The value of faecal N in monitoring dietary quality in desert ungulates: The Arabian oryx as a model

The Arabian oryx (Oryx leucoryx, Pallas 1777) once roamed the Middle Eastern deserts, but went extinct in the wild by 1972. More than 30 years after re-introduction programs were implemented the oryx is still classified as vulnerable on the IUCN Red List. Its poor reproductive performance was determined to be nutrition-related, and the availability of tannin-rich acacia browse has been crucial for the oryx in compensating for unpredictable grass production. Faecal N (FN) is an ubiquitous estimate of nutritional status, but its interpretation depends on dietary tannins that bind with proteins in the gastro-intestinal tract. Using three captive male oryx fed diets varying in the percentage of tannin-rich Acacacia tortilis pods, we demonstrated that dietary polyethylene-glycol-binding tannins (PEG-b-T) were partly recovered in faeces and that faecal PEG-b-T represented qualitative evidence of browsing. Faecal PEG-b-T and FN were positively correlated, but we found no relationship between the dietary percentages of acacia pods, dietary protein, PEG-b-T, in vitro dry matter digestibility, and FN. Beyond the effect of dietary tannins, where the provision of crude protein and energy are sufficient for maintenance, efficient urea recycling might compromise the value of FN as an estimate of dietary status in oryx.