Optimal diet theory: when does it work, and when and why does it fail?

Over the last three decades, many studies have attempted to explain forager diets by using optimal diet theory (ODT). Despite some obvious successes, the utility of this theory remains controversial. We reviewed the results of 134 studies of optimal diet theory to test hypotheses on factors that can explain variation in the ability of ODT to predict diets and diet shifts in response to changes in prey availability. Our major conclusion is that while ODT has generally worked well for foragers that feed on immobile prey, the theory often failed to predict the diets of foragers that attack mobile prey. We found only mixed support for the hypothesis that the theory works better when the study scenario more closely fits the assumptions of the model. Contrary to our a priori predictions, forager types (invertebrate versus ectothermic vertebrate versus endothermic vertebrate) did not differ in their likelihood of corroborating ODT. Two explanations for why optimal diet theory does not work well with mobile prey are that studies on mobile prey often lack information on key parameters that are required to rigorously test ODT, and that with mobile prey, variations among prey in vulnerability (encounter rate and capture success) are often more important than variation in predator active choice in determining predator diets.

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