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

Duet songs of the siamang (Hylobates syndactylus): II. Testing the pair-bonding hypothesis during a partner exchange

T. Geissmann
Institute of Zoology, Tierärztliche Hochschule Hannover, Germany

Key Words: Duet; song; siamang; Hylobates syndactylus; pair bond; gibbons.

Abstract: 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. Siamangs (Hylobates syndactylus) are monogamous apes which produce long and complex duet songs. This study 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.