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

It has repeatedly been suggested for several duetting bird and primate spec ies that duetting might act as a reinforcement of the pair bond. Until now, it has apparently not been demonstrated that the premises underlying the p air-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 siamang s during a forced partner exchange. The duet songs of the siamangs underwen t 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 restric ted to the first stage just after the partner exchange. Some of these chang es 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 effor t by which one partner adapts its duetting behaviour to that of its new mat e. The two newly formed pairs of this report appear to be the first documen ted cases to fulfil the requirements underlying Wickler's (1980) pair-bondi ng hypothesis: The animals under study were showing a stable song pattern w ith pair-specific traits. After the partner exchange, new pair-specific tra its occurred, some of them obviously achieved through a partner-directed ef fort of one or both individual(s). Moreover, the pair-bonding hypothesis ap pears to be one of the few biological functions suggested so far which coul d explain a high degree of duet-complexity as adaptive. However, the loudne ss of the siamang song alone suggests that other functions are also involve d. These are most probably related to territorial advertisement, pair bond advertisement and (possibly) mate attraction.