Growth and sexual dimorphism in orangutan crania: a three-dimensional approach.

Abstract

New insights may be gleaned by taking an ontogenetic approach to investigations of adult dimorphism. Previous work in this area relied on traditional, caliper-based, morphometric methods, and produced conflicting results. This study uses a three-dimensional (3-D) approach for both local and global form comparisons of sex-specific growth and growth patterns. 3-D coordinate data were collected for 20 landmarks on 94 orangutan crania divided into five developmental stages. Data were analyzed using Euclidean distance matrix analysis (EDMA). Results indicate that differences in growth patterns between male and female orangutans exist in the youngest age intervals. Dimorphic patterns are strongest in the face and basicranium at the youngest age intervals, and in the face and neurocranium during adult stages. Females grow substantially more in the cranial base and face during the youngest age groups, while males grow more than females in all anatomical regions later in development. Growth in the palate was similar between sexes. Sexual dimorphism may be produced through the continued growth of one sex relative to the other, representing differences in timing, or growth duration. Dimophism may also result from different growth rates between sexes, where one sex develops faster than the other sex in the same time interval. Orangutan males and females differ in both the rate and duration of their craniofacial development. The data analysis technique used here, EDMA, was integral in identifying dynamic growth processes rather than just the static end results of each developmental stage.