Predators feeding on behaviourally responsive prey: some implications for classical models of optimal diet choice

Hundreds of studies exist on predator foraging behaviour, and the same holds for anti-predator behaviour in prey. However, despite these many studies, almost nothing is known about diet choice by predators that feed on prey with anti-predator behaviour. We addressed this problem theoretically by incorporating anti-predator vigilance into two classical models of diet choice by predators. Vigilance allows prey to detect attack and escape, and prey become more vigilant as the predator focuses more of its attention on a specific prey type. We considered diet models in which predators encounter two types of prey either simultaneously (in mixed-type groups) or sequentially (solitary, randomly distributed prey). In both cases, anti-predator behaviour in prey causes predators to adopt a more generalized diet than one might predict on the basis of classical theory. When prey are encountered simultaneously, increased dietary generalization by predators often reflects broad ‘partial prey preferences’. Here, predators tend to attack both prey types some of the time rather than attack only one prey type all of the time (as suggested by classical theory). When prey are encountered sequentially, increased dietary generalization reflects mainly an increase in the threshold encounter rate necessary to cause the predator to drop the less profitable prey from the diet. However, when sequentially encountered prey require an investment in stalking time (which will be lost to the predator if the prey detects attack), partial preferences may hold over a large part of parameter space. Also possible under this stalking-time scenario is the reversal of prey preferences, in which the inherently less profitable prey is always preferred by the predator. These general deviations from the expectations of classical diet theory reflect, in part, the fact that predators faced with behaviourally responsive prey will often benefit from ‘managing’ the anti-predator responses of those prey.