Orangutan Density Variation and Nest Tree Selection in Logged Bornean Peat Swamp and Sundaland Heath Forest

Orangutan responses to habitat variation are not fully understood, although food availability and forest structure are key features of habitat quality. I examined the effect of these factors on orangutan density in logged peat swamp and kerangas (Sundaland heath) forest in Central Kalimantan. I also investigated nest building patterns since these reflect adaptations to the energetic and mechanical constraints of arboreality and may provide insight into how orangutans manage forest disturbance. I conducted the first surveys of orangutan nests and habitat in Rimba Raya Biodiversity Reserve, a buffer area to Tanjung Puting National Park, established to prevent conversion of forests to oil palm plantations under the United Nations Reducing Emissions from Deforestation and forest Degradation (REDD) program.;I found that orangutan densities were 0.54 -- 2.03 individuals/km2 in heavily logged peat swamp forest, 3.70 -- 4.58 individuals/km 2 in lightly logged peat swamp forest, and 1.61 individuals/km 2 in kerangas forest. Food availability was a significant predictor of orangutan density except where logging intensity was high. I found higher tree taxa richness but lower total food availability in kerangas forest which partially explained lower orangutan density. Logging intensity was an important gradient explaining 27% of forest composition, and orangutan nest density declined significantly with logging intensity. Commercial logging of "gembor" (trees in Lauraceae) had the greatest impact on habitat, explaining 18% of tree species composition, whereas selective logging for local use had no effect on forest composition.;Nest-building patterns varied with logging and forest type. Nests in heavily logged peat swamp forest were built lower in the canopy, in smaller trees and multiple trees were used more often for nest construction (53% of nests). In kerangas forest where preferred nest trees were limited, multiple nest trees were more frequent (18% compared to 4% in peat swamp forest). Orangutans exhibited a strong preference for nesting in Lithocarpus spp. and for nesting in large trees (>40 cm DBH) and a weak preference for nesting in Shorea spp. Tree architecture may be important in nest tree selection and this study found preferential use of Rauh and Roux-Massart hybrid models and avoidance of Roux and Massart models.