The coevolution of parasite virulence and host investment in constitutive and induced defence.

The article focuses on a theoretical analysis of the coevolution between parasite virulence and host investment in constitutive (always active) and induced (activated upon infection) immune defenses, incorporating eco-evolutionary feedbacks and population dynamics. Using an adaptive dynamics framework, the study models how host traits—constitutive defense reducing susceptibility and incurring reproductive costs, and induced defense increasing recovery but causing immunopathology—and parasite growth strategies evolve together under varying host mortality, competition, and parasite effects on host fertility (including castration). Key findings include that coevolution alters predictions compared to host-only evolution models, notably showing that whether parasites castrate hosts critically influences host immune investment strategies: castrating parasites select for host strategies minimizing reproductive costs by modulating constitutive and induced defenses differently than noncastrating parasites. The results align with and extend pace-of-life hypotheses by demonstrating that shorter-lived hosts invest more in constitutive defense, while longer-lived hosts favor induced defense, but these patterns depend on parasite impact on host reproduction and ecological context. This framework highlights the importance of considering host-parasite coevolution and epidemiological feedbacks to understand the diversity of immune strategies and parasite virulence observed in nature, and it calls for empirical testing and further modeling including spatial structure and multiple infections.