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 (δ (15)N(AA)) and bulk-tissue carbon (δ (13)C(bulk)) and nitrogen (δ (15)N(bulk)) isotopes for nine bat species from different feeding guilds (nectarivory, frugivory, sanguivory, piscivory, carnivory, and insectivory). Our objective was to assess the precision of δ (15)N(AA)-based estimates of TP relative to other approaches. TPs calculated from δ (15)N 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 δ (15)N(AA) approach reveals variation in TP within and among species that is not apparent from δ (15)N(bulk) data, and δ (15)N(AA) 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 δ (15)N(AA) provides a reliable approach for addressing questions about variation in the TP of bats that have heretofore proven elusive.