Study of stable carbon, nitrogen, and oxygen isotopes of plants in Sri Lanka reveals complexity in interpreting primate stable isotope ecology
Stable isotopes are powerful tracers of diet and ecology, particularly for primates that are rare, hard to observe, or only represented by historical collections. However, despite early successes in the application of stable isotope analysis to primates, there is a growing awareness of isotopic complexity at the base of primate foodwebs. A new paper published in the American Journal of Primatology, co-authored by Patrick Roberts and Oshan Wedage of the Department of Archaeology, Max Planck Institute for the Science of Human History, provides a comprehensive plant reference dataset for a forest habitat of three primate species in Sri Lanka.
Researchers have suggested that subtle differences in stable carbon, oxygen, and nitrogen isotope analysis of primate tissues can be used to discern detailed ecological niche distinctions, including the eating of different plant parts and feeding at different heights in the canopy. However, the resulting small isotopic distinctions are difficult to interpret given overlapping environmental effects. In this study, Roberts et al. examined the effects of canopy height, plant part, and season on plant isotopes in a seasonally dry tropical forest that is home to the toque macaque, grey langur, and purple-faced leaf monkey that exhibited ecological differences.
The paper found that stable carbon and oxygen isotopes can distinguish between plant parts (fruits and leaves) and canopy height in the Polonnaruwa Nature Reserve forest. Indeed, plant part distinctions will likely drive any isotopic distinctions in the tissues of the different primates living within this forest. For example, the purple-faced leaf monkey demonstrates a greater reliance on leaves that the toque macaque, which will likely be observable in its tissues. Nevertheless, these effects are small and overlapping, while no obvious ecological correlate was identified for nitrogen isotope values.
This work urges caution in the interpretation of subtle differences in the isotopic composition of primate tissues. This is especially the case where less prior ecological information is available, as might be the case for extinct or archaeological primate species. While the authors do not suggest that stable isotopes have little role in studies of primate ecology, attempts should clearly be made to characterize the isotopic variability within a given study area so that confounding influences of different environmental effects can be acknowledged and accounted for.