Duet Songs of the Siamang, Hylobates Syndactylus: Ii. Testing the Pair-Bonding Hypothesis During a Partner Exchange

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 its 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.

Duetting in the Titi Monkey Callicebus cupreus: Structure, Pair Specificity and Development of Duets

“…This is in contrast to newly paired siamangs, which have only been observed to copulate about 50 days after pair formation, although they performed duet songs from the very beginning [15]. Although there are behavioural differences between newly formed and established titi monkey pairs [28 and citations therein], mating and duetting seem not to require a long-standing relationship.…”

“…Only for the siamang (Hylobates syndactylus) does evidence exist from studies in captivity that newly formed pairs need some time to co-ordinate their song contributions and that songs finally develop into pair-specific duets [14][15][16]. In the vast ornithological literature, such pair-specific duets are regarded as the most complex and most highly evolved duet type [17,18].…”

“…Moreover, taking the period of mutual fine tuning between pairmates as concerted investment, pair-specific duets have been discussed as a possible mechanism countering mate desertion [17; but see 19,20]. Whether such theoretical assumptions can also be applied to duetting primate species in general remains to be demonstrated, but this might be the case in siamangs [15,16].…”

The anti-predator behaviour of wild white-handed gibbons (Hylobates lar)

“…Mated pairs give loud and coordinated songs (termed 'duets') that transmit over considerable distances, much beyond the callers' home range, suggesting that these signals have evolved to communicate with other conspecifics that are not part of the immediate social group (Raemaekers and Raemaekers 1985;Geissmann 1999;Geissmann and Orgeldinger 2000). Gibbon songs also perform an important function during predator encounters (Uhde and Sommer 2002;Ellefson 1974).…”

Vocal Responses of Captive Gibbon Groups to a Mate Change in a Pair of White-Cheeked Gibbons (Nomascus leucogenys)

“…Cowlishaw [1992] compared duetting intensity and pair grooming intensity across 6 gibbon species including Kloss' gibbon (H. klossi) and siamangs (Symphalangus syndactylus). The hypothesis was not supported; however, the inclusion of H. klossii (which does not produce duets) and S. syndactylus (which produces the most complex duets) in this comparison is certain to have strongly affected the results [Geissmann, 1999;Geissmann and Orgeldinger, 2000]. Interspecific comparisons between duetting and non-duetting species is not an adequate way to test whether duetting has a pair bond function [Geissmann, 1999;Geissmann and Orgeldinger, 2000].…”