Physiological mechanisms mediating costs of immune responses: what can we learn from studies of birds?

Activating the immune system has associated fitness costs, both immediate costs in the form of reduced current reproduction and long-term costs in the form of reduced life span and future reproduction. This indicates that immune system activation can be an important agent in life history trade-offs. In this review, we evaluate the importance of four currencies generally considered as potential mediators of the costs of immune responses in ecological studies: (1) energetic costs, (2) nutrient costs, (3) autoimmunity and (4) oxidative stress, which may be responsible for these trade-offs. A meta-analysis revealed significant elevation of energy consumption during an immune response; however, the magnitude of this energetic cost was only 5–15%. In a direct comparison using similar immune system activation in tits, energetic savings in terms of lowered feeding rate was seven times higher than energetic costs of mounting an immune response. These results do not support the hypothesis that energy is the key proximate currency mediating the costs of immunity. Nutrient savings from immunosuppression seem to be even less beneficial as this constitutes only a minor part of the daily nutrient turnover in the body. In our view, there are some indications that oxidative stress can be an important currency that could mediate both short-term and long-term costs of immune system activation, although direct evidence is so far limited. The importance of autoimmune responses is at this point hard to evaluate owing to limited empirical studies in wild animals.