Life-cycle complexity in helminths: What are the benefits?

Parasitic worms (i.e., helminths) commonly infect multiple hosts in succession. With every transmission step, they risk not infecting the next host and thus dying before reproducing. Given this risk, what are the benefits of complex life cycles? Using a dataset for 973 species of trophically transmitted acanthocephalans, cestodes, and nematodes, we tested whether hosts at the start of a life cycle increase transmission and whether hosts at the end of a life cycle enable growth to larger, more fecund sizes. Helminths with longer life cycles, that is, more successive hosts, infected conspicuously smaller first hosts, slightly larger final hosts, and exploited trophic links with lower predator–prey mass ratios. Smaller first hosts likely facilitate transmission because of their higher abundance and because parasite propagules were the size of their normal food. Bigger definitive hosts likely increase fecundity because parasites grew larger in big hosts, particularly endotherms. Helminths with long life cycles attained larger adult sizes through later maturation, not faster growth. Our results indicate that complex helminth life cycles are ubiquitous because growth and reproduction are highest in large, endothermic hosts that are typically only accessible via small intermediate hosts, that is, the best hosts for growth and transmission are not the same.Peer Reviewe