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

Abstract

Heterochrony and Sexual Dimorphism in the Skull of the Liberian Chimpanzee (Pan troglodytes verus)AIIometric methods and theory derived from principles of relative growth provide new and powerful approaches to an understanding of the nature and development of sexual d imorphism 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 ontogenetie 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 iacial and mandibular dimensions, and isometry or negative allometry for most neurocranial dimensions. The data confirm previous work in chimpanzees and other anthropoid primates suggesling 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 ontogenetie scaling in the Liberian chimpanzee.

Craniofacial sexual dimorphism in Alouatta palliata, the mantled howling monkey

“…It is also recognized that similar patterns of adult sexual dimorphism can be attained through different ontogenetic mechanisms (Shea, 1986, 1988) that can only be assessed through ontogenetic analysis. One approach is to determine whether morphological differences between adult males and females can be explained as allometric consequences of differences in size, also called ‘ontogenetic scaling’ (Shea, 1986, 1988, 2002; Ravosa & Ross, 1994; Masterson, 1997; Anemone & Swindler, 1999). Accordingly, craniofacial shape differences between adult males and females may be incidental byproducts of selection for differences in size.…”