The Emergence of an Endangered Species: Evolution and Phylogeny of the Trachypithecus geei of Bhutan

Abstract

Golden langurs (Trachypithecus geei) are an endangered primate species in Bhutan. We discuss their evolution in terms of phylogeny, ecology, and biogeography. We test the hypothesis that rivers and mountains in Bhutan isolated a population of capped langurs (Trachypithecus pileatus) that later speciated into the morphologically distinct T. geei. Trachypithecus, the genus to which both capped and golden langurs belong, spread north from a paleorefuge in south China, and Semnopithecus (gray langurs) spread east and northward from a refuge in south India. We show that the 2 genera both arrived in Bhutan and were separated from each other by the Sunkosh River and Black Mountains. Likewise, a population of capped langurs isolated from parental populations by rivers speciated into the distinct golden langurs. We conducted field surveys covering the entire range of langurs in Bhutan. The Sunkosh River and Black Mountain range in west Bhutan isolate gray langurs and golden langurs from each other. In the east, the Manas River system (Manas-Mangde) served as a barrier between golden and capped langurs. However, it is an imperfect barrier and a contact zone between the 2 species occurred on the banks of the Mangde River. Second, we reconstructed the evolutionary history of the langurs of Bhutan via molecular phylogenetic tools. We sequenced the cytochrome b region (cyt b) of the mitochondrial DNA (mtDNA) to model a phylogeny. It revealed the distinct evolutionary paths of the golden, capped, and gray langurs. As predicted, golden and capped langurs are closely related to each other and to other species in Trachypithecus from Southeast Asia. The gray langur of Bhutan grouped into a distinct clade with conspecifics in Semnopithecus from India and Nepal. The south Indian clade of gray langurs is more ancient, with the Bhutan and Nepal gray langurs having diverged later, which fits with the glacial models of ice sheet retreats and colonization of South Asia by gray langurs from south India north toward the Himalayas. Likewise, the golden and capped langur clade are the most derived and divergent from the older groups of Trachypithecus in Southeast Asia, which also fits with paleorefuge models of recolonization by Trachypithecus into the rest of Southeast Asia and north toward the Himalayas from paleorefuges in Southeast Asia. As predicted, golden and capped langurs are closely related to each other and gray langurs are only distantly related to them. The divergence between capped and golden langurs is more recent, while the split between Trachypithecus and Semnopithecus is more ancient and took place before the Pliocene.