American Journal of Primatology 70:690–694 (2008) BRIEF REPORT Fiber-Bound Nitrogen in Gorilla Diets: Implications for Estimating Dietary Protein Intake of Primates JESSICA M. ROTHMAN 1,2Ã , COLIN A. CHAPMAN 2– 4 , AND ALICE N. PELL 1 1 Department of Animal Science, Cornell University, Ithaca, New York 2 McGill School of Environment, McGill University, Montreal, Quebec, Canada 3 Department of Anthropology, McGill University, Montreal, Quebec, Canada 4 Wildlife Conservation Society, Bronx, New York Protein is essential for living organisms, but digestibility of crude protein is poorly understood and difficult to predict. Nitrogen is used to estimate protein content because nitrogen is a component of the amino acids that comprise protein, but a substantial portion of the nitrogen in plants may be bound to fiber in an indigestible form. To estimate the amount of crude protein that is unavailable in the diets of mountain gorillas (Gorilla beringei) in Bwindi Impenetrable National Park, Uganda, foods routinely eaten were analyzed to determine the amount of nitrogen bound to the acid-detergent fiber residue. The amount of fiber-bound nitrogen varied among plant parts: herbaceous leaves 14.578.9% (reported as a percentage of crude protein on a dry matter (DM) basis), tree leaves (16.176.7% DM), pith/ herbaceous peel (26.278.9% DM), fruit (34.7717.8% DM), bark (43.8715.6% DM), and decaying wood (85.2714.6% DM). When crude protein and available protein intake of adult gorillas was estimated over a year, 15.1% of the dietary crude protein was indigestible. These results indicate that the proportion of fiber-bound protein in primate diets should be considered when estimating protein intake, food selection, and food/habitat quality. Am. J. Primatol. 70:690–694, 2008.  c 2008 Wiley-Liss, Inc. Key words: plant protein; ADIN; nutritional ecology; ape nutrition INTRODUCTION Nitrogen (N) is used to estimate protein content because it is an easily measurable component of the amino acids in protein. Crude protein is typically estimated by multiplying the amount of N in a plant by 6.25, because many domesticated plant and animal proteins are known to contain about 16% nitrogen [Van Soest, 1994]. Many authors have estimated the protein content of primate foods on this basis, but few have considered the fact that not all of this crude protein is available [but see Conklin- Brittain et al., 1999; Curtis, 2003; Milton & Dintzis, 1981; Silver et al., 2000; Yeager et al., 1997]. In domesticated plant species, 60–80% of the crude protein is ‘‘true protein’’ that is composed of amino acids [Van Soest, 1994]. First, some of the nitrogen may be bound to the lignified plant cell wall (i.e., fiber-bound N) that is resistant to digestion by animal and microbial enzymes. Second, nitrogen can be bound to secondary compounds, particularly tannins, and thus rendered indigestible. Third, plants contain significant amounts of nonprotein nitrogen in secondary compounds (i.e., alkaloids, glucosinolates, and cyanides), in nucleic acids or ammonia, or as products of amino acid catabolism. Because the 6.25 coefficient is inaccurate for non- amino forms of N, some studies have proposed adjustments to the conversion factor to account for N-containing fractions other than true protein [Levey, 2000; Milton & Dintzis, 1981]; but Conklin- Brittain et al. [1999] suggested that conversion factors might underestimate available protein (AP). There is considerable inter- and intraspecific varia- bility in plant N fractions that cannot be accounted for by a single conversion factor [Van Soest, 1994]. In addition, protein digestibility is affected by an animal’s digestive anatomy, body size, diet quality, and physiological status; therefore, adjustments must be both species- and diet-specific. Nutritionists have persisted in using the 6.25 coefficient to convert N to crude protein for ration formulation as amino acid profiles generally are not available. The utility of Published online 10 April 2008 in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/ajp.20540 Received 4 November 2007; revised 19 February 2008; revision accepted 25 February 2008 Contract grant sponsors: Jane Engel and the Robert G. Engel Family Foundation; Department of Animal Science at Cornell University; Mario Einaudi Foundation; Cornell University Graduate School; Institute of African Development. Ã Correspondence to: Jessica M. Rothman, McGill School of Environment, McGill University, Montreal, Que., Canada H3A 2A7. E-mail: jessica.rothman@mcgill.ca r r 2008 Wiley-Liss, Inc.