Data from: Immune response costs are associated with changes in resource acquisition and not resource reallocation

1. Evolutionary ecologists frequently argue that parasite defence is costly because resources must be reallocated from other life-history traits to fuel the immune response. However, this hypothesis is rarely explicitly tested. An alternative possibility is that immune responses impair an organism's ability to acquire the resources it needs to support metabolism. Here, we disentangle these opposing hypotheses for why the activation costs of parasite resistance arise. 2. We studied fecundity costs associated with immune stimulation in Drosophila melanogaster. Then, by measuring correlated changes in metabolic rate, food consumption and body weight, we assessed whether responses were consistent with immunity costs originating from altered resource allocation or from impaired resource acquisition. 3. Microbial injection resulted in a 45% fecundity decrease. It also triggered a mean decline in metabolic rate of 6% and a mean reduction in food intake of 31%; body weight was unaffected. Metabolic rate downregulation was greater in males than in females, whereas declines in food ingestion were of similar magnitude in both sexes. These physiological shifts did not depend on whether microbial challenges were alive or dead, thus they resulted from immune system activation not pathogenesis. 4. These costs of immune activation are significant for individuals that successfully resist infection and might also occur in other situations when immune responses are upregulated without infection. 5. Whilst we found significant activation costs of resistance, our data provide no compelling evidence for the popularly argued hypothesis that immune deployment is costly because of reallocation of energetic resources to the immune system. Instead, reduction in resource acquisition due to ‘infection-induced anorexia’ may be the principal driver of metabolic changes and fecundity costs resulting from immune response activation.

1. 进化生态学家普遍认为，宿主对寄生虫的防御存在成本，因为机体需要将资源从其他生活史性状重新分配，以支撑免疫应答。然而，这一假说极少得到明确检验。另一种可能的解释是，免疫应答会损害有机体获取维持代谢所需资源的能力。本研究旨在厘清导致寄生虫抗性激活成本产生的两类对立假说。
2. 我们以黑腹果蝇（Drosophila melanogaster）为研究对象，探究了免疫刺激引发的繁殖成本。随后，通过测量代谢速率、食物摄入量与体重的相关性变化，我们评估了免疫成本究竟源于资源分配改变，还是资源获取能力受损。
3. 微生物注射导致宿主繁殖力下降45%，同时使代谢速率平均降低6%、食物摄入量平均减少31%，但体重未受影响。雄性个体的代谢速率下调幅度大于雌性，而两性的食物摄取下降幅度则无显著差异。上述生理变化与微生物挑战物的死活无关，表明其源于免疫系统激活而非致病过程。
4. 对于成功抵御感染的个体而言，免疫激活的这类成本影响显著；在未发生感染但免疫应答被上调的其他场景中，这类成本也可能出现。
5. 尽管我们发现了抗性激活的显著成本，但现有数据并未为广被支持的假说提供有力证据——该假说认为免疫动用的成本源于能量资源向免疫系统的重新分配。相反，由‘感染诱导的厌食症’导致的资源获取能力下降，可能是免疫应答激活引发代谢变化与繁殖成本的主要驱动因素。