Niche differentiation and dietary seasonality among sympatric gorillas and chimpanzees in Loango National Park (Gabon) revealed by stable isotope analysis

The feeding ecology of sympatric great ape species yields valuable information for palaeodietary reconstructions in sympatric early hominin species. However, no isotopic references on sympatrically living apes and their feeding ecology are currently available. Here we present the first isotopic study on sympatric great apes, namely western lowland gorillas (Gorilla gorilla gorilla) and central chimpanzees (Pan troglodytes troglodytes) from Loango National Park, Gabon. We successfully analyzed the stable carbon and nitrogen isotope ratios in a selection of food plants (n = 31) and hair samples (n = 30) retrieved from sleeping nests to test whether niche partitioning among sympatric chimpanzees and gorillas is detectable using isotope analysis of hair. Ape hair strands with roots were sectioned into sequential segments (n = 100) to investigate temporal isotopic variation related to seasonal variations in food resources. We found significant δ13C differences between herbaceous plants and fruits, most likely due to canopy effects. While the δ13C values of chimpanzees indicate the consumption of fruit, the low δ13C values in gorilla hair indicate folivory, most likely the consumption of 13C-depleted herbaceous vegetation. Our isotopic data also confirmed dietary overlap between chimpanzees and gorillas, which varied by season. Gorillas showed significant variation in δ13C values in response to season due to shifting proportions of herbaceous plants versus fruits. In chimpanzees, significant seasonal variation in δ15N was likely related to the seasonal availability of fruit species with particularly high δ15N values. In summary, we found isotopic evidence for niche partitioning and seasonal dietary variation among sympatric great apes at Loango. These findings provide a valuable reference for palaeodietary research on fossil hominins using δ13C analyses, particularly for studies focusing on sympatric taxa and on temporal isotopic variation within incremental tissues such as tooth enamel.