Living apart and having similar trouble: Are frog helminth parasites determined by the host or by the habitat?

Sympatric hosts are exposed to similar ecological conditions, particularly if they are closely related phylogenetically and share some physiological and behavioral traits. We studied the sympatric frogs Leptodactylus chaquensis Cei, 1950 and Leptodactylus podicipinus (Cope, 1862) to investigate the extent to which the helminth parasite communities were influenced by host species’ characteristics or habitat location. We described and compared the helminth communities of 50 L. chaquensis and 40 L. podicipinus collected concurrently from two different study sites in Brazil’s Pantanal floodplain. Similarities in the prevalence and mean abundance of helminths were higher among allopatric populations of the same species than among sympatric populations of different species. The effects of host species, size, and habitat on helminth composition were significant. The amount of variance in the helminth community composition explained by host species and size was greater than that explained by host habitat. These results indicate that the main factors determining similarities in parasite species in this study system are the coevolutionary and biological constraints of the host species, which either limit or allow infection of the parasite species despite the host habitat.