A range of animals, from dinosaurs to armadillos, have a natural form of chain mail armour that is composed of bony plates in the skin called osteoderms. Individual osteoderms each act as miniature shields that are linked together through flexible connections, forming a resilient suit of armour that can reshape with the animal's movements. While natural armour plays an important role in defence against predators, recent work has demonstrated that it serves a range of other functions including thermoregulation. But, does having multiple functions compromise the strength of the armour?
Broeckhoven and colleagues from Stellenbosch University and the African Institute for Mathematical Sciences, in South Africa, sought to test whether increased strength in osteoderms came at the cost of decreased thermoregulatory capabilities by comparing the anatomy and mechanical properties of osteoderms in the armadillo lizard (Ouroborus cataphractus) and giant girdled lizard (Smaug giganteus). Although these lizards may have independently evolved heavy armour to avoid being eaten by mongooses, the armadillo lizard spends a lot of time out in the open in order to feed on termite nests and, therefore, is directly exposed to temperature fluctuations in the environment, whereas the giant girdled lizard inhabits burrows that shield it from the elements. Consequently, these species present an intriguing comparison because their different life history characteristics could influence the selective pressures on their armour.
Sections of skin were removed from the backs of preserved specimens and imaged with a micro-CT scanner to visualize the internal and external anatomy of the tissues. The team then used a computer program to digitally dissect individual osteoderms from the micro-CT scans, calculate their thermal conductivities and determine the stresses and strains (deformations) resulting from a simulated mongoose attack. But how well do these computer simulations match biology? To address this question, they built a mechanical predator in the lab by attaching a canine tooth from a mongoose to a mechanical testing machine and then re-enacted an attack by having the ‘predator’ bite down onto the osteoderm until it broke.
Results from the computer simulations and laboratory experiments indicated that osteoderms were weaker in the armadillo lizard, fracturing at a force that was about 25% lower than that for the giant girdled lizard. In addition, the osteoderms in the armadillo lizard were better insulators as they had lower thermal conductivity and were therefore likely to be better able to maintain the animal's body temperature. Overall, the results seemed to indicate that osteoderms that were more thermally insulated were also weaker against attacks (and vice versa), suggesting a functional trade-off between thermoregulation and strength.
Yet, these lizards may get the best of both worlds by modifying their behaviour. Although the armadillo lizard has weaker osteoderms, it has another line of defence: it bites its tail and rolls up into a ball to avoid being eaten. Similarly, the giant girdled lizard compensates for the lower thermoregulatory capabilities of its osteoderms by hibernating and living in well-insulated burrows to avoid drastic temperature changes. Consequently, testing biomaterials in more ecologically relevant scenarios provides a promising avenue to understand the multi-functional roles of biomaterials and develop better bio-inspired armour.