Ruppell, J. (2007). Vocal diversity and taxonomy of Nomascus in central Vietnam and southern Laos. Master's thesis, Anthropology, Portland State University, vi+81 pp.

Vocal diversity and taxonomy of Nomascus in central Vietnam and southern Laos

Julia Ruppell


Gibbon song vocalizations have been found to be particularly suitable for studying phylogenetic relationships among taxa, because taxon-specific characteristics of songs are not learned but inherited. Based on calls from captive gibbons, Geissmann et al. (2000) suggested that a previously unrecognized taxon of the genus Nomascus (crested gibbons) might exist in Central Vietnam. This is supported by more recent research on the calls of wild crested gibbons suggesting that Nomascus leucogenys siki may consist of more than one taxon, a southern and a northern one. This hypothesis should be tested by comparing calls of all potentially distinct populations of Nomascus leucogenys siki.

The purpose of the present study was to determine if vocal differences exist between localities of Nomascus leucogenys siki occurrence. The study sought to answer the following questions: 1) Are there vocal differences between localities of Nomascus leucogenys siki occurrence? 2) What differences are there between the vocalizations of northern Nomascus leucogenys siki gibbons and southern Nomascus leucogenys siki gibbons?

The songs of seven Nomascus populations in Vietnam and Laos were recorded and examined. Sonograms were used to measure 78 variables of gibbon song which were then analyzed using statistical analysis.

Results of both discriminant analysis and multi-dimensional scaling show that the currently named taxonomic entity Nomascus leucogenys siki can be split into two distinct geographic populations, northern and southern, based on vocal data. This implies that considerable vocal diversity exists between the northern and southern groups. To date, there has been no published description of the vocalizations of wild Nomascus leucogenys leucogenys or northern Nomascus leucgeonys siki, and only one study on calls of southern Nomascus leucgeonys siki. Therefore, this study presents the first evidence that more than one taxon may be involved in what is currently known as Nomascus leucogenys siki. The results are of relevance for gibbon conservation, because discovery of a previously undescribed taxon would require a re-evaluation of conservation priorities for Indochinese gibbons.

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