A perplexing quirk of many host-parasite interactions is that hosts typically reduce their food intake when infected, or merely exposed to, infectious agents. Synthesizing recent studies, we find that the occurrence of this behavior is well-documented. Yet, the ecological factors that influence it, how it affects disease outcomes, or why it evolved remains poorly resolved. Addressing this gap carries important implications for both basic and applied biology; this behavior functions as a ‘master switch’ that governs within-host energetics, physiology, and immune functions that are crucial to host defense mechanisms and the pool of resources available for parasites and pathogens to steal. In some cases, these energetic and physiological changes can function in host defense (a type of ‘self-medication’). However, parasites can also manipulate this behavior to enhance transmission. Thus, identifying contexts where not eating benefits the host, parasite, both, or neither remains challenging.
Results/Conclusions
We use evolutionary epidemiology to reveal the dietary contexts where reduced food intake functions as a host or parasite adaptation, and to examine how this behavior shapes epidemiological and evolutionary outcomes. We show that, depending on dietary contexts (i.e., ratios of specific macro-nutrients), changes in resource acquisition can select for higher or lower virulence. Importantly, we also find that changes in resource acquisition can have disparate effects at the individual- vs. population-level. Accounting for this resource-driven tug-of-war between hosts and parasites can, therefore, reveal novel and unexpected outcomes that are not captured by classical evolutionary theory. Moreover, this data-theory integration proves essential to pinpointing the key evidence needed to rigorously address this evolutionary mystery.