Fischer, J. & Geissmann T., 1989: Group harmony in gibbons: Comparison between white-handed gibbon (Hylobates lar) and siamang (Hylobates syndactylus). Primate Report 25: 11-12 (Abstract only).

Group harmony in gibbons: comparison between white-handed gibbon (Hylobates lar) and siamang (Hylobates syndactylus)

J. Fischer & T. Geissmann

Anthropological Institute, University Zürich-Irchel

The siamang (Hylobates syndactylus) is exceptional among gibbons in that its area of distribution almost completely overlaps with that of gibbons of the lar group, namely the white-handed gibbon (Hylobates lar) and the agile gibbon (Hylobates agilis). The siamang has almost twice the body weight of the gibbons of the lar group (ca. 11 kg vs. 5-6 kg), and it has been suggested that distinct ecological and behavioural differences exist between the siamang and its two sympatric species. The siamang has been claimed to differ from the white-handed gibbon "in the closer integration and greater harmony of group life" (Chivers, 1976, Behaviour 57: 116-135). However, few quantitative data exist in support of this view. In the present study, intra-group interactions in four captive family groups of white-handed gibbons and siamangs (2 groups of each species) were recorded by focal animal sampling. These data failed to show a consistent association between species and any of the behavioural patterns recorded, such as frequency of aggression, frequency of attempted food-transfer, frequency of successful food-transfer, and frequency and duration of social grooming bouts. On the other hand, some of these patterns showed a significant correlation with the parameter group size. Individuals in larger groups tended to show a higher level of these social interactions, with the exception of percent successful food transfers (negative correlation). Our findings indicate that social cohesion within these gibbon groups may be much more flexible according to and depending on social or ecological influences and less rigidly linked to specific gibbon taxons than previously assumed. Another finding of this study - a positive correlation between the frequency of aggression and grooming - is discussed in the light of the functional interpretations commonly attributed to allogrooming behaviour in primates.

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