The costs of mounting and maintaining an immune response lead to trade-offs with investment in (or maintenance of) other organ systems or functions such as reproduction, as has been observed in several taxa, including birds, insects, and mammals. Given these trade-offs, optimal strategies can be affected by biotic and abiotic conditions; however, how these trade-offs are affected by environmental variability remains unclear. Using a deterministic within-host model of disease dynamics, we analyzed the effect of environmental variability (here, the coefficient of variation in the host’s background survival distribution) on the optimal resolution to a trade-off between investment in reproduction and the immune system. We found that higher variability led to increased investment in the immune system at the cost of decreased early-life reproduction when the immune investment strategy was assumed to be constant through life. Since we manipulated the distribution of lifespans while keeping the mean lifespan constant, our results suggest that even the small probability of living far past the mean lifespan for individuals in highly variable environments has a large effect on the optimal investment strategy. When immune investment was able to change over the age of the organism, the optimal investment strategies were again contingent on the level of environmental variability experienced by individuals. Our results support and extend previous work on optimal life-history theory in variable environments by showing that in the context of ecological immunology, the most fit individuals in constant environments are not the most fit individuals in variable environments. The effects of environmental variability on optimal immune investment are likely to have consequences for other aspects of infectious disease such as transmission dynamics, virulence evolution, and the likelihood of epidemics occurring.

中文翻译：

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环境变异影响生态免疫学的最佳权衡

正如在包括鸟类、昆虫和哺乳动物在内的几种分类群中所观察到的那样，建立和维持免疫反应的成本会导致投资（或维持）其他器官系统或功能（如生殖）的权衡。鉴于这些权衡，最佳策略可能会受到生物和非生物条件的影响；然而，这些权衡如何受到环境可变性的影响仍不清楚。我们使用疾病动力学的确定性宿主内模型，分析了环境变异性（这里是宿主背景生存分布的变异系数）对生殖投资和免疫系统之间权衡的最佳分辨率的影响。我们发现，当假设免疫投资策略在整个生命中保持不变时，更高的可变性会导致对免疫系统的投资增加，但代价是生命早期繁殖减少。由于我们在保持平均寿命不变的同时操纵了寿命分布，因此我们的结果表明，即使是个体在高度可变的环境中活得远远超过平均寿命的小概率，也会对最佳投资策略产生很大影响。当免疫投资能够随着生物体的年龄而改变时，最佳投资策略再次取决于个人所经历的环境可变性水平。我们的结果支持并扩展了先前关于可变环境中最优生命史理论的工作，通过表明在生态免疫学的背景下，在恒定环境中最适合的个体并不是在可变环境中最适合的个体。环境变异对最佳免疫投资的影响可能会对传染病的其他方面产生影响，例如传播动态、毒力进化和流行病发生的可能性。