Behavioural observations of bonobos (Pan paniscus) have revealed that their preferred foods are fruit, yet they consume terrestrial herbaceous vegetation (THV) throughout the year. THV accounts for ~2% of bonobo diets but many bonobo sites are characterised by dense understories, obscuring feeding observations on the ground. THV has been viewed as an important food source for P. paniscus, and some evidence suggests that the THV consumed by bonobos bears higher nutritional yields than that consumed by chimpanzees (Pan troglodytes) in East Africa.Stable isotope analysis has been used to supplement feeding observations of nonhuman primates (NHPs) and has helped further our knowledge of the dietary patterns of Pan spp. The δ13C and δ15N values of an animal reflect the foods they ate and are permanently recorded in their tissues or excreta. Since NHPs consume plants, botanical δ13C and δ15N values aid interpretations of their stable isotope compositions. Most tropical grasses use the C4 photosynthetic pathway and have tissue δ13C values between −11‰ and −14‰ . In contrast, trees, shrubs and temperate grasses follow the C3 photosynthetic pathway and have δ13C values averaging about −27‰ and range between −23‰ and −31.5‰. The range of δ13C values in C3 plants is in part due to a “canopy effect.” Plants growing under dense canopy cover have lower δ13C values than plants in open areas due to the incorporation of 13C‐depleted CO2 produced by decaying leaves and lower light intensities. Plant organs also vary in carbon isotope compositions with nonphotosynthetic organs typically exhibiting higher δ13C values compared to leaves. Nitrogen isotopic variation is more difficult to interpret. However, animal tissues are usually 15N‐enriched relative to diet and there is a stepwise increase in δ15N values (~3‰) with each trophic level...