Group harmony in gibbons: Comparison between white-handed gibbon (Hylobates lar) and siamang (H. syndactylus) |
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Authors: | Jan O Fischer Thomas Geissmann |
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Institution: | (1) Anthropological Institute, University Zürich-Irchel, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland |
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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 (two 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/hr. A significant 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 first 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 significant
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 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 taxa than previously assumed. A considerably larger number of gibbon groups would have to
be compared to provide reliable evidence for or against species-specific differences in group cohesion. 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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Keywords: | White-handed gibbon Siamang Hylobates lar Hylobates syndactylus Social cohesion Aggression Food transfer Grooming |
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