Gibbons are distributed in the tropical rain-forests of south-east Asia (Figure 3.1). As a result of climatic and sea level fluctuations during the Pleistocene, individual populations were exposed to repeated phases of gentic isolation, especially in the sunda shelf region. This could help to explain the high number of phenotypically - and probably genetically - distinct populations as well as the unusually nested arrangement of the distribution areas of some of these populations.

The siamang (Symphalangus syndactylus) is the only gibbon species which occurs sympatrically with other gibbons (Hylobates agilis and H. lar, respectively) throughout its whole distribution area. The marked size difference between the siamang and the sympatric species probably is a key factor in enabling this sympatry. The other gibbon species occupy almost completely parapatric distribution areas. Rivers often serve as distribution boundaries.

Figure 3.1. Distribution map of the four gibbon genera Hoolock (hoolock), Hylobates (lar group), Nomascus (crested gibbons or concolor group) and Symphalangus (siamang) (after Geissmann, 1995).

More detailed maps of the individual gibbon species are presented in the systematics section of this website.