trachea and larynx) and estimates the changes in tracheal volume during a dive. These changes are subsequently compared with the results predicted by a corresponding finite element (FE) structural model, itself based on laboratory studies of the trachea of an adult turtle. Boyle's law predicts that the lungs will collapse first during the initial stages of a dive with tracheal compression beginning at much deeper depths after complete air mass expulsion from the lungs. The FE model reproduces the changes extremely well (agreeing closely with Boyle's law estimations) and provides visual representation of the deformed tracheal luminal area. Initially, the trachea compresses both ventrally and dorsally before levelling ventrally. Bulges are subsequently formed laterally and become more pronounced at deeper depths. The geometric configuration of the tracheal structure confers both homogeneity and strength upon it, which makes it extremely well suited for enduring repeated collapse and re-expansion. The structure actually promotes collapse and is an adaptation to the turtle's natural environment in which large numbers of deep dives are performed annually.