All species of gibbons are known to produce elaborate, species-specific and sex-specific patterns of vocalisation often referred to as "songs" (Haimoff, 1984; Marshall & Marshall, 1976). Songs are loud and complex and are mainly uttered at specifically established times of day. In most species, mated pairs may characteristically combine their songs in a relatively rigid pattern to produce coordinated duet songs. Several functions have been attributed to gibbon songs, most of which emphasise a role in territorial advertisement, mate attraction and maintenance of pair and family bonds (Geissmann, 1999; Geissmann & Orgeldinger, 2000; Haimoff, 1984; Leighton, 1987).

Gibbon song vocalisations are typically of pure tone, with the energy concentrated in the fundamental frequency. Depending on species, the fundamental frequency of song vocalisations ranges between 0.2 and 5 kHz.

In recent years, vocal characteristics have been used to assess systematic relationships among hylobatids and to reconstruct their phylogeny (Creel & Preuschoft, 1984; Geissmann, 1993, 2002a, 2002b; Haimoff, 1983; Haimoff et al., 1982, 1984; Marshall et al., 1984).

Female Song Contributions

The most prominent song contribution of female gibbons consists of a loud, stereotyped phrase, the great-call. Depending on species, great-calls typically comprise between 6-100 notes, have a duration of 6-30 s. The shape of individual great-call notes and the intervals between the notes follow a species-specific pattern (Geissmann, 1993, 1995, 2002a; Haimoff, 1983, 1984; Marler & Tenaza, 1977; Marshall & Marshall, 1976; Marshall & Sugardjito, 1986).

Whereas mated females of Hylobates klossii and H. moloch have been reported to produce solo song bouts, mated females of other species usually confine their singing behaviour to duet song bouts only. A female song bout is usually introduced by a variable but simple series of notes termed the introductory sequence; it is produced only once in a song bout. Thereafter, great-calls are produced with an interval of about 2 min. In the intervals, females usually produce so-called interlude sequences consisting of shorter, more variable phrases which in many species bear some resemblance to male phrases described below. These phrases are termed "female short phrases" here. The typical female song bout hence follows the sequential course ABCBCBCBCÖ, where A stands for the introductory sequence, while BCBCBCÖ represent the alternating interlude sequences and great-call sequences (Haimoff, 1983, 1984; Raemaekers et al., 1984). An exception to this rule are the crested gibbons (genus Nomascus), where female song contributions include great-calls or aborted great-calls only, and where no equivalents of introductory sequence and interlude sequences are known (Geissmann et al., 2000; Haimoff, 1983, 1984). Female song bouts usually have a duration of less than 30 min.

Male Song Contributions

As a rule, adult males do not produce great-calls, but "male short phrases" only. Whereas female great-calls remain essentially unchanged throughout a song bout, males gradually build up their phrases, beginning with single, simple notes. As less simple notes are introduced, these notes are combined to increasingly complex phrases, reaching the fully developed form only after several minutes of singing (Mitani, 1988; Raemaekers et al., 1984; Tenaza, 1976). Although fully developed male phrases in most species are more variable than female great-calls, they, too, show species-specific characteristics in note shape and spacing (Geissmann, 2002a; Haimoff, 1983, 1984; Marler & Tenaza, 1977; Marshall & Marshall, 1976; Marshall & Sugardjito, 1986).

Whereas mated males of the dwarf gibbons (genus Hylobates) may produce solo song bouts, mated males of the hoolock gibbons (genus Hoolock), siamangs (genus Symphalangus) and crested gibbons (genus Nomascus) usually sing in duet with their females (described below). Males may engage in uninterrupted song bouts of considerable length, often more than 30 minutes and sometimes up to more than 2 hours.

Duet Songs

During duet songs, mated males and females combine their song contributions to produce complex, but relatively stereotyped vocal interactions (Geissmann, 2002a; Haimoff, 1983, 1984; Marler & Tenaza, 1977; Marshall & Marshall, 1976; Marshall & Sugardjito, 1986). The sequential pattern of duet song bouts is largely similar to that of female song bouts described above (i.e. ABCBCBCBCÖ). Both pair partners contribute to an introductory sequence at the beginning of the song bout (A). Thereafter, interlude sequences (B) and great-call sequences (C) are produced in successive alternation. During interlude sequences, males usually progressively develop their phrases from short, simple to longer, more complex series of notes, similar to the development of their phrases in male solo songs described above. In most species, females participate in interlude sequences with short phrases, as described for their solo songs.

During great-call sequences ó announced by females of the lar group by rhythmical hoots ó the male becomes silent and does not resume calling until near or shortly after the end of the female's great-call, when he will produce a coda which concludes the great-call sequence. The coda resembles other male short phrases, but is more stereotyped. It usually interrupts the progressive building-up of the male short phrases described above by being more advanced in development than those uttered during the interlude sequences. Hylobates pileatus, Hoolock hoolock, H. leuconedys, and Symphalangus syndactylus are unusual among gibbons in that males vocalise not only at the end of the female's great-call, but also during the great-call. Hylobates moloch and H. klossii are unusual in that males of these species are not known to produce codas. There has been some controversy about whether these two species produce duet song bouts at all, but apparently, these species produce no duet songs, only male solo song bouts and female solo song bouts (as discussed in Geissmann, 1993, 2002a). Duet song bouts, like female song bouts, usually have a duration of less than 30 min.

At the climax of a great-call, the female typically exhibits a locomotor display, usually accompanied by her mate in the duetting species. The short and acrobatic bout of vigorous brachiation frequently includes branch shaking and (presumably intentional) breaking off dead branches (e.g. Carpenter, 1940; Chivers, 1974; Ellefson, 1968; Geissmann, 2000; Kappeler, 1984). Not all individuals show this display.