Thermal variability interacts with infection load and the evolution of host-parasite defense

Rising temperatures and unpredictable weather patterns are directly linked to emerging infectious diseases that threaten global biodiversity. Thermal variability affects host-parasite interactions, and understanding how animal hosts respond to temperature variability and parasite infection is paramount for conservation and for predicting zoonotic spillover. Hosts employ two strategies to defend against parasites: resistance (inhibiting or limiting infection) and tolerance (limiting the negative effects of infection). This observation raises a fundamental question: How does thermal variability affect the expression and evolution of resistance versus tolerance? Subsequently, how does the evolution of resistance versus tolerance influence parasite load dynamics? Here, we first review why temperature may differentially affect resistance and tolerance mechanisms, thereby altering host selection and eco-evolutionary disease outcomes. Second, to highlight the importance of these interactions, we present a model that illustrates key potential effects of temperature variability on host defense mechanisms. Our model demonstrates that temperature variability alone could drive lower infection prevalence and loads, but it also selects for host tolerance, ultimately leading to higher net prevalence and loads. We also find widely divergent outcomes depending on how temperature impacts defense strategies. These results highlight key areas for future empirical and theoretical work on the interactions among temperature variability, infection load, and host defense evolution.