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Keywords:

  • biogeography;
  • dispersal;
  • glaciation;
  • Pleistocene;
  • rainforest;
  • speciadon;
  • Sunda shelf

A reappraisal of the zoogeography and systematics of Asian colobines demonstrates marked discontinuities in their distribution. The Bornean proboscis monkey is separated by Sumatra from its sole congener on the Mentawai Islands. Pygathrix species have a discontinuous distribution at the range limit of the Asian Colobinae. The existence in the Himalayas of some disjunct relatives of the south Indian fauna, has obscured a wider disjunction in which for example, the hooded black leaf monkey, Semnopithecus johnii, has one subspecies in southern India and another in north Vietnam. A closely related Vietnamese leaf monkey is a subspecies of the otherwise Indonesian S. auralus. Presbytis comata is disjunct between west Java, northern Sumatra and northern Borneo. The Mentawai Islands P. potenziani is closely related. Biogeographic parallels imply a common cause and previous continuity across the intervening areas. The only wholly compatible explanation is that the disjunct areas alone retained adequate moisture and temperature to support their endemic biota during a cool drought. That not only genera, but species are disjunct, indicates such conditions prevailed recendy, and are probably attributable to the Pleistocene glaciations. The supposition that its maritime climate shielded Asian rainforest from the glacial drought known to have partially deforested Africa and South America, is inapplicable to the Indian subcontinent, and ignores the climatic effects of the emergence of the Sunda and the Sahul shelves. Such known influences, the distribution of drought indicator plants, fossil plants and fossil mammals, grassland birds and freshwater fish, and the anatomical specializations of Nasalis confirm the instability of the Asian environment. The absence of endemic representatives of certain primates in north Sumatra implies the occurrence of two cold droughts. Available evidence appears to correlate the deforestations with the abrupt curtailment of glacial Stages 7 and 5, at about 190 000 years BP and about 80 000 years BP. The greater significance of climatic than topographical barriers in delineating the Oriental zoogeographic region, and a rapid speciation rate, is implied. Morphological change is evidently generated by geographic dispersal, rather than geographic isolation.