Proximate causes of variation in dermal armour: insights from armadillo lizards

Although it is widely assumed that body armour in animals evolved to thwart predator attacks, assessing the role that predators may play in shaping defensive morphologies has proven to be difficult. Recent studies suggest that body armour might be influenced by additional factors besides predation, and/or even by sexual selection. We investigated variation in dermal armour in 13 populations of armadillo lizards Ouroborus cataphractus, spanning the entire distribution range of the species. We obtained thickness measurements of osteoderms - bony plates embedded in dermal layer of the skin - using micro- and nano-computed tomography. Using these data, we examined the effects of predation pressure/risk and climatic variables on dermal armour variation and addressed sexual and ontogenetic influence. Our results show that climate is the only factor affecting variation in dermal armour. Populations inhabiting more arid environments, characterized by low summer precipitation and mild winter temperatures, are relatively more armoured than those present in less arid environments. In contrast to our expectations, predation pressure or perceived predation risk was not associated with osteoderm thickness. The results of our study support the idea that the evolution of defensive traits might not be driven exclusively by predator-prey interactions, but could be moulded by environmental factors. In particular, we highlight the role of dermal armour as a potentially important mechanism to reduce evaporative water loss in arid environments.