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INTRODUCTION
Resource partitioning among potential competitors takes a central place in ecological research. Grass-eating herbivores have received much attention in these studies because of the high numbers of coexisting species, in particular in Africa (Cromsigt & Olff 2006, Prins & Olff 1998, Prins et al. 2006). Around 100 species of herbivore larger than 2 kg occur in Africa that feed to a greater or lesser extent on grass (Prins & Olff 1998), and various studies have argued that African grazer assemblages are structured by patterns of resource partitioning among potential competitors (Arsenault & Owen-Smith 2002). MacArthur & Levins (1967) proposed a simple model to explain species richness as a function of resource availability and niche relationships among species. Based on this niche theory, one might expect the habitat-niche width to decline with an increasing number of sympatric species and decreasing resources. Demment & Van Soest (1985) showed that body size of herbivores shows a relationship along a resource axis of grasses having different quality, expressed as fibre contents. Gut capacity increases linearly with body mass (Clauss et al. 2007, Demment 1982, Demment & Van Soest 1985), whereas energy requirements (expressed as metabolic body size) scale with actual body weight as W0.75 (Kleiber 1975). Since the ratio of metabolic requirements to food processing capacity decreases with increasing size, various authors have argued that smaller-sized herbivores need food of a higher quality than do larger herbivores (Demment & Van Soest 1985, Jarman 1974, Prins & Olff 1998). However, the body mass-food quality relationship has recently been challenged. Pérez-Barberia et al. (2004) could find no significant effect of body mass of ruminants on fibre digestibility, in contrast to previous studies (Iason & Van Wieren 1999, Robbins et al. 1995). In addition, Clauss et al. (2007) argued that gut retention time is largely independent of body mass in grazing ruminants, so that a larger body mass does not necessarily lead to improved digestive efficiency.
In a recent study of the ungulate assemblage of Kruger National Park, South Africa, Codron et al. (2007) showed a significant negative correlation between body mass and the percentage nitrogen (N) in the faeces of herbivores. In addition, evidence was found of...