Ontogenetic patterns of sexual dimorphism in the cranium of Bornean orang-utans ( Pongo pygmaeus pygmaeus)

Abstract

Ontogenetic patterns of cranial sexual dimorphism in a large sample of the Bornean orang-utan ( Pongo pygmaeus pygmaeus) are investigated by means of univariate, bivariate and multivariate statistical analyses. Univariate analyses of 21 linear dimensions reveal that starting at the mid-juvenile stage there is a strong tendency for an increase in number and strength of significant sex differences, all in favor of males. Significant sex differences in the viscero-cranium, reflecting stronger prognathism in males, emerge before those in the neurocranium. Two major growth allometry patterns emerge from bivariate analyses. Ontogenetic scaling is present in 10 dimensions. It is particularly strong in the neurocranium directly associated with brain size, the orbital region and the dental arcade. The heterochronic process of time hypermorphosis is most likely responsible for this pattern. The second growth pattern reflects a departure from ontogenetic scaling with males exhibiting significantly steeper slopes than females. This occurs in 10 cranial dimensions associated with secondary sexual character development: prognathism, canine size and cheek pad area. We suggest that the heterochronic process of acceleration underlies this growth pattern. Principal components analyses reveal two major multidimensional patterns of cranial sexual dimorphism. First, sexual differences at age groups 2 and 3 are primarily the result of differences in principal component II scores, reflecting mainly shape-related differences. Second, age groups 5, 6 and 7 show a trend of stronger size-related shape differences with increasing age in the allometry vector along with decreasing differences in principal component II scores. Age group 4 shows a combination of both patterns. Sexual rather than ecological selection best explains the underlying selective regime of the ontogenetic patterns of cranial sexual dimorphism in orang-utans. Our morphometric analyses clearly corroborate the conclusions of behavioral ecologists in support of the sexual selection theory.