Can Parasites Indirectly Benefit Final Hosts? the Role of Behavioral Manipulation and Trophic Transmission in the Energetic Tradeoffs of a Host-Parasite System

Trophically transmitted parasites often adaptively manipulate their intermediate host’s phenotype to increase transmission to their final host. There may be a net fitness advantage to preying on manipulated food sources when the energy gained from the parasite “delivery service” outweighs the costs of infection. However, no work to our knowledge has directly compared the costs of infection versus the foraging advantages to final hosts of parasites that are known manipulators. Here, we used the trematode parasite Euhaplorchis californiensis (“Euha”), a brain-infecting parasite found in the California killifish (Fundulus parvipinnis), as a model system. Euha increases the frequency of conspicuous behaviors in killifish, which could increase their risk of predation by fish-eating marsh birds, the parasite´s final host. Laboratory studies (using ducks and chicks as proxies for marsh birds) showed no detrimental effects of infection in final hosts, with growth, behavior, immune function, and energy metabolism all maintained. However, field-based mesocosm studies indicated that Euha-infection does not improve predator detection of infected killifish, nor does it improve foraging efficiency. This work suggests that while there is no selective pressure to avoid foraging on prey infected with manipulative parasites, there is no concurrent selective advantage to preferentially foraging on these infected prey