The ontogeny of sexual dimorphism in the cranium of Bornean orang-utans (Pongo pygmaeus pygmaeus): I. Univariate analyses.

Based on a homogeneous sample of 212 individuals spanning all postnatal age periods, we examine the ontogeny of cranial sexual dimorphism in Bornean orang-utans (Pongo pygmaeus pygmaeus) by means of univariate statistics. A distinct pattern emerges at the early juvenile stage and continues at all subsequent stages with males tending to exceed females in all cranial dimensions. In conjunction, starting at mid-juvenile stage, there is a strong tendency for an increase in number and strength of significant cranial sex differences, all of them in favor of males. Significant sex differences in the viscerocranium, reflecting stronger prognathism in males, emerge prior to those in the neurocranium. The total ontogenetic pattern of cranial sexual dimorphism in orang-utans is remarkably similar to that of gorillas, except that there is no evidence of a sex difference in timing of the adolescent growth spurt in the orang-utan. As for other catarrhine species (Wood 1976), male variance of cranial dimensions tends to be greater than that of females, thus lending support to Leutenegger & Cheverud's (1982, 1985) model on the evolution of character dimorphism by means of variance dimorphism.