Facts From Feces: Nitrogen Still Measures Up as a Nutritional Index for Mammalian Herbivores

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 = 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.

Soil ingestion, nutrition and the seasonality of anthrax in herbivores of Etosha National Park

“…Fecal nitrogen variation among species, a result of digestive capabilities, is considerable, and hence, does not necessarily reflect differences in dietary quality (Leslie et al 2008). Since the three study species have very different digestive morphologies and efficiencies, we did not attempt to make comparisons among species in the nutrition parameters.…”

“…Fecal nitrogen is a useful index for the dietary quality of forage (Leslie et al 2008), although tannins (present in the leaves of some dicotyledonous plants) can increase the concentration of fecal nitrogen observed Starkey 1985, Wrench et al 1997), so is a better nutritional proxy for grazers than browsers. Fecal phosphorus correlates with dietary phosphorus, and measurements are not affected by diet selection, so concentrations of this mineral are often included in estimates of dietary quality for mixed-feeding or browsing species (Wrench et al 1997, Grant et al 2000.…”

Causes and Consequences of Broad-Scale Changes in the Distribution of Migratory Caribou (<i>Rangifer tarandus</i>) of Southern Hudson Bay

“…Dietary Quality: Fecal nitrogen has been used as an accurate proxy for dietary nitrogen in more than 150 studies worldwide (Leslie et al, 2008) and is a valid indicator of forage quality Starky, 1985, 1987), including that of caribou forage (Bergerud, 1996;Barten et al, 2001). We expected that in the late period, dietary nitrogen for caribou in the Centre and East would be lower than in the West (Table 1).…”

“…1), during the period of the year that is the most energetically taxing for female caribou (Sadleir, 1984 were located during aerial surveys (Berglund et al, 2014). Eight samples collected at the coast in the Centre were dry; however, nitrogen content remains stable for weeks even when exposed to the elements and most insects (Leslie et al, 2008). Fecal nitrogen values for caribou do not differ between sexes given similar habitat (Bergerud, 1996), caribou have a varied diet (Bergerud, 1977), and we did not collect samples over multiple years or seasons, so we avoided biases resulting from monotypic diets and inappropriate timing of collection (Leslie et al, 2008).…”

Resource partitioning among African savanna herbivores in North Cameroon: the importance of diet composition, food quality and body mass

“…Samples were taken from dung that was estimated to be less than 2 wk old. According to Leslie et al (2007) exposure of faeces to weather or insects does not compromise retention of N for 2-3 wk post defecation. In the wet period dung decay rate was high, so that only fresh dung (up to 3 d old) was collected.…”