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

The relative positions of the orbital and nasal openings in African apes an d humans were studied by a new methodological approach based on the automat ic determination, by image analysis techniques, of horizontal and vertical lines of reference. The material used consisted of Gorilla gorilla (38 male s and 20 females), Part troglodytes (19 males and 13 females), and modern H omo sapiens (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 speci es studied, a medio-lateral orbitonasal overlap was systematically observed . This indicates that nasal breadth is always larger than interorbital dist ance. Medio-lateral overlap was greatest in Gorilla, reduced in Homo, and i ntermediate in Part. By contrast, only Homo presents systematically a verti cal overlap; a vertical overlap was sometimes observed in Pan, but never in Gorilla. Homo presented the greatest vertical overlap, and Gorilla the lea st; the disposition in Pan was intermediate. The interspecific study of the relationships between medio-lateral and vertical overlap in Gorilla, Part, and Homo demonstrated that an increase in vertical overlap was correlated with a decrease of medio-lateral overlap. Sexual dimorphism in orbitonasal relationships was systematically greatest in Gorilla, and reduced in Pan an d Homo, this is also the case for the orbital, nasal, and orbitonasal param eters measured in this study. All these results provide interesting element s for understanding the morphological evolution of the middle face in homin oids.