Within-bout dynamics of diet choice

Conventional diet theories mostly ignore dynamics in prey selectivity during a foraging bout. However, results from experiments with several aquatic predator species showed that, as more prey were eaten, the predators included more of the initially less profitable (small) prey types in their diets. We also found that handling times of the initially more profitable (large) prey types increased with prey sequence, but handling times of the small prey remained constant. Consequently, relative profitability of the large prey declined over the foraging trials. We modeled prey choice by incorporating the change in handling time as a function of prey sequence. The model predicts a shift in diet as the relative prey values change during a foraging period. The predictions qualitatively match the empirical data. In addition, simulations over the foraging bout showed that adopting the strategy using updated profitabilities always gives higher or at least as high total energetic gain as the fixed strategy based on the classical optimal prey-choice model. These results imply that the predators reevaluate prey profitabilities and adjust their selectivity accordingly in the course of foraging, without abandoning rate maximization. We suggest that dynamics of diet choice may in part account for partial preferences frequently observed in studies on prey selection.