Using the axial skeleton as armor: Mechanical behavior of sea turtle carapaces throughout ontogeny.

The shells of turtles serve as protection, yet shell shape and natural history widely vary among turtles. Here we identify the mechanical behavior that provides marine turtles, species characterized with fusiform shells, with biomechanical strength and resilience. The multi-layered carapacial bone structure seemingly serves a protective role for the muscles, nerves and viscera it houses. What are the shell's material properties that provide protection? Most previous work focused on non-marine turtles that differ in natural history and shell morphology from marine species. We measured carapacial mechanical behavior of green turtle (Chelonia mydas), loggerhead (Caretta caretta), and Kemp's ridleys (Lepidochelys kempii) across a range of body sizes in juvenile, subadult and adults. Carapace samples were tested using quasi-static compression to quantify stiffness (Young's modulus), yield strength, and toughness. The mechanical characteristics of marine turtle shells are grossly akin to that of other turtles and driven by the bone's sandwich structure. Yet, the material properties indicate that marine turtle shells are less stiff and strong than those of their freshwater and terrestrial counterparts. We hypothesize that increased flexibility of the shell may reflect tradeoffs for life that includes experiencing pressures from diving somewhat deeply, in marine environments. Shell material properties also differed among species and ontogenetically. Green turtles had the stiffest, strongest, and toughest shells while loggerhead carapaces were the most compliant. Stiffness and yield strength showed positive relationships with body size which were most pronounced in green turtles and Kemp's ridleys. Phylogenetic histories and ecological differences likely drive this interspecific variation.