• complex life cycle;
  • cross-fertilization;
  • parasites;
  • predators;
  • self-fertilization;
  • trophic transmission

Although numerous parasite species have a simple life cycle (SLC) and complete their life cycle in one host, there are other parasite species that exploit several host species successively. From an evolutionary perspective, understanding the mix of adaptive and contingent forces shaping the transition from an ancestral single-host state to such a complex life cycle (CLC) has proved an intriguing challenge. In this paper, we propose a new hypothesis, which states that CLCs involving trophic transmission (i.e. transmission to a predator) evolved because they are an efficient way for parasites to meet a sexual partner, assuming that selective benefits are associated with cross-fertilization. Predators that eat a lot of prey in a relatively short time interval act to concentrate isolated parasites. We use an optimality model to develop our hypothesis and discuss further directions of potential research.