Why Is the Siamang Larger Than Other Hylobatids?

Abstract

Weighing between 10 and 12 kg, the siamang is nearly twice as large as most hylobatids. Why siamangs retained or acquired this larger size is not yet well understood. While small gibbons are allopatric, the siamang is sympatric with two small hylobatid species, agile gibbons on the island of Sumatra and white-handed gibbons in northern Sumatra and the Malaysian peninsula, Indonesia. Increases in body size within evolutionary lineages over time are often seen as expressions of Cope’s rule, which describes the general evolutionary tendency of all living things to increase in size with increasing lineage age, based on assumed, although often unverified, selective advantages associated with larger body size. However, when considering selection pressures that may have lead to a within-lineage body size increase over time, the more favorable mass-to-surface ratio of the larger forms should be taken into account instead of size per se. Using data on body mass and body dimension from the literature, we performed simple calculations of body proportions of small gibbons and the larger siamang and compared these to orangutan and loris body sizes in order to evaluate whether the siamang’s larger size could be explained by a more favorable mass-to-surface ratio. To calculate relative body surface areas we adopted a model in which the body is represented as a combination of five simple cylinders (i.e., body stem, 2 forelimbs, and 2 hindlimbs). As expected, we found that, within the tested primate lineages, the species with the larger mass has the relatively smaller surface area compared to the species with the smaller mass. Thus, our results are in agreement with the idea that a larger body size may have evolved in the siamang due to advantages related to a more favorable mass-to-surface ratio. At present, the exact selective advantage of a lower surface involve per kilogram body weight of siamang is unclear, but we speculate that it may be related to relatively lower energy expenditure, which may have allowed the siamang to successfully inhabit a broader range of habitats, including higher elevation areas, and thereby may have reduced competition with the sympatric smaller gibbons.