Diet-Faeces Trophic Discrimination Factor and Gut Passage Time of an Aotearoa New Zealand Insectivorous Bat, Chalinolobus tuberculatus, Determined via Controlled Feeding Experiment.

Studying bat diet is challenging due to their rarity, cryptic nature, nocturnal habits, and protected status. Stable isotope analysis of bat faeces offers a promising noninvasive and nonlethal method to understand their trophic interactions. However, this requires knowledge of the isotopic difference between food items and resulting faeces, known as the trophic discrimination factor (TDF). Accurate TDF calculations rely on precise dietary information, typically obtained through controlled feeding experiments. We used the rare opportunity of a captive rehabilitating Chalinolobus tuberculatus, a threatened Aotearoa New Zealand insectivorous bat, to conduct a feeding trial to determine its diet-faeces TDF. Given the bat was fed three different food types, we used DNA metabarcoding to match food items to faeces and estimate gut passage time-the time it takes for food to travel from ingestion to excretion. We calculated TDF values for nitrogen and carbon as-0.89 ± 1.19‰ and 0.82 ± 0.23‰, respectively, with both not significantly different from zero. The gut passage time was less than 3-4 h. Altogether, these findings add to our understanding of bat trophic ecology, particularly for Chalinolobus, but also highlight that where TDFs remain poorly constrained and close to zero, applying them may not always improve diet reconstruction accuracy, and uncorrected stable isotope values may be just as informative.