Ecological immunology: life history trade-offs and immune defense in birds

There has been considerable recent interest in the effects of life-history decisions on immunocompetence in birds. If immunocompetence is limited by available resources, then trade-offs between investment in life-history components and investment in immunocompetence could be important in determining optimal life-history traits. For this to be true: (1) immunocompetence must be limited by resources, (2) investment in life-history components must be negatively correlated with immunocompetence, and (3) immunocompetence must be positively correlated with fitness. To gather such empirical data, ecologists need to be able to measure immunocompetence. We review techniques used to measure immunocompetence and how they are applied by ecologists. We also consider the components of the immune system that constitute immunocompetence and evaluate the possible consequences of measuring immunocompetence in different ways. We then review the empirical evidence for life-history trade-offs involving immune defense. We conclude that there is some evidence suggesting that immunocompetence is limited by resources and that investment in certain life-history components reduces immunocompetence. However, the evidence that immunocompetence is related to fitness is circumstantial at present, although consistent with the hypothesis that immunocompetence and fitness are positively correlated. We argue that future work needs to examine the fitness effects of variation in immunocompetence and suggest that artificial selection experiments offer a potentially important tool for addressing this issue.