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Mountain gorilla diet variation documented by stable isotope analysis of feces


PhD student Scott Blumenthal of the City University of New York (CUNY) Graduate School PhD Program in Anthropology and his colleagues have demonstrated that stable isotopes in feces can be used to reconstruct feeding behavior in wild African apes. This study emphasizes the power of using stable isotopes – a method of chemical analysis that is commonly used in anthropology, biology, and geology – to reconstruct some aspects of behavior of individual animals in the wild. Documenting the dietary ecology of African apes provides an important reference baseline for understanding aspects of human evolution, and stable isotopes provides a means to directly compare living and extinct animals.

The work was published in the prestigious Proceedings of the National Academy of Sciences USA with Scott as lead author, a rare honor for a PhD student. This research was supported by several programs at the National Science Foundation (NSF). Scott Blumenthal is a Trainee in the New York Consortium in Evolutionary Primatology, an interinstitutional doctoral training group supported by the NSF IGERT (Integrative Graduate Education and Research Traineeship) program. Prof. Eric Delson of Lehman College is the CUNY PI for this award (0966166). NYCEP combines student and faculty research and training at CUNY, Columbia, NYU, the American Museum of Natural History and its Richard Gilder Graduate School and the Wildlife Conservation Society. Prof. Jessica Rothman of CUNY’s Hunter College Anthropology Department was also involved in this research, supported by an instrumentation grant from NSF’s Directorate for Social, Behavioral & Economic Sciences (0922709). Other colleagues involved in the study include Kendra Chritz (Department of Biology) and Prof. Thure Cerling (Department of Geology and Geophysics) at the University of Utah.

Measuring diet is critical for understanding the ecology, evolution, and conservation strategies of primates; yet traditional methods relying on field observation or visual examination of gut or fecal contents are imperfect measures, particularly if individuals cannot be continuously observed. The timing of diet shifts reconstructed with isotopes in the gorillas match the observational data for these individuals, revealing that a detailed record of dietary flexibility in apes can be measured with stable isotopes. This provides a benchmark for understanding fossil humans, which exhibit isotope variable similar to or greater than what we see in African apes. This means our ancestors displayed a high degree of behavioral flexibility. This is important for understanding how adaptable our ancestors were in the face of changing ancient environments, which helps us understand how the fossil humans eventually became modern.

This study also provides a benchmark for the use of stable isotopes for understanding the ecology of primates in the wild. Most previous studies only included samples of animal tissues or feces, but did not include an analysis of locally available foods. That approach severely limits what you can learn from isotopes and can easily lead to incorrect conclusions. Most African primates consume foods that are similar in isotopic composition to the gorilla foods, suggesting that this approach can be widely applied.

Publication reference: Blumenthal SA, Chritz KL, Rothman JM, and Cerling TE. 2012. Detecting intrannual dietary variability in wild mountain gorillas by stable isotope analysis of feces. Proceedings of the National Academy of Sciences USA 109:21277-21282.

Address Goals

This research uses high-tech methods to study the most mundane of biological materials (fecal matter) in order to better understand diet in mountain gorillas, one of the most iconic of all animals. It has shown the value of stable isotopes in reconstructing small variations in the annual dietary cycle, an approach which can eventually be applied to the fossil record of human evolution. Researchers in biological anthropology, geology and biology combined their knowledge and methods to advance this understanding of dietary ecology and adaptation in one of our closest living relatives, advancing the frontiers of knowledge about their present and our own past. The NYCEP IGERT focuses on the study of primates (including humans) through training doctoral students in evolutionary primatology in multiple departments at several New York City universities, both private and public. NSF investment in research infrastructure through funding the acquisition of advanced instruments such as the system used in this project has clearly paid off in terms of the new and potentially transformative results and approaches developed here.