Gibbons or small apes (the term "lesser apes" should be abandoned) are the most specifically diverse, and the most numerically successful of living apes (Hominoidea) (Fleagle, 1999, p. 236) - with the exception of the genus Homo. Four major groups of gibbons can be identified (Figure 2.1), which are recognized as distinct genera. They differ among each other in cranial shape, in the number of chromosomes and, partially, in their body size. The systematic relationships among the four genera and among the species within the genera are not reliably resolved.

Figure 2.1. Preliminary phylogenetic tree of the gibbons, combining trees based on vocal and molecular data (Geissmann, 2002b; Roos & Geissmann, 2001, and Geissmann, unpublished).

Gibbons are highly specialized, but also very homogenous, in their anatomy. Species differ most markedly in pelage coloration. Species as well as local gibbon populations may strongly differ in the variability of their coloration characteristics, and some species are strongly polymorphic in this respect (polychromatism). In addition, some species exhibit sexually dichromatic pelage coloration, and undergo striking color changes during their ontogeny (see section 7: Ontogeny). Therefore, a reliable species or subspecies identification of gibbons of unknown provenance is often not possible based on pelage coloration criteria alone. Tape-recordings of the gibbon's morning songs (see section 6: Behavior) are often indispensable for reliable identifications.

A classification and species list of the Hylobatidae is shown in Table 2.1.