Parasitic worms (helminths) with complex life cycles divide growth and development between successive hosts. Using data from 597 species of acanthocephalans, cestodes, and nematodes with two-host life cycles, we found that helminths with larger intermediate hosts were more likely to infect larger, endothermic definitive hosts, although some evolutionarily shifts in definitive host mass occurred without changes in intermediate host mass. Life-history theory predicts parasites to shift growth to hosts in which they can grow rapidly and/or safely. Accordingly, helminth species grew relatively less as larvae and more as adults if they infected smaller intermediate hosts and/or larger, endothermic definitive hosts. Growing larger than expected in one host, relative to host mass/endothermy, was not associated with growing less in the other host, implying a lack of cross-host tradeoffs. Rather, some helminth orders had both large larvae and large adults. Within these taxa, though, size at maturity in the definitive host was unaffected by changes to larval growth, as predicted by optimality models. Parasite life-history strategies were mostly (though not entirely) consistent with theoretical expectations, suggesting that helminths adaptively divide growth and development between the multiple hosts in their complex life cycles.

We assessed how parasitic worms (acanthocephalans, cestodes, and nematodes) with two-host life cycles divide their growth and development between hosts. Parasite life-history data were obtained from a life-cycle dataset (see Benesh et al. 2017. Ecology 98: 882), whereas host traits (like mass and endothermy) were compiled from literature sources and databases (see this Dryad repository: https://doi.org/10.5061/dryad.gtht76hmm). More details on how data were compiled, processed, and analyzed can be found in our manuscript (Benesh et al. 2022. Adaptive division of growth and development between hosts in helminths with two-host life cycles. Evolution) and in this GitHub repository: https://github.com/dbenesh82/division_helminth_growth_devo

Four files are included in the repository. “helminths_with_two_host_cycles.csv” is the specie-level dataset without any imputed values. We created 100 datasets in which we imputed missing data. These datasets are in the zip file “helminths_with_two_host_cycles_imputed.zip”. “helminths_with_two_host_cycles_bestimpute.csv” is the data table including the best estimate for missing data from imputation models. “README.txt” describes the columns in the data files.