Intraspecific variation in osteoderm morphology in a cordylid lizard

Osteoderms are bony elements in the dermis of a disparate set of vertebrates, including several taxa of reptiles. Why and to what extent osteoderms vary in occurrence, number, volume, and shape, particularly at the intraspecific level, remains poorly understood. Here we explored the extent of intraspecific variation in the cordylid lizard Hemicordylus capensis, which is broadly distributed across the Western Cape of South Africa. We sampled lizards from three geographically distinct populations along a longitudinal gradient and used micro-computed tomography and 3D geometric morphometrics to quantify osteoderm number, volume, and shape. Our results show substantial variation across populations, sexes, and body regions. Specifically, the northernmost population has significantly fewer and smaller osteoderms than the central and southern populations. We also observed sexual dimorphism: males have more and larger osteoderms than females. Interestingly, sexual shape dimorphism was restricted to the northern population, where males exhibit flatter, ‘pancake’-shaped osteoderms and females more elongated, ‘sausage’-like forms. Additionally, osteoderm morphology varied across body regions, with the neck bearing thicker, keel-shaped (often spiked) osteoderms, contrasting with the ‘pancake’- or ‘sausage’-shaped osteoderms on the trunk. We discuss potential functional interpretations for this variation—including roles in protection, thermoregulation, and hydroregulation—that merit further investigation.