Isotopic analysis of tooth enamel carbonate from modern North American feral horses: implications for paleoenvironmental reconstructions

The accuracy of paleoenvironmental reconstructions based on isotope analyses of equid teeth is currently uncertain because the exact relationship between the isotope composition of modern feral equids and their environment has not been thoroughly studied. We analyzed the carbon and oxygen isotope values (δ 13C and δ 18O) of tooth enamel carbonate and the δ 13C values of fecal samples from modern feral horses. We compared those values with the δ 13C values of local vegetation and the δ 18O values of local waters. Herds were studied in two contrasting localities: eastern Oregon, where grasslands consisted of 100% C 3 species, and New Mexico, where >95% of the grasses were C 4 species. Carbon isotope analyses of fecal material and tooth enamel suggest that horses consumed primarily grass, but some New Mexico horses also consumed significant amounts of shrubs and/or forbs. Microhistological analyses of fecal samples show that Oregon horses consumed 95% grass, and Oregon enamel δ 13C values are consistent with a diet containing 100% C 3 plants. Microhistological analyses of fecal samples from New Mexico indicate a diet averaging 75% grass, while enamel δ 13C values suggest that diets averaged 85% C 4 plants (range=72–97%). Thus, reconstructions of the C 3/C 4 ratio of grasses in ancient grasslands that are based on the δ 13C values of fossil equid teeth may underestimate the abundance of C 4 grasses. The mean δ 18O values of tooth enamel paralleled the trends observed in the mean δ 18O values of precipitation. However, the mean δ 18O values of enamel carbonate from Oregon and New Mexico differed by only 3.3‰, which is less than the difference in the mean δ 18O values of precipitation (6.5‰). In addition, the range of δ 18O values within New Mexico enamel samples (6.5‰) was greater than the difference between mean enamel δ 18O values at each site. Calculated values for the δ 18O of water ingested by horses are 2–3‰ more positive than mean δ 18O values for corresponding precipitation, suggesting that horses consumed waters that were enriched in 18O due to evaporation. While our results confirm that local climatic and hydrological conditions can influence the δ 18O values of equid enamel, they also show that the δ 18O values of equid teeth are not always a direct proxy for the isotope ratios of precipitation.