BASICRANIAL FLEXION - RELATIVE BRAIN SIZE - AND FACIAL KYPHOSIS IN NONHUMAN-PRIMATES

Numerous hypotheses explaining interspecific differences in the degree of basicranial flexion have been presented. Several authors have argu ed that an increase in relative brain size results in a spatial packin g problem that is resolved by flexing the basicranium. Others attribut e differences in the degree of basicranial flexion to different postur al behaviors, suggesting that more orthograde animals require a ventra lly flexed pre-sella basicranium in order to maintain the eyes in a co rrect forward-facing orientation. Less specific claims are made for a relationship between the degree of basicranial flexion and facial orie ntation. In order to evaluate these hypotheses, the degree of basicran ial flexion (cranial base angle), palate orientation, and orbital axis orientation were measured from lateral radiographs of 68 primate spec ies and combined with linear and volumetric measures as well as data o n the size of the neocortex and telencephalon. Bivariate correlation a nd partial correlation analyses at several taxonomic levels revealed t hat, within haplorhines, the cranial base angle decreases with increas ing neurocranial volume relative to basicranial length and is positive ly correlated with angles of facial kyphosis and orbital axis orientat ion. Strepsirhines show no significant correlations between the crania l base angle and any of the variables examined. It is argued that prio r orbital approximation in the ancestral haplorhine integrated the med ial orbital walls and pre-sella basicranium into a single structural n etwork such that changes in the orientation of one necessarily affect the other. Gould's (''Ontogeny and Phylogeny.'' Cambridge: Belknap Pre ss, 1977) hypothesis, that the highly flexed basicranium of Homo may b e due to a combination of a large brain and a relatively short basicra nium, is corroborated.