Can Fecal T3 Metabolite Level Fluctuations in European Roe Deer (<i>Capreolus capreolus</i>) Give Insights on Body Condition and Thermal Stress?

Mammals can use a variety of physiological mechanisms to adapt to changes in their environment. Thyroid hormones (THs) are key modulators of growth and mediators of environmental conditions by regulating developmental processes and metabolism in animals. In recent years, advancements in non-invasive sampling have allowed monitoring of the fluctuations of THs and their metabolites in wild mammals. Triiodothyronine (T3) represents the major metabolite of THs excreted in feces so that it can be monitored in fecal samples. In this study, fecal samples collected during the hunting season from legally culled European roe deer (Capreolus capreolus; n = 160) were assayed to investigate the potential fluctuations of fecal TH metabolites (FTMs) in response to environmental (e.g., the temperature, local densities) and individual (e.g., sex, age, body, and nutritional conditions) variables. For this aim, we validated a TH enzyme immunoassay in the feces of roe deer. Our results show that FTMs can be successfully measured with satisfactory accuracy and precision. Extraction recovery (70%-120%), intra- and inter-day repeatability (<15%), linearity dilutions (80%-120%), and parallelism (<20%) were consistent with international guidelines. Environmental temperature (p < 0.001) showed a strong inverse correlation with FTM levels. THs can thus represent a reliable indicator in studying animals' adaptative responses to environmental temperature changes, providing perspectives for the study of the impact of climate change on ungulates and mammals. Further analyses, comparing samples collected all year round, are needed to investigate the correlations of TH values versus the other investigated variables.