Fischer, J.O. & Geissmann, T., 1990: Group harmony in gibbons: Comparison between white-handed gibbon (Hylobates lar) and siamang (H. syndactylus). Primates 31: 481-494.

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

J. O. Fischer & T. Geissmann
Anthropological Institute, University Zürich-Irchel, Switzerland

Key words: White-handed gibbon; siamang; Hylobates lar; Hylobates syndactylus; social cohesion; aggression; food transfer; grooming.

Abstract: The siamang (Hylobates syndactylus) is exceptional among gibbons in that its area of distribution almost completely overlaps those of other gibbons, namely the white-handed gibbon (H. lar) and the agile gibbon (H. agilis) of the lar group. 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, p. 132). However, few quantitative data exist to support this hypothesis. In the present study, intra-group interactions in 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 most of the behavioural patterns recorded, such as frequency of aggression, percentage of successful food transfer, frequency of social grooming bouts, and duration of social grooming / animal / h. A signifcant difference was found for only two of the variables: Individual siamangs in this study showed longer grooming bout durations, and made fewer food transfer attempts than lar individuals. Only the frst of these two differences is consistent with the hypothesis mentioned above, whereas the lower frequency of food transfer attempts in siamangs is the opposite of what should be expected under the hypothesis. On the other hand, two of these behavioural patterns showed a signifcant correlation with the parameters group size and individual age: Both individuals in larger groups and younger individuals tended to show shorter grooming bouts and a smaller proportion of successful food transfers. Our fndings 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 specifc gibbon taxa than previously assumed. A considerably larger number of gibbon groups would have to be compared to provide reliable evidence for or against species-specifc differences in group cohesion. Another fnding 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.