RESEARCH ARTICLE Investment in Constitutive Immune Function by North American Elk Experimentally Maintained at Two Different Population Densities

Natural selection favors individuals that respond with effective and appropriate immune re-sponses to macro or microparasites. Animals living in populations close to ecological carry-ing capacity experience increased intraspecific competition, and as a result are often in poor nutritional condition. Nutritional condition, in turn, affects the amount of endogenous resources that are available for investment in immune function. Our objective was to under-stand the relationship between immune function and density dependence mediated by trade-offs between immune function, nutritional condition, and reproduction. To determine how immune function relates to density-dependent processes, we quantified bacteria killing ability, hemolytic-complement activity, and nutritional condition of North American elk (Cer-vus elaphus) from populations maintained at experimentally high- and low-population densi-ties. When compared with elk from the low-density population, those from the high-density population had higher bacteria killing ability and hemolytic-complement activity despite their lower nutritional condition. Similarly, when compared with adults, yearlings had higher bac-teria killing ability, higher hemolytic-complement activity, and lower nutritional condition