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Thomas Geissmann
Gibbon Research Lab., Hannover

It has repeatedly been suggested for several duetting bird and primate species that duetting might act as a reinforcement of the pair bond. Until now, it has apparently not been demonstrated that the premises underlying the pair-bonding hypothesis are met by any duetting species. A recent set of three publications provide first support for a pair bonding function of duet singing in siamangs (Hylobates syndactylus).

Siamangs are monogamous apes which produce loud and long song bouts which are mostly exhibited by mated pairs. Typically, mates combine their partially sex-specific repertoire in relatively rigid, precisely timed and complex vocal interactions to produce well-patterned duets. The first study (Geissmann, 2000) presents a detailed description of singing behaviour, repertoire, and song and duet organisation of several adult pairs of captive siamangs, revealing that the organisation of both the song bout as a whole, as well as the great call sequence in particular, is more complex than has been believed previously.

The second study (Geissmann, 1999) analyses the changes in duet structure in two pairs of siamangs during a forced partner exchange. The duet songs of the siamangs underwent many notable changes during partner exchange. Of 33 different variants of the great call sequence, 29% in one pair and 21% in the other were restricted to the first stage just after the partner exchange. Some of these changes were certainly due to individualistic traits of the new partner, and for some other changes, this possibility cannot be reliably excluded. At least two changes, however, can only be interpreted in terms of a learning effort by which one partner adapts his duetting behaviour to that of its new mate. The two newly formed pairs of this report appear to be the first documented cases to fulfil the requirements underlying Wickler's (1980) pair-bonding hypothesis: The animals under study were showing a stable song pattern with pair-specific traits. After the partner exchange, new pair-specific traits occurred, some of them obviously achieved through a partner-directed effort of one or both individual(s). Moreover, the pair-bonding hypothesis appears to be one of the few biological functions suggested so far which could explain a high degree of duet-complexity as adaptive. However, the loudness of the siamang song alone suggests that other functions are also involved. These are most probably related to territorial advertisement, pair bond advertisement and (possibly) mate attraction.

The third study (Geissmann & Orgeldinger, 2000) provides additional support for the pair bonding hypothesis by documenting a relationship between pair bonds and duet singing in siamangs. As a working hypothesis, the authors assumed that if duetting were related to pair bonding, one might expect to see a relationship between duetting intensity and indicators of pair bond strength. The authors recorded daily frequency and duration of duetting and three generally accepted indicators of pair bond strength (mutual grooming, behavioural synchronization and distance between mates) in 10 siamang groups in zoos. Duetting activity was positively correlated with grooming activity and behavioural synchronization, and negatively correlated with distance between mates. These results suggest that the production of coordinated duets by siamang pairs is related to pair bonding.

References

Geissmann, T., 1999: Duet songs of the siamang, Hylobates syndactylus: II. Testing the pair-bonding hypothesis during a partner exchange. Behaviour 136: 1005-1039.

Geissmann, T., 2000: Duet songs of the siamang, Hylobates syndactylus: I. Structure and organisation. Primate Report 56: 33-60.

Geissmann, T. & Orgeldinger, M., 2000: The relationship between duet songs and pair bonds in siamangs, Hylobates syndactylus. Animal Behaviour 60: 805-809.

Wickler, W., 1980: Vocal dueting and the pairbond. I. Coyness and partner commitment. A hypothesis. Zeitschrift für Tierpsychologie 52: 201-209.