Immune Response, Not Immune Maintenance, Is Energetically Costly in Wild White‐Footed Mice (Peromyscus leucopus)

Abstract Understanding the cost of immune function is essential for more accurate characterization of energy budgets of animals and better understanding of the role of immunity in the evolution of life‐history strategies. We examined the energetic cost of maintaining a normally functioning immune system and mounting a mild immune response in wild male white‐footed mice (Peromyscus leucopus). To evaluate the cost of maintaining immunocompetence, we compared resting and daily metabolic rates (RMR; DMR) and masses of body organs of mice whose immune systems were suppressed by cyclophosphamide with those of control mice. To evaluate the cost of mounting an immune response, we measured RMR, DMR, and organ masses in mice whose humoral and cell‐mediated immune responses had been stimulated by injections of sheep red blood cells and phytohemagglutinin, respectively. Immunosuppression resulted in a significant reduction in circulating leukocytes, by 225%, but no significant effect on metabolic rates or organ masse...