The choice of two prey types that minimises the probability of starvation

Summary In this paper we investigate the optimal diet of a forager faced with two prey types. Classical optimal foraging theory, based on the maximization of the mean net rate of energetic gain , predicts that the optimal policy is either to take only the more profitable prey type or to take both prey types. The decision between these policies does not depend on the forager's energy reserves or the time available for foraging. We develop two alternative models, based on the minimization of the probability of starvation. In the first model, foraging occurs continuously, and it is optimal to take a prey type if and only if it increases the forager's energy reserves. In the second model foraging stops at dusk, and the forager dies during the night if its reserves at dusk are too low. The optimal policy, which has to be found numerically by dynamic programming, depends on the forager's reserves and the time left till dusk. In general the optimal policy is either to take both types or to take only the more profitable type. Taking both types is optimal when reserves are low, and there is some evidence that this occurs. The models show that factors that have been ignored in classical models may be of importance.