Helminth parasite body size scaling with host species size

Host and parasite body size frequently scale positively (often referred to as Harrison's rule), potentially as a result of fundamental constraints on large parasite species simply being morphologically incapable of infecting small host species, or through a variety of alternative proposed mechanisms. Regardless of mechanism, Harrison's rule does have some empirical support. Here, we leverage several trait databases and a large set of host-helminth parasite interactions to gauge support for Harrison's rule in parasitic helminths. We fit generalized additive models from both the host perspective (do larger host species tend to have larger average parasites?) and the parasite perspective (do larger parasite species tend to infect larger average host species?) across a range of host taxa including mammals, birds and fish. We find support for both relationships in mammals, moderate support in fish and very limited support in birds, relative to a null model. Together, we demonstrate positive host-parasite body size scaling in parasitic helminths, suggesting the potential for fundamental morphological constraints on host-parasite interactions, while also highlighting the clear differences in body size scaling across host taxa.