Scaling up morphological differentiation of pangolin scales: Serial, ontogenetic and evolutionary variation

Abstract

Pangolins are the most heavily trafficked mammals in the world, largely because of the high demand for their keratinous scales from the traditional Chinese medicine market. While seizures of pangolin material are largely composed of isolated scales, efficient approaches to reach species-level identification are missing. This mostly originates from the lack of comparative studies on the shape of pangolin scales, resulting in knowledge gaps on the imbricated effects of serial, ontogenetic, and evolutionary variations. Using a large sample of micro-CT scan data, we performed the first quantitative 3D analysis of scale shape variation among the eight species of extant pangolins. Our geometric morphometric approach suggests that pangolins grossly share similar trends of serial variation, with scale shape and size being similarly distinct across the different body regions. Relative elongation is by far the main component of scale shape variation at the intraspecific level, although degree of asymmetry and shape of scale bed area also allow distinction among different body areas, especially in adults. At the evolutionary level, Phataginus is the most distinctive genus, with P. tricuspis having the most elongated scales overall. Scales of the back (dorsum) appear to be the best discriminator between species, providing one of the few scale shape differences recorded between the genera Smutsia and Manis. Our results provide an unprecedented, upstream understanding of broad differentiation patterns across the scaled body of pangolins. Together with other yet-to-be-explored morphological traits (e.g., scale size, ornamentation, and thickness), scale shape could provide a valuable matrix of information for forensic applications.