Is cell-mediated immunity related to the evolution

53 According to life-history theory, the development of immune function should be balanced, through evol54 utionary optimization of the allocation of resources to reproduction, and mechanisms that promote sur55 vival. We investigated interspecific variability in cell-mediated immune response (CMI), as measured by 56 the phytohaemagglutinin (PHA) assay, in relation to clutch size, longevity, and other life-history traits in 57 50 species of birds. CMI exhibited significant repeatability within species, and PHA responses in chicks 58 were consistently stronger than in adults. Univariate tests showed a variety of significant relationships 59 between the CMI of both chicks and adults with respect to size, development period, and lifespan, but 60 not clutch size or prevalence of blood parasites in adults. Multivariate analyses confirmed these patterns 61 but independent variables were too highly correlated to isolate unique influences on CMI. The positive 62 relationship of chick CMI to nestling period is further complicated by a parallel relationship of chick CMI 63 to the age at testing. However, multivariate analysis showed that chick CMI varies uniquely with length 64 of the nestling period. Adult CMI was associated with a strong life-history axis of body size, development 65 rate, and longevity. Therefore, adult CMI may be associated with prevention and repair mechanisms 66 related to long lifespan, but it also may be allometrically related to body size through other pathways. 67 Neither chick CMI nor adult CMI was related to clutch size, contradicting previous results linking para68 site-related mortality to CMI and the evolution of clutch size (reproductive investment) in birds.