# The impact of sterility-mortality tolerance and recovery-transmission trade-offs on host–parasite coevolution

## TL;DR

This study explores how host and parasite trade-offs influence their coevolution, revealing that these trade-offs can lead to complex evolutionary outcomes like branching and population cycles.

## Contribution

The paper introduces a coevolutionary model incorporating both host sterility-mortality and parasite recovery-transmission trade-offs, which has not been explored in prior models.

## Key findings

- Evolutionary branching in the host can lead to branching in the parasite population.
- Population cycles can prevent coexisting strains from reaching extreme trait values.
- Coevolution reduces parasite recovery rate as crowding increases.

## Abstract

Understanding the coevolutionary dynamics of hosts and their parasites remains a major focus of much theoretical literature. Despite empirical evidence supporting the presence of sterility-mortality tolerance trade-offs in hosts and recovery-transmission trade-offs in parasites, none of the current models have explored the potential outcomes when both trade-offs are considered within a coevolutionary framework. In this study, we consider a model where the host evolves sterility tolerance at the cost of increased mortality and the parasite evolves higher transmission rate at the cost of increased recovery rate (reduced infection duration), and use adaptive dynamics to predict the coevolutionary outcomes under such trade-off assumptions. We particularly aim to understand how our coevolutionary dynamics compare with single species evolutionary models. We find that evolutionary branching in the host can drive the parasite population to branch, but that cycles in the population dynamics can prevent the coexisting strains from reaching their extremes. We also find that varying crowding does not impact the recovery rate when only the parasite evolves, yet coevolution reduces recovery as crowding intensifies. We conclude by discussing how different host and parasite trade-offs shape coevolutionary outcomes, underscoring the pivotal role of trade-offs in coevolution.