Gibbon Systematics and Species Identification

by Thomas Geissmann

This section offers a comprehensive review of gibbon systematics. Parts of the following text have previously been published in the following papers, but the text has been (and continues being) updated for the web version.
Geissmann, T. 1994: Systematik der Gibbons. Zeitschrift des Kölner Zoo 37, 65-77.
Geissmann, T. 1995: Gibbon systematics and species identification. International Zoo News 42, 467-501.

Contents:

Abstract
1. Introduction
2. Gibbon Systematics
3. Adopting a Systematic Framework
4. Gibbon Distribution
5. Identifikation Key
6. References

Abstract

A study of wild and captive gibbons and museum specimens, and a survey of the literature, suggests that gibbons (genus Hylobates) include 16 species, which form four distinct groups (genera Hylobates, Hoolock, Nomascus, and Symphalangus): these are the dwarf gibbons or lar group (7 species); the hoolocks (Hoolock, 2 species); the crested gibbons or concolor group (6 species); and the siamang (Symphalangus syndactylus, 1 species). A key for the identification of adult gibbons based on visual characteristics is presented, together with distribution maps of all recognised species. A detailed description of fur colour variation and colour photographs of all species are presented in the Photo Gallery. A detailed description of vocal characteristics, together with sonagrams and sound examples are presented in the Sound Gallery.

1. Introduction

The gibbons, or lesser apes (genus Hylobates), are a relatively small and uniform group of primates. One might assume that the systematical relationship within this group were relatively simple and easily resolved. This does not appear to be the case, however. Although several revisions of gibbon systematics have been published (e.g. Groves, 1972; Marshall & Sugardjito, 1986; Pocock, 1927) and various scenarios have been proposed to describe the radiation of this group (e.g. Chivers, 1977; Groves, 1993; Haimoff et al., 1982), the phylogenetic relationships even among the main divisions of the gibbons are unclear (see below).

Nevertheless, research on gibbons during recent years has considerably increased our knowledge on these apes. For instance, a number of field studies have been devoted to social structure and ecology of most gibbon species (review in Leighton, 1987). The occurrence of hybrid zones between some gibbon species are relatively recent discoveries of considerable scientific interest for gibbon systematics (e.g. Brockelman & Gittins, 1984; Mather, 1992). Marshall and Marshall (1976) systematically described, and collected tape-recordings of, the territorial songs of various gibbon species in the wild and demonstrated their importance for species identification. That publication stimulated a large number of additional studies on gibbon vocalisation whose results are of considerable value for gibbon systematics (e.g. Geissmann, 1993, 2002a,b; Haimoff et al. 1982, 1984; Marshall et al., 1986; Mitani, 1987).

Today, gibbons may be considered one of the better studied groups of primates. Many results of the gibbon research from the last 20 years are, however, not available to the non-specialist. These findings have usually been published in hardly-accessible scientific reports and theses, and most of them seem to be very reluctant to find their way into more popular text books and teaching books. For instance, only the siamang is often being referred to a distinct genus (Symphalangus), although other gibbon groups (such as the hoolocks or the crested gibbons) should also be raised to genus rank (i.e. Hoolock and Nomascus, respectively) if Symphalangus were recognised as a genus. Kloss's gibbon (H. klossii) - although not being closely related to the siamang - is still occasionally referred to as "dwarf siamang" or Symphalangus klossii, and species such as Mueller's gibbon (H. muelleri) and the pileated gibbon (H. pileatus) are sometimes ignored or listed as a subspecies of the white-handed gibbon (H. lar) (e.g. Berger & Tylinek, 1984). More recently, the adoption of Cracraft's phylogenetic species concepts for primate taxonomy (Groves, 2001) led to the recognition of several gibbon taxa as full species that had previously been identified as subspecies (Hylobates albibarbis, Hoolock leuconedys, Nomascus siki, N. hainanus).

The identification of the various gibbon forms often appears to pose an even major problem, irrespective of the adopted nomenclature. A reliable identification of some gibbon species and subspecies based solely on fur colouration may not be feasible even for the specialist. In such cases, the analysis of vocal characteristics almost always resolves the uncertainty, at least when species identification is required. Unfortunately, it may be too late for a species diagnosis in the proper sense for many zoo gibbons: While visiting European and American zoos, I frequently met hybrid gibbons. In many cases, the owners did not know that their "Javan gibbons," "lar gibbons" or "hoolocks" were, in reality, mixed pairs or hybrid offspring of such pairs. Believing that they had been breeding pure taxa, some institutions had for years sent their surplus gibbon offspring as pure species to other gibbon holders, and, by doing so, had unknowingly helped spreading the species mixture in the captive population even further.

Apparently, a serious lack of information exists. In the present report I would like to summarise current views on gibbon evolution and systematics and provide an identification key for all currently recognised species. Elsewhere on this site, colour photographs of all species and a description of their songs together with some sound bits are presented as an additional aid for species identification.

Some other topics which would also be relevant in this context, such as the identification of subspecies and hybrid gibbons, and colour changes in young and maturing gibbons, are not fully covered here. They may be added to this site in the future. At this point, some comments regarding subspecific fur characteristics and the ontogenetic changes in fur colouration (together with photographic examples), are provided in the photo gallery.