Optimal diets: simultaneous search and handling of multiple-prey loads by salamander larvae

Summary A key assumption of conventional opitmal diet theory is that foragers cannot search for prey while already handling one prey item. Some foragers, however, can handle multiple-prey loads; i.e., they can search for, attack and handle further prey when already handling one or more prey. We examined diet selection by small-mouthed salamander larvae, Ambystoma texanum, that can search while handling up to two prey at a time. We gave A. texanum larvae a choice between two size classes of Daphnia pulex at two prey densities. Larval A. texanum diet choice did not fit the predictions of conventional optimal diet theory, but fit very well with the predictions of a multiple-prey model. At low prey density, A. texanum larvae were nonselective. At high prey density, larvae were non-selective when their mouths were empty, but showed a strong preference for larger, more valuable prey when larvae already had prey in their mouths. In 16 out of 18 instances, foragers either accepted or rejected small prey in keeping with a multiple-prey model's predictions.