Aim: Interspecific variation among metazoans often follows a latitudinal pattern, with species at higher latitudes being larger-bodied than related species from lower latitudes (Bergmann’s rule). For parasitic species, body sizes within any higher taxon often correlate with the body sizes of their hosts (Harrison’s rule). Whether temperature-driven latitudinal effects or host-driven resource constraints act independently or additively to shape interspecific variation in parasite body sizes remains unknown. We use a comparative approach to test the effects of latitude and host body size on parasite body sizes in two taxa of parasitic worms showing convergent life cycles.

Location: Global.

Time period: Contemporary.

Major taxa studied: Hairworms (Nematomorpha) and mermithids (Nematoda) parasitic in arthropods.

Methods: With 223 records for mermithids and 258 for nematomorphs worldwide, we used linear mixed effects models to test the effects of latitude and host body size on parasite length, intraspecific length variation, parasite egg diameter, and variation in egg diameter. Further, we modelled parasite length with local mean annual temperature as predictor instead of latitude, as a direct test of underlying mechanisms. All models took into account host and parasite taxonomic structure within the datasets.

Results: For both taxa, host body size was clearly the main determinant of parasite body length, with neither latitude nor local temperature (annual mean or range) having an effect. No predictor affected intraspecific length variation, whereas egg diameter was positively associated with parasite length and variation in egg diameter was negatively associated with latitude.

Main conclusions: Our results support a strong role for host traits in shaping the evolution of parasite body sizes (Harrison’s rule), but no role for latitude (Bergmann’s rule), even though these parasites infect ectothermic hosts. At a mechanistic level, the evolutionary driving force of external temperature on parasite physiology seems to be eclipsed by the availability of resources from the host.

Data compiled from primary literature sources. See article for details.

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