Data from: Invasion history alters the behavioural consequences of immune system activation in cane toads.

1. Acute activation of the immune system often initiates a suite of behavioural changes. These ‘sickness behaviours’—involving lethargy and decreased activity—may be particularly costly on invasion fronts, where evolutionary pressures on dispersal favour individuals that move large distances. 2. We used a combination of field and laboratory studies to compare sickness behaviours of cane toads from populations differing in invasion history. To do this we stimulated immune system activation by injecting lipopolysaccharide (LPS) to mimic bacterial infection. 3. We predicted that LPS would result in less severe sickness behaviour in toads from invasion-front populations because they had undergone selection for rapid and sustained dispersal (activities in conflict with lethargy and decreased activity). 3. Contrary to our prediction, LPS injection caused a greater reduction in dispersal-relevant traits in invasion-front individuals than in conspecifics from the range core. 4. Our data suggest that the rapid invasion of cane toads through tropical Australia has seen an evolutionary shift in the magnitude of sickness behaviour elicited by pathogen infection. The increased sickness behaviour among invasion-front toads suggests a shift away from pathogen tolerance (seen in range-core populations) towards resistance to pathogen attack. But as a consequence, when pathogens do become successfully established, toads from invasion-front populations may have less capacity to tolerate their ill-effects.

1. 免疫系统的急性激活通常会引发一系列行为变化。这类被称为“疾病行为（sickness behaviours）”的反应——表现为嗜睡与活动量减少——在入侵前沿（invasion fronts）环境中往往代价尤为高昂：在此类环境中，针对扩散（dispersal）的进化压力更倾向于选择长距离移动的个体。
2. 本研究结合野外调查与实验室实验，对比了具备不同入侵历史的蔗蟾（cane toads）种群的疾病行为表现。具体而言，我们通过注射脂多糖（lipopolysaccharide，LPS）以模拟细菌感染，以此激活免疫系统。
3. 我们曾预测，入侵前沿种群的蔗蟾在经LPS处理后，其疾病行为严重程度会更低——因为这些种群经历了针对快速且持续扩散的选择压力，而该选择压力与嗜睡、活动减少的行为存在冲突。
3. 但与我们的预测相反，注射LPS后，入侵前沿个体在扩散相关性状上的下降幅度，显著大于分布范围核心区（range core）的同种个体（conspecifics）。
4. 本研究数据表明，在蔗蟾快速入侵澳大利亚热带区域的过程中，病原体感染所诱导的疾病行为强度已发生进化偏移。入侵前沿种群蔗蟾的疾病行为增强，意味着其策略已从分布核心区种群所具备的病原体耐受（pathogen tolerance）转向病原体攻击抗性。但随之而来的代价是，当病原体成功定植后，入侵前沿种群的蔗蟾对其有害效应的耐受能力可能有所下降。