Chimpanzee isotopic ecology: a closed canopy C3 template for hominin dietary reconstruction

The most significant hominin adaptations, including features used to distinguish and/or classify taxa, are critically tied to the dietary environment. Stable isotopic analyses of tooth enamel from hominin fossils have provided intriguing evidence for significant C 4 /CAM (crassulacean acid metabolism) resource consumption in a number of Plio-Pleistocene hominin taxa. Relating isotopic tooth signatures to specific dietary items or proportions of C 3 versus C 4 /CAM plants, however, remains difficult as there is an ongoing need to document and quantify isotopic variability in modern ecosystems. This study investigates the ecological variables responsible for carbon isotopic discrimination and variability within the C 3 -dominated dietary niche of a closed canopy East African hominoid, Pan troglodytes , from Ngogo, Kibale National Park, Uganda. δ 13 C values among C 3 resources utilized by Ngogo chimpanzees were highly variable, ranging over 13‰. Infrequent foraging on papyrus (the only C 4 plant consumed by chimpanzees at the site) further extended this isotopic range. Variation was ultimately most attributable to mode of photosynthesis (C 3 versus C 4 ), food type, and elevation, which together accounted for approximately 78% of the total sample variation. Among C 3 food types, bulk carbon values ranged from −24.2‰ to −31.1‰ with intra-plant variability up to 12.1‰. Pith and sapling leaves were statistically more 13 C depleted than pulp, seeds, flowers, cambium, roots, leaf buds, and leaves from mature trees. The effect of elevation on carbon variation was highly significant and equivalent to an approximately 1‰ increase in δ 13 C for every 150 m of elevation gain, likely reflecting habitat variability associated with topography. These results indicate significant δ 13 C variation attributable to food type and elevation among C 3 resources and provide important data for hominin dietary interpretations based on carbon isotopic analyses.