USE OF CARBON AND NITROGEN ISOTOPES FOR ASSESSING DIETARY DIFFERENCES IN BIG BROWN BATS (EPTESICUS FUSCUS) FROM DIFFERENT GEOGRAPHIC REGIONS IN NORTH AMERICA

Stable isotopes have been used in ecological research to investigate dietary habits and food webs. These studies are useful for establishing the tropic structure of an entire ecosystem or the dietary patterns of a single species, including both aquatic and terrestrial organisms. Several approaches have been used evaluate dietary habits using stable isotopes, including the analysis of different tissues, organs, fecal matter, shells, etc. For this study, we primarily analyzed wing tissue from big brown bats (Eptesicus fuscus), an insectivorous species that ranges from coast to coast and from Canada to Mexico, and thus can be expected to reflect differences in isotopic values in different environments. Because big brown bats commonly roost in man-made structures (houses, churches, schools, etc.), in both urban and rural settings, they are ideally suited for making geographic comparisons. We tested the hypothesis that bats from agricultural and non-agricultural sites in North America should show marked differences in isotope ratios of carbon and nitrogen, based on the consumption of herbivorous insects available in these different environments. We first examined muscle tissue and wing tissue from the same individual bats, and found no significant difference in isotope ratios. Wing tissue was therefore used in subsequent analyses. Wing tissue was sampled from several populations of big brown bats across the United States, represented by both urban and rural habits, and analyzed for their carbon and nitrogen stable isotopes. A statistically significant difference was found in the isotopic values of carbon and nitrogen between bats living in agricultural versus non-agricultural areas, whereas areas characterized by a combination of these two environments could not be statistically distinguished. To confirm these findings, two additional populations of big brown bats were sampled, one from an agricultural monoculture (corn) in western Indiana, and the other from a forested region in southern New Hampshire. For the latter comparison, we analyzed wing tissue, blood plasma, compacted blood cells, hair, and feces. Although our analyses of the latter data are incomplete, preliminary results support our original findings indicating marked differences in isotopic values between agricultural and non-agricultural regions.