Morphological Integration and Variation in Facial Orientation in Pongo pygmaeus pygmaeus: A Geometric Morphometric Approach via Partial Least Squares

The orientation of the face relative to the cranial base is an important aspect of intraspecific and interspecific variation in hominoids. In contrast to African apes, Pongo is characterized by a structural condition known as airorhynchy, an upwardly rotated face relative to the basicranium. In this study we investigated patterns of covariation between facial orientation and basicranial and orbital shape. The sample comprised 43 adult specimens of Pongo pygmaeus pygmaeus. The shape of the craniofacial complex is represented by 105 three-dimensional landmarks and semilandmarks digitized on images obtained with computed tomography (CT). We investigated covariation patterns and facial orientation using partial least squares analysis of basicranial and orbital shape vis-à-vis maxillary shape. We found strong integration between the shape of the cranial base, orbits, and the shape and orientation of the palate and premaxilla. Facial orientation in our sample is also highly variable. As a dorsally rotated palate is associated with a ventral rotation of the orbits but not with a reduced basicranial angle, the “facial block” hypothesis cannot explain the variation in facial orientation in Pongo. Our results also indicate that mature males have a more ventrally oriented lower face that may be due to a reconfiguration of the craniofacial complex during the extended male growth phase. These findings demonstrate that there is strong integration between the cranial base and face and a high degree of variation in facial orientation in Pongo.