Combining isotopic analyses and satellite telemetry to assess geographic variability in the trophic ecology of Amazon River dolphins Inia geoffrensis

The Amazon River dolphin Inia geoffrensis is considered a generalist consumer, feeding on a wide variety of fishes in the Amazon and Orinoco basins. Most studies on their trophic ecology have relied on stomach content analysis. In this study, we analyzed dolphin skin for bulk-tissue δ 13 C and δ 15 N values and conducted compound-specific stable nitrogen isotope analysis of amino acids (δ 15 N-AA) to investigate the diet and trophic position (TP) of 8 satellite-tracked river dolphins in 2 areas of the Peruvian Amazon. Potential fish prey were also sampled and analyzed for bulk δ 15 N and δ 13 C. Our results showed contrasting TPs when using bulk δ 15 N versus δ 15 N-AA data. Bulk δ 15 N indicated similar TPs for dolphins. In contrast, δ 15 N-AA data revealed differences among dolphins; those with smaller home ranges had congruent TPs from both methods, but dolphins that traveled more extensively had varying TPs based on δ 15 N-AA values, suggesting a diet of fish prey of lower TP. Differences in feeding behavior may reflect variations in habitat and prey availability, though our small sample size limits population-level inferences. By integrating bulk stable isotope analysis with δ 15 N-AA and satellite telemetry, we gained deeper insights into the trophic behavior of river dolphins in the Peruvian Amazon. This study provides the first δ 15 N-AA values for these dolphins, serving as a baseline for future research on their diet and habitat use.