Non-lethal fungal infection could reduce aggression towards strangers in ants

Many parasites interfere with the behaviour of their hosts. In social animals, such as ants, parasitic interference can cause changes on the level of the individual and also on the level of the society. The ant-parasitic fungus Rickia wasmannii influences the behaviour of Myrmica ants by expanding the host's nestmate recognition template, thereby increasing the chance of the colony accepting infected non-nestmates. Infected ants consistently show an increase of the alkane tricosane (n-C23) in their cuticular hydrocarbon profiles. Although experimental application of single compounds often elicits aggression towards manipulated ants, we hypothesized that the increase of n-C23 might underlie the facilitated acceptance of infected non-nestmates. To test this, we mimicked fungal infection in M. scabrinodis by applying synthetic n-C23 to fresh ant corpses and observed the reaction of infected and uninfected workers to control and manipulated corpses. Infected ants appeared to be more peaceful towards infected but not uninfected non-nestmates. Adding n-C23 to uninfected corpses resulted in reduced aggression in uninfected ants. This supports the hypothesis that n-C23 acts as a 'pacifying' signal. Our study indicates that parasitic interference with the nestmate discrimination of host ants might eventually change colony structure by increasing genetic heterogeneity in infected colonies.

诸多寄生虫会干预宿主的行为。在群居动物（如蚂蚁）中，寄生干预可同时在个体层面与社会层面引发改变。寄生蚂蚁的真菌Rickia wasmannii可通过扩大宿主的巢伴识别模板，影响红蚁属（Myrmica）蚂蚁的行为，进而提升蚁群接纳受感染非巢伴的概率。受感染蚂蚁的表皮碳氢化合物谱中，烷烃类物质二十三烷（n-C23）的含量持续升高。尽管单独施加单种化合物的实验常会引发针对处理过蚂蚁的攻击行为，但我们推测，n-C23的含量升高或许是受感染非巢伴更易被接纳的核心原因。为验证这一假说，我们通过向新鲜蚂蚁尸体涂抹合成n-C23，模拟糙红蚁（Myrmica scabrinodis）的真菌感染状态，并观察受感染与未受感染的工蚁对对照组尸体与处理组尸体的反应。受感染工蚁对受感染的非巢伴表现得更为亲和，而非未受感染的非巢伴。向未受感染的蚂蚁尸体添加n-C23，可降低未受感染工蚁的攻击行为。这一结果支持了“n-C23可作为安抚信号”的假说。本研究表明，寄生干预对宿主蚂蚁巢伴识别能力的影响，最终可能通过提升受感染蚁群的遗传异质性，改变蚁群的种群结构。