Data from: Survival after pathogen exposure in group-living insects: don’t forget the stress of social isolation!

A major cost of group-living is its inherent risk of pathogen infection. To limit this risk, many group-living animals have developed the capability to prophylactically boost their immune system in the presence of group members and/or to mount collective defenses against pathogens. These two phenomena, called density dependent prophylaxis and social immunity, respectively, are often used to explain why, in group-living species, individuals survive better in groups than in isolation. However, this survival difference may also reflect an alternative and often overlooked process: a cost of social isolation on individuals’ capability to fight against infections. Here, we disentangled the effects of density-dependent prophylaxis, social immunity and stress of social isolation on the survival after pathogen exposure in group-living adults of the European earwig Forficula auricularia. By manipulating the presence of group members both before and after pathogen exposure, we demonstrated that the cost of being isolated after infection, but not the benefits of social immunity or density-dependent prophylaxis, explained the survival of females. Specifically, females kept constantly in groups or constantly isolated had higher survival rates than females that were first in groups and then isolated after infection. Our results also showed that this cost of social isolation was absent in males, and that social isolation did not reduce the survival of non-infected individuals. Overall, this study gives a new perspective on the role of pathogens in social evolution, as it suggests that an apparently non-adaptive, personal immune process may promote the maintenance of group-living under pathogenic environments.

群居生活的核心代价之一，是其固有的病原体感染风险。为规避此类风险，诸多群居动物演化出两类应对能力：一是在群居同伴存在时预防性强化自身免疫系统，二是针对病原体发起集体防御。这两种现象分别被称为密度依赖性预防（density-dependent prophylaxis）与社会免疫（social immunity），二者常被用于解释群居物种的个体为何在群体中的存活率高于单独存活时的存活率。然而，这一存活率差异或许也可归因于另一种常被忽视的过程：社交隔离对个体抗感染能力所造成的代价。本研究以欧洲球螋（Forficula auricularia）的群居成虫为研究对象，通过操控病原体暴露前后的同伴存在情况，厘清了密度依赖性预防、社会免疫以及社交隔离压力对其病原体暴露后存活率的影响。研究证实，能够解释雌性个体存活率差异的，是感染后被隔离的代价，而非社会免疫的益处或密度依赖性预防的作用。具体而言，始终处于群体中或始终被隔离的雌性个体，其存活率均高于先处于群体中、随后在感染后被隔离的雌性个体。研究结果同时显示，社交隔离的代价仅存在于雌性个体中，雄性个体并未受此影响；且社交隔离并未降低未感染个体的存活率。总体而言，本研究为病原体在社会演化中的作用提供了全新视角：研究表明，一种看似非适应性的个体免疫过程，或可促进群居生活在病原环境下的维持。