Interspecific competition, whereby one species suffers due to exploitation or interference by another, is the most common interaction among organisms. If two species occupy the same ecological niche and geographic area, unless the two diverge in some dimension of niche use, one will be competitively excluded. Previous primate studies have upheld these predictions, whereby species consume different foods or divide the habitat structurally. However, sympatric hylobatids (gibbons and siamangs) pose a conundrum, broadly overlapping in ecology, using the same part of the canopy and eating the same foods. Due to higher body mass, siamangs are expected to win access to resources during encounters with smaller-bodied gibbons. So, the question remains how these species can coexist. While the greatest competition is expected between ecologically-similar taxa, primates must also contend with other local species. For hylobatids, langurs are one such taxon. They approximate small gibbons in body mass, but their physiology allows them to subsist on more fibrous, lower-quality foods, which might reduce competition. However, langurs live in larger groups and may therefore be able to displace gibbons. The proposed study will investigate competition among three primate species, siamangs (Symphalangus syndactylus), agile gibbons (Hylobates agilis) and mitered leaf monkeys (Presbytis melalophos) at Way Canguk, Bukit Barisan Selatan National Park, Indonesia. For 12 consecutive months resource availability will be monitored and focal data will be collected on feeding, locomotion, and space use. Based on intergroup encounters and playback experiments group ranks will be determined. To estimate energy intake food samples will be collected and analyzed for nutritional content. Thus, this study will explore 1) key factors influencing niche use, 2) dominance relationships between heterospecific groups, 3) how dynamic interactions promote coexistence, and 4) energetic consequences of rank. The intellectual merit of the proposed study is that it will increase understanding of ecological systems by extending models of same-species competition to interspecific competition. Energy intake and expenditure will be estimated to examine the impacts of differential behavioral tactics on individual fitness. Specifically, it will be investigated if subordinate species act as "fugitives" by fleeing direct competition and more rapidly utilizing renewed food patches than ecologically-similar dominants. Playback experiments will be used to understand how acoustic signals mitigate competition between species. The broader impact of this study includes the continuation of a collaborative tradition at Way Canguk among local assistants, Indonesian scientists, and international researchers. Three assistants have been employed to complete habituation of the study groups and help in data collection, and Indonesian students will be supervised. By providing the first data on agile gibbons and langurs on-site, this study will pave the way for future studies on these two species. The data collected will be shared with other researchers on-site for future collaborative work. Detailed examinations of energetic balance across animal communities may reveal differential impacts of habitat loss on species' survival and will therefore be integral in designing plans to conserve the primate community.
