Are they both the same shit? Winter faeces of roe and red deer show no difference in nutritional components

Herbivorous ungulate diets affect population performance and overall forest health through balanced interactions on plant-herbivore relations; therefore, understanding them is critical. Faeces are frequently used in ungulate nutritional ecology because they can provide information about animals' digestive efficiency. Roe deer (Capreolus capreolus) and red deer (Cervus elaphus) have different morpho-physiological and ecological constraints, and these differences should be reflected in their faeces. On the other hand, the lack of information about the animal (sex, age, reproductive status, diet selection, etc.) may be challenging for such studies. This study aimed to detect species' different susceptibility to these factors reflected in animals' faeces. Thus, we hypothesised that near-infrared reflectance spectrometry (NIRS) could distinguish between the faecal nutrients of two cervids. We collected 94 usable faeces from both species along the forest transect in Bohemian forests in the Czech Republic, covering 2 500 ha. Roe and red deer overlap was determined using the four faecal nutritional components on two axes. No discrimination occurred, refuting our hypothesis and highlighting that out-of-control variables are critical for faecal studies in uncontrolled settings. Fibrous parts explained the most variance (48%), indicating animals' strong reliance on nutrition quality. Apparently, uncontrolled supplementary feeding produced similar faecal nutrient outcomes during the nutrition-limiting winter, which was theoretically supported by the animal's response to predation and hunting pressure. The inability of NIRS to identify the source of N in faeces may also explain the lack of discrimination.