Amy Markeson

It has been hypothesized that changes in atmospheric CO₂ levels and temperatures cause changes in global vegetation patterns, particularly changes in the ratio of C₃ to C₄ vegetation. Vegetational changes may in turn affect diet composition of mammalian herbivores. Shifts in diet can be monitored through isotopic analysis of carbon in the tissues of herbivores once the fractionation between the diet and tissue of interest is established. We investigated the diets of herbivorous mountain voles (Microtus montanus) in the field. We first established the fractionation of carbon between the diet and hair using a captive colony of prairie voles (Microtus ochrogaster). Voles were fed rabbit chow (primarily alfalfa, C₃) since conception. The fractionation between diet and hair was determined by measuring the dietary and hair d ¹³C values. The fractionation was 1.9‰. To establish the ratio of C₃ and C₄ vegetation in the diets of wild mountain voles, we have been live-trapping Microtus montanus monthly since May 2000. Hair samples of voles and vegetation were collected and analyzed for d ¹³C. We have determined that the voles forage on primarily C₄ vegetation June-September, but they eat primarily C₃ vegetation during February, March, and May. Hair samples of voles taken from museum specimens collected decades ago show a similar seasonal pattern in diet to samples collected recently. Our results demonstrate that stable isotope analysis can be a valuable tool to determine diets of living animals as well as museum specimens.