Form variation in the postnatal facial skeleton of the African apes

This thesis explores the intraspecific and interspecific form variation in the facial skeleton of the African apes from an ontogenetic perspective. Three dimensional coordinate data of 28 landmarks were obtained from the facial skeletons of sexed cross-sectional ontogenetic series of G. g. gorilla (75 juveniles, 62 adults), P. paniscus (61 juveniles, 34 adults) and P. t. troglodytes (57 juveniles, 58 adults). Statistical shape analysis was carried out using geometric morphometric techniques. The first analytical section of this thesis assesses the degree and nature of sexual dimorphism in the adult facial skeleton of each species and tests the hypothesis that intraspecifically, males and females are ontogenetically scaled. Sexual dimorphism was found to be very low in P. paniscus, and of increasing degree in P. troglodytes and G. gorilla, respectively. Within each species the males and females share the same scaling trajectory prior to adulthood. In each species the adult males and female diverge from this shared juvenile trajectory, only in G. gorilla is any component of these diverged trajectories ontogenetically scaled. The second analytical section explores the interspecific differences by testing the hypothesis that interspecific shape differences are due to ontogenetic scaling. This hypothesis was falsified, although there are a number of interspecific shape similarities, clear shape differences are present between all three species at all sizes during postnatal life. The conclusion of this thesis is that size is a primary determinant of intraspecific shape differences in the facial skeleton in all three species prior to adulthood. However, size is not the primary determinant of species differences in the shape of the facial skeleton