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We have very few data on the distribution, behavior, and socioecology of several gibbon species, including the Javan gibbon ( Hylobates moloch: Hylobatidae). In this dissertation, I will examine the evolutionary implications of the expression of individual variation and behavioral plasticity for the emergent relationships among individuals and groups in hylobatid communities. The primary data set is derived from the systematic study of a small population of Javan gibbons (N=8 groups) in the Cagar Alam Leuweung Sancang (CALS, or the Sancang Forest Nature Reserve), West Java. These data encompass the overall ecology of the reserve, including: a population assessment, an evaluation of habitat quality in both disturbed and undisturbed areas, and a summary of anthropogenic influences. I report group densities, and the age/sex composition of groups based on range mapping methodology, including fixed-point counts. The exclusive home range (territory) of one group, at 6.25 ha, is the smallest within the sample (χ=14.86, SD=3.89) and well below the reported average of 17 hectares for the species. I describe historical and ongoing patterns of human disturbance, and discuss implications for both species-specific conservation strategies and socioecological flexibility within the Hylobatidae. The dissertation will assist in the refinement of models for the evolution of social and mating systems in the Hylobatidae. The majority of intragroup behavioral data were extracted from a single cohesive social group consisting of two adult males, a single adult female with a physiologically dependent infant, and an adolescent female. These data are contrasted with those of a neighboring uni-male/uni-female group with a single, dependent infant. Behavioral profiles and intragroup social relationships are discerned from focal group scan data and patterns of group movement. Results from the analysis of proximity maintenance data, patterns of group movements, and intergroup interactions indicate subtle, but important, differences in the social and ecological relationships between group members. These findings elucidate aspects of individual behavioral plasticity, and subsequent variability in social organization, in the context of human disturbance on a variety of scales. Finally, I synthesize the published data on hylobatid ecology and social behavior to elucidate the commonalities and unique aspects among hylobatid populations. A better understanding of gibbon ecology, life histories, and intra- and intergroup relationships is crucial to our definition of "preferred social units", or viable community compositions, in any strategy involving the conservation of social organisms. This dissertation includes co-authored materials.
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