Carbon and Nitrogen Stable Isotopic Profiling of Chimpanzees and Monkeys in Kalinzu Forest Reserve, Uganda

Stable isotope analysis is a widely used tool in primate ecology for investigating diet and environment, with numerous studies focusing on chimpanzees. However, few studies have used this method to explore the dietary niche of chimpanzees in comparison to other primates or examined intra-individual dietary variability. This limitation hinders the understanding of the comparability of stable isotopic data with the wealth of behavioral observational data in primate ecology. We report the carbon and nitrogen stable isotope ratios of hairs from wild eastern chimpanzees (Pan troglodytes schweinfurthii) and four other primate species (Cercopithecus mitis, Cercopithecus ascanius, Allochrocebus lhoesti, and Colobus guereza) in the Kalinzu Forest Reserve, Uganda. Bulk analysis revealed that both plant foods and chimpanzees in Kalinzu exhibited lower carbon stable isotope ratios than expected for the region's rainfall. Inter-species comparison of bulk stable isotope ratios and preliminary compound-specific nitrogen stable isotope analysis of amino acids revealed that chimpanzees in Kalinzu have a lower degree of faunivory than the sympatric monkeys. Furthermore, ultra-fine sectioning of a hair sample was conducted to investigate dietary variation over daily to weekly timescales. In one adult male chimpanzee, carbon and nitrogen stable isotope ratios fluctuated by more than 1‰ within approximately 10 days. These findings highlight the importance of recognizing uncontrolled ecological variability and hidden intra-individual dietary changes when interpreting stable isotope data in relation to behavior and environmental traits.