Many organisms evolved natural armour to protect themselves from threats, such as predators. A common strategy is to layer the armour with materials possessing different functions in structures known as ‘hierarchical bio-composites’ that serve a wider range of purposes than an individual material. While many invertebrate skeletons and seashells have a hard exterior to protect a soft interior, the chink in this armour design is that the harder outer layers can also be brittle, making them more prone to damage if an impact is too intense. Turtles, armadillos and alligators take an alternative approach; their body armour is composed of hard internal layers covered by a bi-layer of soft materials (keratin and collagen), but what benefits could such a soft-coated armour provide? Yaniv Shelef and Benny Bar-On from Ben-Gurion University, Israel, sought to answer this question by studying the mechanical properties and engineering of turtle shells.
Although cushioned armour may sound like a bad idea, the benefit of this strategy is that the softer, outer bi-layer serves as a shock absorber that minimises damage to the internal layers. Shelef and Bar-On wanted to understand how the soft bi-layer produces these shock-absorbing properties. They studied the upper portion of the shell (the carapace) in red-eared slider turtles (Trachemys scripta elegans), which is composed of a deep bony layer covered by soft skin containing collagen towards the middle (the dermis) and keratin on the surface (the epidermis). They used mechanical testing equipment that made small dents in the surface to quantify stiffness and hardness, and then built a digital model of the turtle shell to simulate how these tissue layers were affected by hydration, the sharpness of the object used to dent the surface and changes to the tissues’ stiffness and hardness. These simulations were done using finite element analysis, which quantified the stresses and strains (deformations) that the shells would experience during an assault or impact.
The authors found that the skin protected the underlying bone by absorbing the energy from impacts and localising damage to the surface. In car terms, the keratin layer acts as a bumper that crushes as it absorbs most of the impact energy and then the collagen layer acts as an air bag that compresses to further buffer damage to the inner regions. This ‘bumper-buffer’ function decreases stress to the bones by 50% and persists regardless of how sharp the object impaling the tissues is and whether the tissues are wet or dry. The multifunctional skin of the shell also produces different responses depending upon the type of impact. For instance, the damage to the surface of the keratin layer was greatest when the impact object was sharper and the collagen layer sustained the greatest damage when the impact object was blunter. Overall, the individual layers of the skin exhibited unique functions that, together, could protect the turtle from a range of attacks.
This study of the resilience of turtle shells identifies the benefits of soft-coated armour and demonstrates how the coordinated functions of biomaterials can produce diverse defence mechanisms; so the secret of turtle shell strength may only be skin deep.