CONDITION, DISEASE AND IMMUNE DEFENSE

Life-history traits and secondary sexual characters often demonstrate condition-dependence, and reproductive success thus ultimately appears to be determined by condition. Here we test the hypothesis that anti- parasite defence is condition-dependent and thus ultimately limits fit ness. Animal hosts defend themselves against parasites by an efficient immune system that changes its activity level depending on level of i nfection. Since immune function is costly, as demonstrated by several field studies, we predicted that large immune defence organs should be maintained when the costs of an elevated immune response were reduced , or when the benefits were increased. Hence, the size of immune defen ce organs was predicted to increase in response to disease due to incr eased benefits of investment in immune function, and the; size was pre dicted to increase in response to high body condition because of reduc ed costs of investment in immune function. A comparative study of bird s demonstrated that the size of the spleen was significantly increased among individuals suffering from parasitic infections and signs of di sease as compared to healthy individuals. Furthermore, we found eviden ce for a positive association between spleen size and body condition. These findings are consistent with the hypothesised cost of immune fun ction and hence a cost of anti-parasite defence.