Immune challenge affects basal metabolic activity in wintering great tits

The costs of exploiting an organism's immune function are expected to form the basis of man) life-history trade-offs.. However, there has been debate al,out whether such costs can he paid in energetic and nutritional terms. W e addressed this question in a study of wintering, free-living, male great tits by injecting them with a novel, non-pathogenic antigen (sheep red bloo d cells) and measuring the changes in their basal metabolic rates and vario us condition indices subsequent to immune challenge. The experiment showed that activation of the immune system altered the metabolic activity and pro file of immune cells in birds during the week subsequent to antigen injecti on: individuals mounting an immune response had nearly 9% higher basal meta bolic rates, 8% lower plasma albumin levels and 37% higher heterophile-to-l ymphocyte ratios (leucocytic stress indices) than sham-injected control bir ds. The) also lost nearly 3% (0.5 g) of their body mass subsequent to the i mmune challenge. Individuals that mounted stronger antibody responses lost more mass during the immune challenge. These results suggest that energetic expenditures to immune response may have a non-trivial impact upon an indi vidual's condition.