| While immune responses can clear diverse pathogens, such defense comes at a significant cost in terms of energy, protein, time, and damage to host tissue. Hosts must therefore balance these costs and benefits against other crucial life history traits, such as reproductive investment. Striking an optimal balance among such investments, while accounting for other variables such as pathogen abundance, is thought to drive the enormous diversity in immune defenses among wild animals. However, the physiological mechanisms underlying such differential investment in immune responses remain unknown. Revealing such mechanisms is crucial in understanding how and where selection acts to optimize investment in immunity against other costly life history traits. Here, I present evidence of geographical variation in immune defenses among three populations of song sparrows (Melospiza melodia) living along a latitudinal gradient in western North America: one in Southern California, one in Washington, and one in Alaska. Most notably, I studied the widely effective, yet highly costly acute phase immune response, a rapid reaction to infection that includes the induction of fever, antimicrobial peptides, and sickness behaviors such as fever and lethargy. Using behavioral observations, blood sampling, and radiotelemetry in the wild, I showed that differences in the acute phase immune response and two types of antibody defenses among these populations were consistent with predictions based upon both life history theory and presumed pathogen abundance. These differences in the acute phase response persisted when birds from California and Washington were brought into a common laboratory environment. This result suggests that these population differences do not result from immediate environmental conditions alone, and may therefore reflect evolutionary divergence. Additionally, laboratory experiments showed that pro-inflammatory cytokines, immune system signaling molecules, likely play an important role in determining these population differences at the molecular level. This represents a crucial first step in revealing which mechanisms underlie differential investment in immune defenses among populations. |
