Unlike the classical models of optimal diet (Emlen 1966; Schoener 1971), it has been proposed that large generalist herbivores should not feed to maximize energy intake per unit time, but rather to optimize the nutrient mix within a given bulk of food (Westoby 1974). The model proposes that an animal’s dietary choice is constrained by its need for a certain minimum amount of various nutrients, as well as the overall limit on the amount of food it can process (obtain or digest) within a given space of time - thus requiring it to balance the different nutritional
benefits against the various costs, rate restrictions and various diet components. Another such model of an optimal diet under nutrient constraints was proposed by Pulliam (1975). These models seem far more appropriate for herbivores (as compared to carnivores), as their food may vary considerably in terms of nutrient content and digestive constraints over space and time (Crawley 1983). Furthermore, according to Milton (1979), herbivores should be strongly influenced by nutritional features, such as the quality or secondary compounds (antiquality) of potential foods.
The nutritional quality of ingested food is determined by its composition in terms of the proportion of cell wall to cell contents, by the concentrations of protein, soluble carbohydrates, minerals and other nutrients in the cell contents, as well as the degree of lignification of the cell wall (Owen-Smith 2002; O’Connor et al. 2007). Additionally, many plants contain secondary compounds which act to reduce the nutrients available to herbivores (Robbins 1983; Robbins, Mole, Hagerman & Hanley 1987; Owen-Smith 2002) or have toxic effects (Milton 1979).
Protein is regarded as one of the most important and often limiting components of wildlife diets. The continuous availability of dietary nitrogen, an estimate of crude protein (% N x 6.25), is essential for the growth and maintenance of all animal tissues (Robbins 1983). Faecal nitrogen as an indicator of diet quality has been used in a number of herbivore studies (Sinclair 1974; Mould & Robbins 1981; Hall-Martin et al. 1982; Wofford et al. 1985; Leslie & Starkey 1987; Grant
et al. 1995; Wrench et al. 1997; Van der Waal et al. 2003; Osborn 2004), and has
been found to be correlated to dietary protein (Mould & Robbins 1981; Wofford et
al. 1985; Howery & Pfister 1990; Grant et al. 1995; Wrench et al. 1997).
It is not adequate, however, to explain dietary quality only in terms of these components. The dietary fibre component is equally important in determining the quality of forage. “Dietary fibre” is often used to refer to the plant cell wall, which consists mainly of cellulose and hemicellulose (Robbins 1983). These
components are not susceptible to enzymatic digestion in vertebrates, but can be broken down by gastrointestinal flora found in the digestive tracts of most herbivorous animals (Robbins 1983). Herbivores have a tendency to selectively feed on less fibrous plants and plant parts (Bell 1971; Owen-Smith & Cooper 1987; Cooper et al. 1988). Possible reasons for this tendency include:
1) Cell wall components prevent the extraction of cell nutrients (Keys et al. 1969; Bell 1971; Janis 1976), therefore the more fibrous the food, the less available the nutrients within cells.
2) Fibre and lignin content has been negatively correlated with digestibility in ruminants and non-ruminants (Schneider et al. 1951; 1952; Owen-Smith 2002; Hatt et al. 2005).
A high fibre content is therefore associated with low quality forage (Erasmus et al. 1978; Demment & Van Soest 1985; Owen-Smith 1988). Additionally, a high fibre content is generally associated with a low protein content, as protein concentration is largely determined by the cell contents to cell wall ratio (Erasmus et al. 1978; Owen-Smith 2002).
In addition to these requirements, herbivores also need a variety of minerals to maintain cellular activity and body functions (Robbins 1983). Furness (1988) drew attention to the role of mineral nutrition on diet selection by herbivores, which lead to the suggestion that minerals should be included in optimal foraging models, especially relating to seasonal requirements (Bazely 1989). Deficiencies or imbalances of minerals play an important role in determining animal condition, fertility, productivity and mortality, and it is thought that marginal deficiencies may be easily overlooked in favour of more obvious factors causing population fluctuations, such as severe weather, food shortages, parasites or infectious agents (Underwood 1977). Of all essential minerals, sodium and phosphorus are increasingly being shown to be the two minerals governing the body condition and diet selection of large herbivores when soil fertility is primarily low (McNaughton 1988; 1990; Freeland & Choquenot 1990; Ben-Shahar & Coe 1992). Both are essential for normal growth and reproduction (Weir 1972;
Groenewald & Boyazoglu 1980; Grassman & Hellgren 1993; Ulrey et al. 1997; Wrench et al. 1997; Dörgeloh et al. 1998; Seydack et al. 2000). According to the Department of Environmental Affairs and Tourism’s Eastern Cape state of the environment report (DEAT 2004), a large portion of the Eastern Cape has a naturally high salinity in both its ground and surface water. As elephants are known to drink large volumes of water on a daily basis (Weir 1972), it is thus unlikely that sodium would prove to be a limiting nutrient in this area. A previous study by Koen et al. (1988) on the macronutrients available to elephants in the AENP has noted, however, that the phosphorus levels in both the plants species and faeces analysed was low, with a very high calcium: phosphorus ratio, which would hamper the absorption of the little phosphorus available, suggesting that phosphorus levels may affect the growth and reproduction of elephants in this region.