Time Constraints and Diet Choice: Different Predictions from Different Constraints

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Abstract
The time that an animal spends foraging may often be constrained by simultaneous requirements for the expression of other behaviors. For example, the need for mating, vigilance, or antipredator escape responses may significantly alter the time that an animal allocates to foraging behavior. I show here that the time alloated to foraging behavior may, in turn, significantly alter predictions concerning diet choice. I review predictions from the two classes of models that specially address the influence of time constraints on prey selection. Bout length for each class of model is treated either as a known constant or as a random variable with either of two frequency distributions (normal or negatively exponential). Two major predictions are derived from the models. (1) If foraging-bout length is such that only a single prey is taken per bout (class 1), the forager should decrease prey selectivity as the time left in the bout decreases. If more than one prey may be taken (class 2) and if the bout length is known, then selectivity exhibited within the bout depends on the relative value of available prey. If prey are similar in rank, then the forage should generalize. If prey differ markedly in rank, then selectivity should decrease as the time left in the bout decreases. It is predicted that intermediate differences generate oscillations in selectivity toward the end of the bout from a generalist policy to a specialist policy, then back to a generalist policy. Thus, both classes of model generally predict an end-of-bout change in preference. This end-of-bout effect, however, declines as the variance in bout length increases, and vanishes altogether when the time of the end of the bout is unpredictable. (2) In addition to the end-of-bout effect, mean selectivity exhibited throughout the bout should differ between bouts that differ in length; mean selectivity should generally decline with a decrease in bout length. This between-bouts effect is predicted regardless of the variance in bout length. Several studies from the literature report either an end-of-bout effect or a between-bouts effect for animals foraging under time constraints.