OPTIMAL DIET CHOICE FOR LARGE HERBIVORES - AN EXTENDED CONTINGENCY-MODEL

1. A more general contingency model of optimal diet choice is develope d, allowing for simultaneous searching and handling, which extends the theory to include grazing and browsing by large herbivores. 2. Foragi ng resolves into three modes: purely encounter-limited, purely handlin g-limited and mixed-process, in which either a handling-limited prey t ype is added to an encounter-limited diet, or the diet becomes handlin g-limited as it expands. 3. The purely encounter-limited diet is, in g eneral, broader than that predicted by the conventional contingency mo del, 4. As the degree of simultaneity of searching and handling increa ses, the optimal diet expands to the point where it is handling-limite d, at which point all inferior prey types are rejected, 5. Inclusion o f a less profitable prey species is not necessarily independent of its encounter rate and the zero-one rule does not necessarily hold: some of the less profitable prey may be included in the optimal diet. This gives an optimal foraging explanation for herbivores' mixed diets. 6. Rules are shown for calculating the boundary between encounter-limited and handling-limited diets and for predicting the proportion of infer ior prey to be included in a two-species diet, 7. The digestive rate m odel is modified to include simultaneous searching and handling, showi ng that the more they overlap, the more the predicted diet-breadth is likely to be reduced.