Borneo as a biogeographic barrier to Asian-Australasian migration

Widespread deforestations during cool glacial climates carved major discontinuities in the distribution of primates and other fauna and flora. Primates present and absent on Sumatra, Indonesia, indicate that two dry periods moulded their modern distribution. The earlier one eliminated all primates from Sumatra. The second allowed primates which had meanwhile returned, to survive in the wetter north of the island. These recolonizers first spread to Borneo during the interval between the deforestations, with subsequent postglacial diversification. Borneo’s previous impoverished primate fauna suggests that faunal barriers between Borneo and western SE Asia had been as formidable as those which isolate Sulawesi from Borneo. Butterflies and Pleistocene stegodonts dispersing from Asia to Sulawesi, bypassed Borneo via the Philippines or Java. The failure of the leopard, tiger and wild dog to reach Borneo, discredits the notion that suitable habitats linked it and Sumatra the glacial exposure of the Sunda Shelf. The belief that certain Malay mammals, which made it to the north Natuna islands, had been by Sundaland drainage, from continuing their journey to Borneo, ignores indications that this did not deter the stink from reaching the Natuna islands from Bomeo, and ignores the available dispersal route between the northern and eastern Sunda drainage. Even if this route was blocked, it remains inexplicable that a river system could obstruct a wild boar, but not a gibbon. Over twice as many mammals in Malaya and Sumatra also range to Java rather than Borneo, although the strait between Sumatra and Java is deeper than that between Sumatra and Borneo. This pattern of distribution is incompatible with free overland dispersal. Colonization of the Malay archipelago was evidently primarily by rafting, but the shallow strait between Borneo and Sumatra was not the only barrier. Much more significant was the cool climate which, exacerbated by the desiccative effects of the glacial emergence of the Sunda Shelf, induced a severe and prolonged drought in south Borneo and south Sumatra. This caused the Pleistocene extirpation of south-west Bornean freshwater fish which were replaced by a younger fauna as the climate moderated. Bird distribution indicates grassland persisted in south Borneo after it had succumbed to forestation elsewhere in SE Asia. The principal Bornean glacial rainforest refugium extended from Mt Kinabalu to Mt Dulit. Two mammal genera, Hylomys and Melogale now restricted in Borneo to that area, were preglacially lowland in distribution. A similarly restricted Bornean monkey is seemingly montane only because its descendents have altered as they postglacially reoccupied areas. By the time rainforest had reattained the south coast, potential colonizers from Sumatra had first to negotiate the sea crossing. Moist climates in south Borneo were presumably infrequent before the last interglacial.