Macaque evolution and dispersal in insular south-east Asia

The evolution of primates is usually approached from the standpoint of adaptation and interspecific competition. However, climatic and eustatic changes associated with periodic glaciations have had a profound influence on their geographical distribution, favouring the intervention of contingency in evolution. This paper deals with the role of chance and competition in the dispersal and stocking of macaques in the islands of south-east Asia. The genus Macaca is unique among non-human primates for the range of habitats colonized, from continents to islands. We first review current knowledge about the zoogeography of macaques in Sundaland. We point out the inconsistencies present in the hypotheses thus far proposed to account for the colonization of shallow- and deep-water islands. We then propose a new perspective of macaque dispersal through the Indonesian archipelago, which takes into account sea-level changes, as well as latitudinal and altitudinal rainforest shifts following climatic cycles during the Quaternary. We envision three steps: (i) dispersal and partial disappearance of the first radiation of macaques; (ii) primary mainland recolonization by pigtailed and longtailed macaques; and (iii) secondary sea-rafting colonization by longtailed macaques. This model implies that liontailed, Sulawesi and Mentawai macaques stemmed from pre-glacial remnant populations, whereas pigtailed macaques originated in post-glacial populations that diverged later on. The model accounts for the distribution of longtailed macaques throughout the Indonesian archipelago. The riverine habits of this species would have favoured its dispersal by sea rafting, which was otherwise extremely rare for other primate species. Stocking would have been successful for longtailed macaques only in islands where no other macaque competitors were already present.