ETHOLOGICAL ASPECTS OF PARASITE TRANSMISSION

In parasitic life cycles, transmission events, that is, the passage of a parasite from an "upstream" host to a "downstream" host, often involve behavioral adaptations that result in an increase of the chances of the parasite's being actually transmitted. These behaviors stem from evolutionary changes in the parasites' genomes: either the parasites select responses (their own responses or those they provoke in upstream hosts) to stimuli arising from downstream hosts or from the environment where they live, or they select stimuli (arising from themselves, or from alterations they provoke in upstream hosts) that are addressed to downstream hosts. The various processes are differentiated on the basis of behavioral adaptations and illustrated by examples. The difficulty of demonstrating that the traits are actually the result of a selective process favoring transmission and several unresolved questions are discussed.