Stable carbon isotope reconstruction of ungulate diet changes through the seasonal cycle

We analysed stable carbon isotope ratios (δ13C) in faeces of 11 African ungulate species from three South African savanna environments to determine whether this approach is sufficiently sensitive to record short-term seasonal diet changes in browsers (BR), mixed-feeders (IM), and grazers (GR). At monthly intervals, faecal δ13C revealed variations in proportions of C3 (browse) to C4 (grass) biomass consumed that were not detected by broader dry versus wet season comparisons, including subtle diet shifts amongst BR and GR. However, trends in faeces were influenced by changes in C3 and C4 plant isotope composition of up to 3‰. Nonetheless, faeces and plants showed strongly similar patterns of variation through the seasonal cycle, so that small diet shifts can be reliably inferred, provided that the variations in plants are controlled for. Faecal δ13C of BR may be further influenced by consumption of isotopically different plant parts such as foliage versus fruit and flowers, and GR faeces may reflect differential utilization of grass following different photosynthetic sub-pathways. Future studies will need to incorporate data that capture isotopic variations in herbivore food sources, and if this is achieved, the approach may well become adopted as a routine addition to traditional methods for assessing diet, habitat use, and habitat condition.