Sexual dimorphism in the dermal armour of cordyline lizards (Squamata: Cordylinae)

While sexual dimorphism in offensive weaponry is a well-established phenomenon, few studies have looked at sexual differences in defensive body armour. In this study, we investigate sexual dimorphism in the expression of osteoderms - bony elements embedded in the skin - in four species of cordyline lizards by using high-resolution micro-computed tomography. Our results unambiguously show that only in the species with an intermediate phenotype characterized by isolated osteoderms without any clear ecological function do males have a higher degree of osteoderm expression than females. By contrast, no significant sexual differences are present in the species with an armoured phenotype consisting of a continuous layer of imbricating osteoderms. These findings suggest that sexual selection acting on dermal armour might be ubiquitous, but that in the armoured phenotype, its effect is weak because of a stronger selection pressure towards improved defence or physiological function.