Geissmann, T. & Orgeldinger, M. 1998: Duet or divorce! - Duett oder Scheidung! Folia Primatologica 69: 283 (Abstract only).

Duet or Divorce! - Duett oder Scheidung!

T. Geissmann¹, M. Orgeldinger<sup>2

1</sup>Institut für Zoologie, Tierärztliche Hochschule, Hannover, und
² Zoologisches Institut, Ruprecht-Karls-Universität, Heidelberg, Deutschland

Key Words: Duet · Pair-bonding · Song · Gibbons · Hylobates syndactylus · Siamang

One of the most commonly cited functional explanations for duet songs includes strengthening of the pair bond. Nevertheless, this function has apparently not yet been demonstrated in any animal species that sings.

Gibbons (Hylobatidae) are monogamous arboreal apes living in East-Asian rain-forests. Most gibbon species produce loud, long and well coordinated duet songs. A recent study concluded that duetting in gibbons is unrelated to pair-bonding [Cowlishaw, Behaviour 1992; 121:131ff], because the Kloss gibbon (Hylobates klossii) does not produce duets but apparently shows strong pair bonds like the duetting siamang (H. syndactylus). We doubt, however, that the function of duetting can be tested by comparing duetting activity between duetting and non-duetting species.

Our hypothesis is: If duetting is related to pair-bonding, we should expect to find a relationship between duetting activity and potential indicators of pair bond strength. In order to test this prediction, data on duetting activity and three behavioural variables believed to be possible indicators of pair bond strength were collected in 10 siamang groups observed in various zoos. In accordance with the hypothesis under test, we found duetting activity to be positively correlated with grooming activity and behavioural synchronisation between mated siamangs, and negatively correlated with inter-individual distance between mates. These results suggest that the production of coordinated duets by siamang pairs is related to pair-bonding.

The presence of a pair bond, in the absence of duetting, in Kloss gibbons does not imply that duetting is unrelated to pair-bonding in gibbons in general (contra Cowlishaw, 1992), but merely that it is unrelated to pair-bonding in Kloss gibbons. The differences among various gibbon species in song structure, the amount of solo singing of either sex, the amount of duetting, and the complexity of vocal coordination all suggest that the functions of singing and their importance are not the same across all gibbon species.