ISOTOPIC PATTERNS IN EXTANT MAMMALIAN HERBIVORE TEETH FROM EASTERN AFRICA

The desire to develop carbon and oxygen isotopic ratios of fossil mammal teeth as paleoenvironmental indicators has driven a wide-ranging effort to sample modern terrestrial systems. The results of this effort show that carbon isotope ratios (δ¹³C) of extant mammalian herbivores vary with environment and are primarily functions of the δ¹³C values of ingested plants, while physiology plays a secondary but important role. These modern data have been critical analogs for interpreting δ¹³C values of fossil enamel, primarily in studies that explore the relative proportion of C₃ and C₄ resources in a fossil animal’s diet and in some cases distinctions within both the C₃ and C₄ dietary end-members. Interpretation of oxygen isotopes in mammalian herbivore tooth enamel is less tractable, however modeling and empirical data show that the oxygen isotope ratios (δ¹⁸O) of tooth enamel from some taxa correlate with environmental variables such as aridity and the isotopic composition of local waters, providing a template for interpreting δ¹⁸O values from fossil teeth. Here we present a large isotopic dataset of tooth enamel from extant mammalian herbivores that lived in a range of habitats in eastern Africa, including closed-canopy forest, montane forest, moorland, grassland, and scrubland environments. We use these data to investigate isotopic distributions within individual ecosystems and across environmental gradients. Each of these communities of mammalian herbivores fits into a distinct δ¹³C-δ¹⁸O space that reflects their environment. At the taxon level, we observe isotopic variation that is indicative of both environment and behavior. For example, while this and other datasets document higher δ¹⁸O values with increased aridity for some taxa, δ¹³C values decrease along this same aridity gradient for several herbivores. The change in δ¹⁸O values is likely due an increase in δ¹⁸O values of water in more arid settings, whereas the decrease in δ¹³C values in xeric environments may be due to increased dietary intake of C₃ plants or arid-adapted C₄ grasses. With these and other examples we provide additional documentation on the variability of δ¹³C and δ¹⁸O values in teeth from extant mammalian herbivores and propose ways that they can be used as analogs for fossil systems.