Mistletoes have much higher transpiration rates than their hosts. This is the basis for the ecological “compatibility hypothesis” for host choice. The inability of mistletoes to penetrate a host and connect to its vascular system is the competing “immunity hypothesis”. Epi-parasites would have to transpire faster than their host mistletoe, which must transpire faster than its host. Therefore, epi-parasites should be able to parasitise their host's host. I surveyed epi-parasite host choice at a site and found this did not happen. Globally about a quarter of all epi-parasites are obligate. This supports the immunity hypothesis.

中文翻译：

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表寄生槲寄生不会寄生寄主的宿主，这支持了宿主选择的免疫假说

槲寄生的蒸腾速率比寄主高得多。这是宿主选择的生态“相容性假说”的基础。槲寄生无法穿透宿主并连接到其血管系统，这是相互竞争的“免疫假说”。表层寄生虫必须比寄主槲寄生蒸腾得更快，而寄主槲寄生也必须比寄主更快地蒸腾。因此，外寄生虫应该能够寄生在其宿主的宿主上。我调查了一个地点的表外寄生虫宿主选择，发现这并没有发生。全球约四分之一的表外寄生虫是专性寄生虫。这支持了免疫假说。