Sexual dimorphism of ontogeny in the crab-eating macaque (Macaca fascicularis)

We present a new method for the statistical testing of similarity in growth patterns and apply it to the study of sexual dimorphism of ontogeny in Macaca fascicularis crania. Three-dimensional coordinates of 35 landmarks were located and digitized from the craniofacial skeleton of a large cross-sectional sample of M. fascicularis and used as raw data for an analysis of sexual dimorphism of growth. Our results show that sexual dimorphism in adult craniofacial morphology is the product of sexually dimorphic patterns of growth. Growth of the face and basicranium is most similar between the sexes during the first age interval. After that time, female basicranial growth lags behind male growth in magnitude but the overall pattern of growth remains similar between the sexes through time. The neurocranium shows relatively little postnatal growth and little similarity in growth patterns between the sexes. Skull morphology of development age 6 (oldest) females is statistically indistinguishable from the skulls of developmental age 5 males. Our results suggest that a combination of delayed appearance of developmental markers in males and rate hypermorphosis operating during postnatal growth are responsible for sexual dimorphism of the craniofacial skeleton in M. fascicularis. Those features particularly influenced by rate hypermorphosis include the anterior protrusion of the male snout, the more anteriorly hafted face of the male, and the larger surface of the male planum occipitalis. Given the novelty of our analytical approach to the study of heterochrony, we discuss our results with reference to: (1) the implications of a composite characterization of local aspects of craniofacial growth; (2) the local and global aspects of heterochrony; and (3) the interplay of functional and structural demands with an overall change in rate and/or timing of development.