Sexual dimorphism in osteoderm expression and the role of male intrasexual aggression

The functional significance of osteoderms, body plates embedded in the skin of various extinct and extant tetrapods, has been studied widely in the past, leading to the advancement of a plethora of hypotheses to explain their presence. Whereas the emphasis of most studies is on the role of osteoderms as protection against predators, alternative hypotheses remain largely unexplored. This study investigates whether male intrasexual aggression might contribute to variation in osteoderm expression, using the Cape cliff lizard Hemicordylus capensis as model organism. Micro-computed tomography was used to examine ontogenetic variation and sexual dimorphism in osteoderm expression in two study populations that were assumed to display contrasting levels of male aggression. Our results show that osteoderms in the trunk develop at the onset of sexual maturity, or after, in a lateral to medial fashion. A clear difference in osteoderm volume between males and females was detected, with osteoderm volume being significantly higher in males than in females, regardless of locality. Higher levels of intrasexual aggression, inferred from bite force data, appear to be present in individuals from the southern locality, and this appears to coincide with high osteoderm expression. In summary, although osteoderm expression has long been regarded a result of natural selection, our findings suggest that these structures might instead be sexually selected.