The ischial callosities of Sulawesi macaques

Sulawesi island has a high level of endemism, including the seven species of monkey from the genus Macaca (macaques). These monkeys have a pair of sitting pads, termed ischial callosities that have diverse shapes and previously were described verbally only. Although useful, these verbal descriptions cannot fully describe shape variation and are somewhat subjective, and cannot directly be used to analyze relationships among species. Here, we report a quantitative analysis of shape of Sulawesi macaque ischial callosities using geometric morphometric tools to optimally describe shape variation and objectively reconstruct general pattern of callosity shapes. By quantification of shape variation, we compare the relationships of each Sulawesi macaque species with each other and with the two geographically neighboring macaque species, M. nemestrina and M. fascicularis, by consensus coordinates of the callosity outlines. The Sulawesi macaques have a wider degree of variation compared with M. fascicularis and M. nemestrina; variation exists in the dorsal part and in the bending of the callosity. There are three general types of callosity shape in Sulawesi macaques: oval without bending (M. tonkeana and M. maurus), oval with outward bending (M. ochreata and M. brunnescens), and oval or reniform with inward bending (M. hecki, M. nigrescens, and M. nigra). These types are congruent with their geographical distribution. The pathway of shape change may have started from oval without bending in the center and the southern peninsula, to outward bending in the southeastern species, and to oval or reniform with inward bending in the northern species.