A geometric approach to cranial sexual dimorphism in the orang-utan

Adult craniofacial morphology is quantified and compared using Euclidean distance matrix analysis (EDMA), a three-dimensional morphometric method for the comparison of forms, which localizes form differences between comparative groups. Results indicate that the number and magnitude of differences between male and female crania are striking. The face, basicranium and neurocranium exhibit the most dimorphism, while the palate shows the least. Significant differences also exist between young adult and fully adult individuals, especially males, supporting the delayed onset of sexual maturity and secondary sex characteristics in males. As one of the many new morphometric techniques available, EDMA was useful for identifying local form difference and provides insights into the understanding of sexual dimorphism in this species beyond that obtained from traditional statistical methods based on linear caliper measurements.