Shape of the orbital opening: individual characterization and analysis of variability in modern humans, Gorilla gorilla, and Pan troglodytes

The description of the human orbital shape is principally qualitative in th e classical literature, and characterised by adjectives such as circular, r ectangular or quadrangular. In order to provide a precise quantification an d interpretation of this shape, a study based on automatic image analysis a nd Fourier analysis was carried out on 45 human skulls (30 males, 15 female s), and for comparison on 61 skulls of Gorilla gorilla (40 males, 21 female s), and 34 skulls of Pan troglodytes (20 males, 14 females). Sexual dimorph ism in the shape of the orbital opening was not demonstrated. Its dominant morphological features could be characterized by Fourier analysis; elliptic al elongation and quadrangularity were dominant morphological features of t he shape of the orbital opening in the three species. Elliptical elongation was more marked in humans and Pan, whereas quadrangularity was particularl y emphasized in Gorilla. An intraspecific variability of the shape of the o rbital opening existed in humans, Gorilla and Pan, and seemed close in the three species. Interspecific partition between humans, Gorilla and Pan was demonstrated despite the variability observed in the three species studied. Interspecific differences between Gorilla and the Pan-humans group were pr incipally explained by the differences in quadrangularity, and by differenc es in orientation of triangularity and pentagonality. Differences in the sh ape of the orbital opening between humans and Pan were principally explaine d by differences in hexagonality, and by differences in orientation of quad rangularity. A closeness of shape between some humans and some individuals in Pan and, to a lesser degree, with some individuals in Gorilla was observ ed, demonstrating the existence of a morphological continuum of the shape o f the orbital opening in hominoids.