Tools and Technology Article Facts From Feces: Nitrogen Still Measures Up as a Nutritional Index for Mammalian Herbivores DAVID M. LESLIE, JR., 1 United States Geological Survey, Oklahoma Cooperative Fish and Wildlife Research Unit and Department of Natural Resource Ecology and Management, Oklahoma State University, Stillwater, OK 74078-3051, USA R. TERRY BOWYER, Department of Biological Sciences, 921 S 8th Avenue, Stop 8007, Idaho State University, Pocatello, ID 83209-8007, USA JONATHAN A. JENKS, Department of Wildlife and Fisheries Sciences, South Dakota State University, Brookings, SD 57007, USA ABSTRACT Fecal nitrogen (FN) has been applied widely as an index of dietary quality in studies of nutritional ecology of free-ranging and captive vertebrate herbivores, particularly ruminants. Three related articles in the Journal of Wildlife Management (JWM; Leslie and Starkey 1985, 1987; Hobbs 1987) have been cited (n ¼ 150) in 87 publications and 39 peer-reviewed journals. The critique by Hobbs (1987) and the reply by Leslie and Starkey (1987) on limitations and appropriate applications of FN have been used to justify use of FN or negate its value as a nutritional proxy. We undertook a retrospective analysis of FN applications since 1985, largely because we sensed that methodological cautions noted in the 3 JWM publications were not being followed, leading to faulty conclusions and management applications, and that application protocols needed updating. From January 1986 through July 2007, the 107 species-by-continent applications of FN, citing the 3 JWM publications singly or in any combination, were diverse; FN was used in various ways on 5 continents and for 50 wild and domestic species. Cumulative rates of departure from recommended FN applications increased in recent years, largely in studies that compare different species while failing to fully acknowledge that differences likely reflected digestive capabilities rather than differences in some aspect of dietary intake. Post-1985 research on plant secondary compounds (e.g., tannins) has refined limitations to the application of FN, permitting more straightforward protocols than were possible in 1985. Although use does not necessarily reflect value, the number of published applications during the past 22 years indicates that peer reviewers from a variety of scientific disciplines view FN as a suitable proxy for nutritional status, which can be used to contrast study units when carefully defined by the study design. Any index can have shortcomings, and there are still circumstances when application of FN is problematic. Precise prediction of intake with FN under field conditions is still hampered by inherent variability, but contrasts of comparable study units and species can be appropriate. Published protocols for FN, as amended herein, should be adhered to, and conclusions are strengthened by the use of multiple nutritional indices. ( JOURNAL OF WILDLIFE MANAGEMENT 72(6):1420–1433; 2008) DOI: 10.2193/2007-404 KEY WORDS fecal index, fecal nitrogen, mammalian herbivores, nutritional ecology, nutritional index, plant secondary compounds, ruminants, tannins, ungulates. Because studies of nutritional ecology of free-ranging herbivores can be time consuming and expensive to conduct, some investigators used fecal indices (e.g., Putman 1984, Kohn and Wayne 1997) to evaluate relationships of nutrition with diet, demography, and behavior. Animal scientists were among the first to establish relationships of fecal constituents, notably nitrogen (N), with quality and quantity of diets of domestic herbivores (e.g., Kotb and Luckey 1972, Cordova et al. 1978, Holechek et al. 1982). Although some wildlife management treatises advocated use of fecal N (FN; Caughley and Sinclair 2002), others warned of its shortcomings (Robbins 1983, Servello et al. 2005). After evaluating effects of plant secondary compounds (e.g., Freeland and Janzen 1974), specifically tannins, on protein digestion and excretion in 5 ungulate species, Robbins et al. (1987:103) offered what stands as the strongest criticism of FN by concluding that it ‘‘is not a precise indicator of any dietary parameter and should not be used in ecological studies.’’ Fecal material of many free-ranging herbivores is readily available, making an index based on its components appealing. Under conditions lacking extensive dung beetle activity, as is common in Africa (Wrench et al. 1996), exposure of feces to weather and other insects does not compromise retention of N for 2–3 weeks postdefecation, making FN a useful index under field applications when relatively fresh feces can be obtained (Jenks et al. 1990, Osborn et al. 2002, Kamler et al. 2003a). Nevertheless, debate over the accuracy and application of FN has not abated. Following earlier investigations on wild ruminants (e.g., Arman et al. 1975), Leslie and Starkey (1985) demonstrated positive relationships of FN to dietary N, dietary phospho- rus, and dietary dry matter digestibility in Roosevelt elk (Cervus elaphus roosevelti) and Columbian black-tailed deer (Odocoileus hemionus columbianus). Those authors cautioned that applications of FN were constrained by differences in species-specific digestive capabilities, study areas, and study conditions and the potential consequences of consuming plant secondary compounds (Robbins et al. 1987, Dearing et al. 2005). After re-evaluation of data in Sinclair et al. (1982) and Leslie and Starkey (1985), Hobbs (1987) echoed concerns of Robbins et al. (1987) and concluded that FN was not a reliable predictor of dietary quality because of the influence of intra-seasonal and inter-animal variability and plant secondary compounds. Nevertheless, Hobbs (1987:319) also concluded that ‘‘specific regressions [could] be useful in specific locations.’’ Neither Robbins et al. (1987) nor Hobbs (1987) offered any definition of precision. Leslie and Starkey (1987) countered by outlining condi- 1 E-mail: cleslie@usgs.gov 1420 The Journal of Wildlife Management  72(6)