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 has focused on non-marine turtles, which 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 juveniles, subadults 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 those 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 include experiencing pressure from diving somewhat deeply in marine environments. Shell material properties also differ among species and ontogenetically. Green turtles have the stiffest, strongest and toughest shells while loggerhead carapaces are the most compliant. Stiffness and yield strength show positive relationships with body size which are most pronounced in green turtles and Kemp's ridleys. Phylogenetic histories and ecological differences likely drive this interspecific variation.