What Should a Clever Ungulate Eat?

Abstract

An optimal foraging model applicable to large herbivores is developed. This transforms primary data on vegetation composition, defined in terms of the standing biomasses, nutrient contents, and eating rates offered by a range of food types, into an expression for nutrient ingestion and assimilation rates. A "clever ungulate" is defined as a short-term optimizer for foraging performance alone. An optimal dietary range is identified, which varies for different target nutrients and for time-minimizer versus profit-maximizer tactics. Foraging performance is more sensitive to changes in food quality and in bite sizes than to differences in overall quantitative food abundance. Model predictions are compared with the observed foraging behavior of kudus under natural conditions. Real kudus are both selective and responsive in their food choice, and a rough correlation exists between the acceptances shown for particular plant species and their leaf crude protein contents. However, discrepancies suggest that bite-size differences are also significant, and that energy rather than protein is the target nutrient. The effects of vegetation patch structure remain unresolved, but four patch scales may be operative. Key problem areas identified for further field research include the significance of bite-size differences, potential target nutrients and their rates of digestive assimilation, the effects of plant secondary chemicals, and the influence of varying scales of patch structure.