9706653 Amundson The C and O isotope composition of structural carbonate ( and phosphate) in the tooth enamel of fossil Equids has become widely used in Cenozoic paleoenvironmental studies in south Asia, Africa, and North America. Yet, with respect to modern horse (Equus), there is insufficient knowledge of (1) what portion of the surrounding ecosystem the horses utilize for their diet, (2) the relationship of the isotopic composition of dietary intake to the isotopic composition of body water and dissolved CO2 in blood, and (3) the inter-and intra-tooth isotopic variability in a given individual. A better understanding of these uncertainties is important to decipher the environmental significance of the C and O isotope composition of enamel. We propose to conduct a two pronged study to answer these questions through: (1) controlled dietary studies on experimental horses at the UC Davis School of Veterinary Medicine and (2) a two year study of two feral horse herds in which we (a) isotopically characterize the surrounding ecosystem, (b) monitor the monthly plant species composition, and isotopic composition, of the horse diet, (C ) monitor the O isotope composition of body water, (d) examine inter-tooth isotopic variability in selected individuals, and (e) examine intra-tooth morphological and isotopic variations. One impetus for this research are the observations reported in the animal science and paleontological literature (as well as recent findings by isotope chemists) that Equus teeth form over a significant period of an individual's life. While the exact chronology is somewhat uncertain, we believe that teeth may potentially record both annual and seasonal variations in the isotopic composition of diet and water intake. If this is true, Equid teeth may provide a more detailed isotopic record of the past than previously recognized. Equally important, potential isotopic variations within an individual caused by physiology, rather than dietary intake, must be recognized for ade quate interpretation of this isotopic proxy. This research builds on pioneering studies of the relationship between environmental water and the O isotope composition of enamel phosphate, and on recent studies of in-tooth isotopic variability in fossil and modern organisms. Our purpose here is to consolidate this existing knowledge into a cohesive experimental design that will adequately answer the question that remains about Equus tooth enamel and its relevance as an isotopic proxy for environmental conditions.
