Tracing the diet of herbivores and omnivores through isotopic analysis

Understanding the movement of herbivores across a landscape and ecological dynamics through time is important for current conservation and ecosystem management, while providing valuable insights into evolution and adaptation. Documented historical evidence in the relationship between organisms and their surrounding vegetation is generally limited and thus isotopic analysis of fossils can provide valuable insights into this important area of research. 

Plant species have specific isotopic signatures reflecting their geographic origin as well as their metabolic abilities to utilize carbon during photosynthesis. These isotopes can be systematically traced through the food chain based on the fact that food consumers incorporate these isotopic signatures into their skeletal tissues. As a result, dietary isotopes in animal tissues are an archive of their diet status and changes throughout their lifetime. This theory can also be applied to the dietary variability in human populations, however, the use of imported foods complicates the interpretation of results (Sealy, 2001). 

Types of dietary isotopes

Strontium isotopes (⁸⁷Sr/⁸⁶Sr) can be used to interpret the origin of plant material based on the fact that geological units underpinning soil horizons and groundwater reservoirs have distinct strontium isotope signatures which are picked up by overlying vegetation. This type of isotopic analysis provides information in a very detailed local scale as long as the reference/background ⁸⁷Sr/⁸⁶Sr range is known for the site in question. 

Carbon isotopes (𝛿¹³C) can be used to distinguish between different types of plants (C3, C4 or marine vegetation) and thus approximate the relative contributions of each in an individual’s diet profile, while providing information on climate variability. 

Oxygen isotopes (𝛿¹⁸O) can be measured to investigate large scale variability in precipitation sourcewater, which is taken up by vegetation through the roots and integrated into plant tissue. In this way, such isotopes can be used to constrain general regions in which individuals lived or migrated from, particularly when the 𝛿¹⁸O signature differs from local individuals. 

Nitrogen isotopes (𝛿¹⁵N) are often used in combination with carbon isotopes as they provide further information on the trophic level of the organism in question since an enrichment in 𝛿¹⁵N occurs at each progressively higher trophic level (Sealy, 2001; Post, 2002). In this way, one can assess the structure of a paleo-ecosystem through the analysis of dietary isotope. From a perspective of human evolution, 𝛿¹⁵N isotopic analysis can provide information about whether hominids were predominantly vegetarian or carnivorous in nature. 

Sample selection for dietary isotopes

The dietary isotopes can be analysed on a variety of different bone structures, depending on research interests. The rate of turnover in collagen material varies depending on the bone in question, which influences which diet periods are integrated into the bone and can be interpreted through isotopic analysis. For example, teeth are developed during childhood in humans and therefore isotopic signatures from teeth would provide information on diet in the first years of life. It also provides information about the environmental condition, mainly for water sources, in which individuals spent their childhood.

On the other hand, other bones (e.g. femurs) continue to develop during adolescence and therefore would provide information about diet at a slightly later life stage. Finally, some bone structures (such as ribs) turnover collagen every few years, and would therefore record the diet from the final years of life. This concept of relative bone age is also used in radiocarbon dating, when teeth are used to help identify age of the deceased individual whereas a bone of rapid turnover is used to estimate year of death. While some of these relationships between bone development and life stage are known, this is a research area that is still continuing to develop.  

Examples in research

Strontium isotopes from mammoths and other large mammals were investigated by Esker et al. (2019) in an attempt to track the movement of these vertebrates across central Texas during the Late Pleistocene. ⁸⁷Sr/⁸⁶Sr analysis indicated that the group of mammals moved into a watershed area prior to their ultimate demise, indicating a possible shortage of resources due to drought in their home range, forcing them to migrate for survival. 

A study by Chenery et al. (2010) investigated strontium and carbon isotopes in human bones at a Roman archaeological site in Gloucester (UK) to investigate diet and mobility of the local population. Using these combined isotopes, the authors found a diverse diet among the population, with some local Gloucester residents as well as others who had recently migrated to the region. Specifically, elevated levels of dentine 𝛿¹⁸O in some individuals pointed to the hypothesis that they grew up in a warmer climate (e.g. Wales or southern England) compared to those with lower levels of 𝛿¹⁸O who grew up in a cooler UK environment. This was further complemented by the fact that these potential migrants also demonstrated higher dentine ⁸⁷Sr/⁸⁶Sr signatures which did not match the local Gloucester range, proving they likely ate vegetation from another location during their earlier years. 

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Duration: 2 minutes, 40 seconds || Speaker: Arash Sharifi, PhD
This video excerpt is part of Isobar Science’s webinar: Geochemistry and Application of Strontium Isotopes.
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Conclusion 

While there are many opportunities to utilise strontium, oxygen, carbon and nitrogen isotopes to trace diet through space and time, the correlation between isotopes and the landscape is not always straightforward (Bentley et al. 2004). As a result, the use of this research pathway depends on the local site and quality of isotopic analysis of the landscape as well as the aim of your research.

References 

Bentley, R.A., Price, T.D. and Stephan, E., (2004). Determining the ‘local’ 87Sr/86Sr range for archaeological skeletons: a case study from Neolithic Europe. Journal of Archaeological Science, 31(4), pp.365-375. DOI: 10.1016/j.jas.2003.09.003

Chenery, C., Müldner, G., Evans, J., Eckardt, H. and Lewis, M., (2010). Strontium and stable isotope evidence for diet and mobility in Roman Gloucester, UK. Journal of archaeological Science, 37(1), pp.150-163. DOI: 10.1016/j.jas.2009.09.025

Esker, D., Forman, S.L., Widga, C., Walker, J.D. and Andrew, J.E., (2019). Home range of the Columbian mammoths (Mammuthus columbi) and grazing herbivores from the Waco Mammoth National Monument,(Texas, USA) based on strontium isotope ratios from tooth enamel bioapatite. Palaeogeography, Palaeoclimatology, Palaeoecology, 534, p.109291. DOI: 10.1016/j.palaeo.2019.109291

Sealy, J., (2001). Body tissue chemistry and Palaeodiet in: Brothwell, D.R., Pollard, A. M., (Eds.), Handbook of Archaeological Science. John Wiley & Sons, Chichester, pp. 269-279.

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