Pathogen prevalence modulates medication behavior in ant Formica fusca

Ants face unique challenges regarding pathogens, as the sociality which has allowed them to form large and complex colonies also raises the potential for transmission of disease within these colonies. To cope with the threat of pathogens, ants have developed a variety of behavioral and physiological strategies. One of these strategies is self-medication, in which animals use biologically active compounds to combat pathogens in a way which would be harmful in the absence of them. Formica fusca ants are to date the only species of ants proven to successfully self-medicate against an active infection caused by a fungal pathogen by supplementing their diet with food containing hydrogen peroxide. Here, we build on that research by investigating how the prevalence of disease in colonies of F. fusca affects the strength of the self-medication response. We exposed either half of the workers of each colony or all of them to a fungal pathogen and offered them different combinations of diets. We see that workers of F. fusca engage in self-medication behavior even if exposed to a low lethal dose of a pathogen, and that the strength of that response is affected by the prevalence of the disease in the colonies. We also saw that the infection status of the individual foragers did not significantly affect their decision to forage on either control food or medicinal food as uninfected workers were also foraging on hydrogen peroxide food, which opens up the possibility of kin medication in partially infected colonies. Our results further affirm the ability of ants to self-medicate against fungal pathogens, shed new light on plasticity of self-medication and raise new questions to be investigated on the role self-medication has in social immunity.

蚂蚁在应对病原体时面临独特挑战：尽管社会性使其得以构建庞大且复杂的蚁群，但这也提升了病原体在群体内部传播的风险。为应对病原体威胁，蚂蚁演化出了多种行为与生理层面的防御策略。其中一类策略便是自我药疗（self-medication）：动物借助具有生物活性的化合物对抗病原体，而这类化合物在无病原体存在的情况下，可能对自身造成危害。截至目前，褐林蚁（Formica fusca）是唯一被证实可通过补充含过氧化氢的食物，成功对真菌病原体引发的活动性感染实施自我药疗的蚂蚁物种。本研究以此前的研究为基础，探究褐林蚁蚁群的疾病流行率如何影响其自我药疗反应的强度。我们将每个蚁群中半数或全部工蚁暴露于真菌病原体环境，并为其提供不同配方的食物组合。研究结果显示，即便暴露于低致死剂量的病原体环境，褐林蚁工蚁仍会表现出自我药疗行为，且该反应的强度会受到蚁群疾病流行率的影响。我们还发现，单个觅食蚁的感染状态并不会显著影响其选择取食对照食物还是药用食物——未感染的工蚁同样会取食含过氧化氢的食物，这表明在部分感染的蚁群中，存在通过亲缘个体实施药疗的可能性。本研究结果进一步证实了蚂蚁对抗真菌病原体的自我药疗能力，为自我药疗的可塑性提供了新的认知，并为探究自我药疗在社会免疫（social immunity）中的作用提出了新的研究方向。