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Brockelman / Oral

New findings and models of gibbon social organization, and parallels with humans

Warren Y. Brockelman, Institute of Science and Technology for Research and Development and Faculty of Science, Mahidol University, Salaya, Putthamonthon, Nakhon Pathom 73170, Thailand

     Until the 1990's, the gibbons (Family Hylobatidae) were regarded as adhering invaryingly to a simple pattern of social organization consisting of monogamous, territorial, nuclear families. As such, their social system appeared to be remote from that of humans in many ways and a poor model for human evolution. Our view of gibbon society is now changing rapidly, for two main reasons: long term data from at least two sites which are starting to contradict the traditional view, and a somewhat belated application of Neodarwinian natural selection theory toward explaining gibbon social structure and its evolution. Theories about gibbon social evolution are being set forth which can only be tested with more refined data than have been collected in the past. Recent empirical findings about dispersal, pair formation, and group stability indicate that gibbon group structure is more variable than has been noted in the past, and that delayed dispersal with subadults remaining in their family groups and helping their siblings and parents is common. Gibbons are also found to have a dual mating strategy consisting of investment in a long term mate and territory, and extra-pair copulation, as has been found in "monogamous" birds. We still lack a clear understanding of the benefits of territoriality to each sex in gibbons, and its relation to food resources. Some ideas about why territorial defense of resources benefits both sexes will be proposed, which need to be tested with further observations on foraging and diet in gibbons. In light of the new findings, gibbons are seen in several ways to provide a good model for human family structure, even though humans may never have evolved through a stage with the same pattern of forest canopy resource exploitation.

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