No Single Association of Constitutive Immune Defenses with Fat Condition or Protein Content

Wildlife populations increasingly encounter unpredictable environmental conditions and reduced resource availability, influencing energy and protein available for immune defenses. According to resource-constraint hypotheses, central to ecoimmunology, organisms with more resources should have the ability to invest more into immune defenses. We investigated whether fat condition or protein content influenced three constitutive immune defenses-bacterial killing ability, hemolytic complement activity, and total antioxidant capacity-in mule deer (Odocoileus hemionus). We used hundreds of samples from individuals captured during 2014-2021 and generalized linear models fit in a Bayesian framework to determine the probability of the direction of a relationship. Bacteria-killing ability had greatest probability (0.87) of a positive relationship with fat condition, and a 0.84 probability of a negative relationship with protein. In contrast, hemolytic complement activity had a 0.93 probability of being negatively associated with protein content and antioxidant capacity had a 0.94 probability of a positive association with protein content. Neither had a clear directional relationship with fat condition. Still, absolute effect sizes were small, and immune differences between a deer in poor nutritional condition and one in good nutritional condition were minor. The small effect sizes were contrary to the assumptions of many studies and might arise because the immune defenses in this study were constitutive, highly regulated by negative feedback cycles, and had low energy and resource costs. These nuanced results, however, support different, albeit small, effects of protein and fat reserves on immune defenses. Our results highlight the importance of assessing the assumed relationship between immune defenses and nutritional reserves in each population.