DOI: 10.5061/dryad.zkh1893mj
title: Data from: Using the axial skeleton as armor: mechanical behavior of sea turtle carapaces throughout ontogeny

description: 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.
description: 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<sup>-1</sup>.  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<sup>®</sup> 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 (σ<sub>y</sub>) and toughness
 (<em>U</em><sub>r</sub>) 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.
description: # Material Properties of Sea Turtle Shells
 [https://doi.org/10.5061/dryad.zkh1893mj](https://doi.org/10.5061/dryad.zkh1893mj) ## Description of the data and file structure Individual shell samples from all carcasses are listed in the dataset, along with variables and units.  ### Files and variables #### File: SeaTurtleFullData\_Dryad\_(1).xlsx **Description:**  * Cells which are highlighted in gray represent "data not captured". Samples which did not yield have no yield strength or toughness values in the data set and are highlighted in gray.