An Experimental Study in Wild Wood Mice Testing Elemental and Isotope Analysis in Faeces to Determine Variations in Food Intake Amount

SIMPLE SUMMARY: Elemental and stable isotope analyses are useful and common methods for wildlife diet studies, e.g., for characterizing diets and trophic relationships. However, little is known about the potential applicability of these techniques to address other aspects of feeding ecology. Here, we evaluated whether faecal elemental (carbon and nitrogen) and/or isotopic values (δ(13)C, δ(15)N) can determine variations in the amount of food intake. Overall, elemental analyses and δ(15)N values failed in reporting significant differences, but preliminary outcomes support the potential use of faecal δ(13)C values as an indicator to detect short-term slight food intake changes. The results of this work provide, for the first-time, reference data for interpreting faecal elemental and isotopic patterns in free-ranging wood mice (Apodemus sylvaticus), as well as new insights into the additional applicability of isotopic analysis in feeding ecology studies. ABSTRACT: The analysis of carbon and nitrogen elemental (C, N) and isotopic compositions (δ(13)C, δ(15)N) in faeces are considered reliable methodologies for the study of diet in wildlife. Here, we tested the suitability of these techniques to detect variations in the amount of food intake. We captured wild wood mice (Apodemus sylvaticus) with Sherman live traps where bait access was initially free, and later it was experimentally limited inside by four different devices to cause intended variations in the amount ingested. The total C and N (%) and stable δ(13)C and δ(15)N isotopic values were determined for the bait and in mice faecal samples. Faecal values were lower than bait ones except for N, likely due to animal matter ingested before capture. No significant differences in total C, N and δ(13)C were found due to individual traits. However, breeding males showed higher δ(15)N values than breeding females, probably due to differences in energy and protein demands between both sexes during the breeding season. Only δ(13)C detected food intake variations (≥2 g). Despite further research being needed, these results initially support the potential of δ(13)C to provide information on the amount ingested, thus being useful to complement trophic ecology studies.