Trophic position and dietary breadth of bats revealed by nitrogen isotopic composition of amino acids

Bats perform important ecosystem services, but it remains difficult to quantify their dietary strategies and trophic position (TP) in situ. We conducted measurements of nitrogen isotopes of individual amino acids (δ15NAA) and bulk-tissue carbon (δ13Cbulk) and nitrogen (δ15Nbulk) isotopes for nine bat species from different feeding guilds (nectarivory, frugivory, sanguivory, piscivory, carnivory, and insectivory). Our objective was to assess the precision of δ15NAA-based estimates of TP relative to other approaches. TPs calculated from δ15N values of glutamic acid and phenylalanine, which range from 8.3–33.1‰ and 0.7–15.4‰ respectively, varied between 1.8 and 3.8 for individuals of each species and were generally within the ranges of those anticipated based on qualitative dietary information. The δ15NAA approach reveals variation in TP within and among species that is not apparent from δ15Nbulk data, and δ15NAA data suggest that two insectivorous species (Lasiurus noctivagans and Lasiurus cinereus) are more omnivorous than previously thought. These results indicate that bats exhibit a trophic discrimination factor (TDF) similar to other terrestrial organisms and that δ15NAA provides a reliable approach for addressing questions about variation in the TP of bats that have heretofore proven elusive.