Does clutch size evolve in response to parasites and immunocompetence?

Parasites have been argued to influence clutch size evolution, but past wor k and theory has largely focused on within-species optimization solutions r ather than clearly addressing among-species variation. The effects of paras ites on clutch size variation among species can be complex, however, becaus e different parasites can induce age-specific differences in mortality that can cause clutch size to evolve in different directions. We provide a conc eptual argument that differences in immunocompetence among species should i ntegrate differences in overall levels of parasite-induced mortality to whi ch a species is exposed, We test this assumption and show that mortality ca used by parasites is positively correlated with immunocompetence measured b y cell-mediated measures. Under life history theory, clutch size should inc rease with increased adult mortality and decrease with increased juvenile m ortality. Using immunocompetence as a general assay of parasite-induced mor tality, we tested these predictions by using data for 25 species. We found that clutch size increased strongly with adult immunocompetence, In contras t, clutch size decreased weakly with increased juvenile immunocompetence. B ut, immunocompetence of juveniles may be constrained by selection on adults , and, when we controlled for adult immunocompetence, clutch size decreased with juvenile immunocompetence. Thus, immunocompetence seems to reflect ev olutionary differences in parasite virulence experienced by species, and di fferences in age-specific parasite virulence appears to exert opposite sele ction on clutch size evolution.