The R code used to analyse the data from In an arms race between host and parasite, a lungworm's ability to infect a toad is determined by host susceptibility not parasite preference

Evolutionary arms races can alter both parasite infectivity and host resistance, and it is difficult to separate the effects of these twin determinants of infection outcomes. We used a co-introduced, invasive host–parasite system (the lungworm Rhabdias pseudosphaerocephala and cane toads Rhinella marina), where rapid adaptation and dispersal have led to population differences in infection resistance. We quantified behavioural responses of parasite larvae to skin-chemical cues of toads from different invasive populations, and rates at which juvenile hosts became infected following standardized exposure to lungworms. Chemical cues from toad skin altered host-seeking behaviour by parasites, similarly among populations. The number of infection attempts (parasite larvae entering the host's body) also did not differ between populations, but rates of successful infection (establishment of adult worm in host lungs) were higher for range-edge toads than for range-core conspecifics. Thus, lower resistance to parasite infection in range-edge juvenile toads appears to be due to less effective immune defences of the host rather than differential behavioural responses of the parasite. In this ongoing host–parasite arms race, changing outcomes appear to be driven by shifts in host immunocompetence.进化军备竞赛可同时改变寄生虫的感染能力与宿主的抗性，且难以区分这两大感染结局决定因素的效应。本研究采用共同引入的入侵性宿主-寄生虫体系（肺线虫*Rhabdias pseudosphaerocephala*与蔗蟾*Rhinella marina*），该体系因快速适应与扩散演化出了感染抗性的种群差异。我们量化了寄生虫幼虫对不同入侵种群蔗蟾皮肤化学信号的行为响应，以及标准化暴露于肺线虫后幼龄宿主的感染率。蔗蟾皮肤释放的化学信号可改变寄生虫的觅宿主行为，且不同种群间该效应并无显著差异。不同种群间的感染尝试次数（即进入宿主体内的寄生虫幼虫数）同样无显著差异，但分布边缘种群蔗蟾的成功感染率（即成虫在宿主肺内定植的比例）显著高于分布核心种群的同种个体。由此可见，分布边缘幼龄蔗蟾对寄生虫感染的抗性更低，这一现象并非源于寄生虫的行为响应差异，而是缘于宿主自身免疫防御效能的减弱。在这场持续进行的宿主-寄生虫进化军备竞赛中，感染结局的变化似乎由宿主免疫能力的改变所驱动。