Heterochrony and sexual dimorphism in the skull of the Liberian chimpanzee (Pan troglodytes verus)

Allometric methods and theory derived from principles of relative growth provide new and powerful approaches to an understanding of the nature and development of sexual dimorphism among living primates. The Frankfurt collection of Liberian chimpanzee skulls and mandibles provides a large skeletal sample from a single natural population of wild shot animals, including individuals of all ages and both sexes, and allows investigation of allometric and heterochronic patterns of sexual dimorphism. Univariate, bivariate, and multivariate analyses are utilized in this study in order to ascertain the ontogenetic nature of male-female differences in the skull of the Liberian chimpanzee. The results of univariate and multivariate analyses indicate that, while overall levels of sexual dimorphism in the chimpanzee skull are small, the greatest differences are in dimensions of the viscerocranium, while neurocranial dimensions and orbital size tend to be less dimorphic. Bivariate regressions of 21 cranial variables against basicranial length document positive allometry in many facial and mandibular dimensions, and isometry or negative allometry for most neurocranial dimensions. The data confirm previous work in chimpanzees and other anthropoid primates suggesting that males and females are “ontogenetically scaled” in most cranial traits. That is, males and females share the same cranial growth trajectories, although ending up at different points. Both rate and time hypermorphosis are suggested as underlying causes of ontogenetic scaling in the Liberian chimpanzee.