Gibbon Systematics and Species Identification
Courtesy of Dr Thomas Geissmann
A study of wild and captive gibbons and museum specimens, and a survey of the literature, suggests that gibbons (genus Hylobates) include 12 species, which form four distinct groups (genera Hylobates, Bunopithecus, Nomascus, and Symphalangus): these are the 44-chromosome gibbons (including the Hylobates lar group and H. klossii: 5 species); the hoolock (Bunopithecus hoolock, 1 species); the crested gibbons or concolor group (4 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 (12). 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 of all species 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;
Haimoff et al. 1982, 1984; Marshall et al., 1986; Mitani, 1987).
Today, gibbons may be considered one of the best 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 hoolock or the crested gibbons) should also be raised to genus rank (i.e. Bunopithecus and Nomascus, respectively) if Symphalangus were recognised as a genus. Kloss's gibbon (H. klossii) ñ although not being closely related to the siamang ñ is still occasionally referred to as "dwarf siamang" or Symphalangus klossii, and species such as Mueller's gibbon (Hylobates muelleri) and the pileated gibbon (H. pileatus) are often ignored or listed as a subspecies of the lar gibbon (H. lar) (e.g. Berger & Tylinek, 1984).
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 covered here. They may be added to this site in the future.
Site by Thomas Geissmann for further information see: http://www.tiho-hannover.de/gibbons
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