Costs of parasite generalism revealed by abundance patterns across mammalian hosts.

Most parasites are able to infect several host species, yet generalism is anticipated to be a costly strategy. Parasite abundance provides an important indicator of fitness because it is promoted by longevity and fecundity, which are correlated with a parasite's basic reproductive number. Using a global database of parasitic helminths of terrestrial mammals, we show that the abundance of parasitic helminths is phylogenetically structured across helminth species. High parasite abundance can be achieved through three separate components: infecting abundant host species, having a high infection success (prevalence) or achieving high parasite abundance withinhost individuals (infection intensity). Both phylogenetically specialist and generalist strategies can reach high abundances but through different strategies. Relatively specialist parasites achieve higher abundance than generalists on their principal host species but rely on host relatedness to infect multiple species. For generalist parasites, geographic overlap with and diet similarity to principal host species are better predictors of parasite abundance than host relatedness. This study characterizes constraints on parasite host range and provides novel context to evaluate the potential for parasites to establish in new host species.