IMPLICATIONS OF CARBON ISOTOPIC VARIABILITY IN MODERN RODENTS IN THE GREAT PLAINS, USA FOR INTERPRETING DIETS OF FOSSIL RODENTS

Stable isotope analysis is a standard tool for reconstructing mammalian paleoecology. Paleodiet datasets are typically interpreted based on isotopic patterns in modern ecosystems and working hypotheses of ancient ecosystems. While laser ablation isotope techniques permit analysis of fossil small mammal teeth (e.g. rodents and lagomorphs), our understanding of isotopic variation in rodents across modern environments is still limited, hindering interpretation of isotopic data from ancient ecosystems. We have compiled a dataset of >500 δ¹³C values from 14 modern rodent species with geographic ranges that span the southern Great Plains and that vary in dietary ecology. For each species, specimens were sampled with uniform spacing across the geographic range in the region and isoscapes of δ¹³C values were compared statistically to regional patterns of δ¹³C values of surface soil organic matter, biome types, vegetation, land use, and climate variables. Pairwise statistical tests of δ¹³C values indicate statistically significant differences among both species and dietary categories. Folivores have significantly lower δ¹³C values than all other dietary categories, granivores exhibit the most positive δ¹³C values, and omnivores and rootivores have δ¹³C values that span the spectrum of diets derived from C₃ and C₄ resources. Insectivores integrate δ¹³C values of both C₃ and C₄ plants on the landscape resulting in an averaged value of the two resources. The δ¹³C values of all individuals taken together slightly increase with δ¹³C values of surface soils, an indicator of C₄ plant biomass, and variance also increases as soil δ¹³C values increase. Granivores and rootivores incorporate more C₄ resources with increasing C₄ biomass while other species predominately consume only C₃ resources. Rodent δ¹³C values do not differ significantly by land type, however the median and range increase in warmer and drier biomes. The relationships revealed here are critical for interpreting fossil small mammal diets in the Great Plains, and provide a reference for interpreting small mammal diets in general.