The ontogeny of sexual dimorphism in the cranium of Bornean orang-utans (Pongo pygmaeus pygmaeus): II. Allometry and heterochrony.

Based on a homogeneous sample of 212 individuals spanning all postnatal periods, we examine the ontogeny of cranial sexual dimorphism in Bornean orang-utans (Pongo pygmaeus pygmaeus) by means of allometric analysis and in terms of heterochrony. The bivariate growth allometries of 20 cranial dimensions against basicranial length yield two major patterns. Confirming the null hypothesis, strong ontogenetic scaling, where growth regressions of both sexes fall along a single ontogenetic continuum, and where shape differences between adult males and females result from the extension of relative growth in the smaller females to larger size in males, is found in 10 cases. Ontogenetic scaling is particularly strong in proportions of (1) the neurocranium directly associated with brain size, (2) the orbital region, and (3) the dental arcade. In terms of heterochrony such a pattern most likely is the result of a process termed "time hypermorphosis", i.e. an extension of the growth period in time in males. The second major pattern seen in the remaining 10 cases shows a departure from ontogenetic scaling, with males exhibiting a significantly steeper slope than females. Departures from ontogenetic scaling, where size and shape are dissociated with adult males being disproportionately larger than adult females, are found in proportions of cranial regions directly associated with secondary sexual character development: prognathism, canine size, and cheek pad area. In terms of heterochrony such a pattern most likely is the result of a process termed "acceleration", i.e. the rate of shape change is increased in males.