Figure 1.

(a) Proximate (within-species) and evolutionary (among-species) relationships between immunocompetence and parasite virulence, which is defined as parasite-induced mortality. Studies show that individuals with higher immunocompetence have reduced mortality within species (33–40). Given this benefit, then species that are exposed to higher parasite virulence and, hence, that experience higher parasite-induced mortality should evolve greater immunocompetence. (b) Assuming that immunocompetence measured for juveniles vs. adults reflects levels of parasite-induced mortality experienced, then species with higher adult immunocompetence should evolve larger clutches, whereas species with higher juvenile immunocompetence should evolve smaller clutches under life history theory.