Nematode parasite species richness and the evolution of spleen size in birds

Abstract

Some of the interspecific variation in spleen sizes among bird species can be explained by interspecific variation in the proportion of birds infected by parasitic nematodes. Because prevalences of parasitic infections vary considerably in space and time within a host species, other variables may provide better measures of the selective pressure exerted by parasites on their hosts. For instance, the number of parasite species (species richness) exploiting a host population or species provides a more reliable index of the pressure exerted by parasites across generations. Among bird species, relative spleen size correlated positively with the species richness of nematode parasites exploiting a host species. This relationship was found after correcting for avian body mass (g), avian phylogeny, and sampling effort. A possible trade-off between investment in resistance against parasites and investment in reproduction was highlighted by a negative relationship between relative spleen size and relative testis mass. Parasitic nematodes could influence the trade-off, increasingly favoring investment in resistance as their species richness increases. The results of this comparative analysis and of previous ones suggest a causative role for parasitic nematodes in the evolution of avian spleen size.