Grazing mammals, ungulates, pose two evolutionary puzzles as helminth hosts. First, why do some helminths infect intermediate hosts prior to infecting ungulates, given that grazers could directly consume propagules on vegetation? Second, ungulates are large and long-lived, so why are they occasionally intermediate instead of definitive hosts, as in taeniid cestodes? We comprehensively surveyed helminth life cycles and transmission involving ungulates. We identified six transmission routes and found that ungulate helminth parasitism has evolved some 25 times. Direct egg transmission to ungulates is rare, and we suggest this is due to a transmission barrier caused by ungulate faecal avoidance. Our survey confirmed that ungulates are almost always definitive hosts, and we discuss the exceptional cases when they are not.

中文翻译：

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蠕虫蠕动传播和两个进化难题。

作为蠕虫宿主，放牧哺乳动物，有蹄类动物构成了两个进化难题。首先，鉴于放牧者可以直接食用植物上的繁殖体，为什么有些蠕虫在感染有蹄类动物之前先感染中间宿主？其次，有蹄类动物又大又长寿，那么为什么它们有时像中间的eni足类动物那样居于中间而不是确定的宿主呢？我们全面调查了蠕虫的生命周期和涉及有蹄类动物的传播。我们确定了六种传播途径，发现有蹄类蠕虫寄生虫已经进化了约25次。鸡蛋直接传播到有蹄类动物的情况很少，我们建议这是由于避免有蹄类动物的粪便引起的传播障碍。我们的调查证实，有蹄类动物几乎总是确定的宿主，我们讨论了非有蹄类动物的例外情况。