MODELS TO PREDICT FORAGING BEHAVIOR OF BIG BROWN BATS (EPTESICUS FUSCUS) USING STABLE ISOTOPE SIGNATURES δ13C AND δ15N OF SKIN TISSUE

Stable isotopes are commonly used to evaluate trophic relationships, nutrient state, and temporal and spatial variation in diet, food webs, and behavior both within and between species. Here we describe the development and application of models to predict the foraging behavior of a common insectivorous bat (Eptesicus fuscus) based upon δ¹³C and δ¹⁵N signatures of skin tissue. We used a 42-specimen sample collected from three well-characterized ecogeographies disparate both in photosynthetic mechanism and fertilizer use to generate the models. Significant differences between these three sites in terms of δ¹³C (F_2,39=112.92***) and δ¹⁵N (F_2,39=97.06***) of bat skin samples, as well as a significant interaction effect of these two signatures (Wilk’s Λ=0.032, F_4,76=87.02***), made it possible to develop three models using Fisher’s linear discriminant functions: (1) A model predicting if bats forage in C₃ or C₄ sites, (2) A model predicting if bats forage in agricultural areas which utilize natural fertilizers, and (3) A synthesis model using both variables to predict specific foraging topology. We present the results of model application to a dataset of 310 bats sampled from 9 states that included a broad range of δ¹³C (-26.53‰ < δ¹³C< -17.20‰) and δ¹⁵N (6.36‰ < δ¹⁵N < 15.60‰) signatures in bat samples. We validated the use of skin tissue samples (from wing membranes) in the model by comparing each site across five tissue types. We found significant correlations (p-values range from 0.0184 to <0.0001 for each of the twenty comparisons conducted) between the following tissue types: skin, hair, whole blood, plasma, and feces and selected skin samples for model development due to consistently low variance within this tissue type. Model predictions are discussed in terms of implications for pest management and bat conservation.