Migration can allow individuals to escape parasite infection, which can lead to a lower infection probability (prevalence) in a population and/or fewer parasites per individual (intensity). Since individuals with more parasites often have lower survival and/or fecundity, infection intensity shapes the life-history tradeoffs determining when migration is favored as a strategy to escape infection. Yet, most theory relies on susceptible-infected (SI) modeling frameworks, defining individuals as either healthy or infected, ignoring details of infection intensity. Here, we develop a novel modeling approach that captures infection intensity as a spectrum, and ask under what conditions migration evolves as function of how infection intensity changes over time. We show that relative timescales of migration and infection accumulation determine when migration is favored. We also find that population-level heterogeneity in infection intensity can lead to partial migration, where less-infected individuals migrate while more infected individuals remain resident. Our model is one of the first to consider how infection intensity can lead to migration. Our results frame migratory escape in light of infection intensity rather than prevalence, thus demonstrating that decreased infection intensity should be considered a benefit of migration, alongside other typical drivers of migration.

中文翻译：

---

寄生虫强度和迁徙行为的演变

迁移可以让个体逃避寄生虫感染，这会导致种群中感染概率（流行率）和/或每个个体的寄生虫（强度）降低。由于寄生虫较多的个体通常存活率和/或繁殖力较低，因此感染强度会影响生活史权衡，决定何时将迁移作为逃避感染的策略。然而，大多数理论依赖于易感感染 (SI) 建模框架，将个体定义为健康或感染者，而忽略了感染强度的细节。在这里，我们开发了一种新的建模方法，将感染强度捕获为一个频谱，并询问在什么条件下迁移会随着感染强度如何随时间变化而演变。我们表明，迁移和感染积累的相对时间尺度决定了何时有利于迁移。我们还发现，感染强度的人口水平异质性会导致部分迁移，即感染较少的个体迁移，而感染较多的个体仍留在原地。我们的模型是第一个考虑感染强度如何导致迁移的模型。我们的结果根据感染强度而不是流行率来构建迁徙逃逸，因此表明感染强度降低应与其他典型的迁徙驱动因素一起被视为迁徙的好处。我们的模型是第一个考虑感染强度如何导致迁移的模型。我们的结果根据感染强度而不是流行率来构建迁徙逃逸，因此证明感染强度降低应与其他典型的迁徙驱动因素一起被视为迁徙的好处。我们的模型是第一个考虑感染强度如何导致迁移的模型。我们的结果根据感染强度而不是流行率来构建迁徙逃逸，因此证明感染强度降低应与其他典型的迁徙驱动因素一起被视为迁徙的好处。