Relative orbitonasal overlap in african great apes and humans quantified by the automatic determination of horizontal and vertical lines of reference

Abstract

The relative positions of the orbital and nasal openings in African apes and humans were studied by a new methodological approach based on the automatic determination, by image analysis techniques, of horizontal and vertical lines of reference. The material used consisted ofGorilla gorilla (38 males and 20 females),Pan troglodytes (19 males and 13 females), and modernHomo spaiens (51 males and 41 females). This allowed the relative positions of the orbital and nasal openings to be quantified by the determination of medio-lateral and vertical orbitonasal indices of overlap. In all the species studied, a medio-lateral orbitonasal overlap was systematically observed. This indicates that nasal breadth is always larger than interorbital distance. Medio-lateral overalp was greatest inGorilla, reduced inHomo, and intermediate inPan. By contrast, onlyHomo presents systematically a vertical overlap: a vertical overlap was sometimes observed inPan, but never inGorilla. Homo presented the greatest vertical overlap, andGorilla the least; the disposition inPan was intermediate. The interspectific study of the relationships between medio-lateral and vertical overlap inGorilla, Pan, andHomo demonstrated that an increase in veritical overlap was correlated with a decrease of medio-lateral overlap. Sexual dimorphism in orbitonasal relationships was systematically greatest inGorilla, and reduced inPan andHomo, this is also the case for the orbital, nasal, and orbitonasal parameters measured in this study. All these results provide interesting elements for understanding the morphological evolution of the middle face in hominoids.