A Comparative Study of the Primate External Ear Using Three-Dimenstional Geometric Morphometrics

The cartilaginous external ear plays a fundamental role in the physiology of audition and understanding the evolution of primate hearing can shed light on primate sensory ecology. Nevertheless, variation of external ear morphology in primates has received little attention to date from anthropologists. In this study, three-dimensional geometric morphometrics, complemented by linear metrics, was used to analyze interspecific variation in the shape of the external ear (pinna and concha) of sixteen primate species relying on physical ear molds and 3D models derived from CT scan data. The results indicate that while the helix shows the greatest variability at the attachment sites to the head and at the posterior border, there is not a clear taxonomic pattern to the overall shape variation of this structure. Shape variation of the concha more closely follows the major taxonomic division with the primate order. The concha of platyrrhines is characteristic of a narrow mid-section, with additional narrowing in the inferior portion. In catarrhines, the concha becomes more rounded inferiorly while the hominid concha has an irregular shape, particularly in the anterior structures. Comparing the size of the concha in relation to the pinna did not show any consistent pattern across the primate order, suggesting that these two structures vary independently. However, the size of these structures are shown to increase linearly with body size increase across taxa. This study demonstrates the value of geometric morphometrics as a tool which may aid further hearing studies in primatology and biological anthropology