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.

Methods
Samples with averaged dimensions of 9mm x 9mm (length x width) were tested dorso-ventrally, scute side up, under compression. Sample height was variable and depended upon shell thickness. Quasi-static compression of samples was conducted using an Instron E1000 Material tester at a displacement rate of 0.02 mm s-1.  Tests continued until the maximum force capacity of the Instron (1KN) or densification was reached, whichever came first.

To characterize the mechanical properties of marine turtle shells, we plotted mechanical testing data as force-displacement curves using Bluehill® Software (Instron, Norwood, MA, USA). These curves were converted into engineering stress-strain curves and used to calculate the mechanical properties of interest (Young's modulus, yield strength, and toughness). The Young’s modulus describes a material’s ability to resist deformation (i.e., stiffness). We calculated stiffness as the slope of the linearly elastic region of the stress-strain curve. For samples that reached a yield point, yield strength (σy) and toughness (Ur) were also calculated. Yield strength refers to the minimum stress a material can withstand to experience permanent deformation. We evaluated the yield strength as the point on the stress strain curve where the material transitioned from elastic to plastic deformation. Toughness describes the amount of work done by a material per unit volume (i.e., how much energy is absorbed). Toughness was calculated as the area under the stress-strain curve up to the yield point.