Parasites determine a predator's optimal feeding strategy

Summary The diet selected by three-spined sticklebacks (Gasterosteus aculeatus) depends on the degree of parasitization by one or both of two parasite species (Schistocephalus solidus, Glugea anomala). Uninfested fish prefer the more profitable of two different size classes of prey (Daphinia magna). Fish parasitized by Glugea or Schistocephalus attack both prey types equally often, whereas sticklebacks infested by both parasite species prefer to attack the less profitable prey. The diet selected is optimal under the condition that parasites decrease their host's competitive ability.