Centre for Social Evolution, Department of Biology, University of Copenhagen, DK-2100 Copenhagen, Denmark.
Proc Natl Acad Sci U S A. 2013 Sep 24;110(39):15752-7. doi: 10.1073/pnas.1311654110. Epub 2013 Sep 9.
The ants are extraordinary in having evolved many lineages that exploit closely related ant societies as social parasites, but social parasitism by distantly related ants is rare. Here we document the interaction dynamics among a Sericomyrmex fungus-growing ant host, a permanently associated parasitic guest ant of the genus Megalomyrmex, and a raiding agro-predator of the genus Gnamptogenys. We show experimentally that the guest ants protect their host colonies against agro-predator raids using alkaloid venom that is much more potent than the biting defenses of the host ants. Relatively few guest ants are sufficient to kill raiders that invariably exterminate host nests without a cohabiting guest ant colony. We also show that the odor of guest ants discourages raider scouts from recruiting nestmates to host colonies. Our results imply that Sericomyrmex fungus-growers obtain a net benefit from their costly guest ants behaving as a functional soldier caste to meet lethal threats from agro-predator raiders. The fundamentally different life histories of the agro-predators and guest ants appear to facilitate their coexistence in a negative frequency-dependent manner. Because a guest ant colony is committed for life to a single host colony, the guests would harm their own interests by not defending the host that they continue to exploit. This conditional mutualism is analogous to chronic sickle cell anemia enhancing the resistance to malaria and to episodes in human history when mercenary city defenders offered either net benefits or imposed net costs, depending on the level of threat from invading armies.
蚂蚁在进化过程中演化出了许多谱系，这些谱系利用密切相关的蚂蚁社会作为社会寄生虫，但亲缘关系较远的蚂蚁进行社会性寄生则很少见。在这里，我们记录了一种共生真菌蚂蚁宿主、一种永久性共生的巨猛蚁属寄生客蚁以及一种掠夺性的农业捕食者之间的相互作用动态。我们通过实验表明，寄生客蚁使用生物碱毒液来保护其宿主蚁群免受农业捕食者的袭击，这种毒液的效力比宿主蚂蚁的叮咬防御强得多。相对较少的寄生客蚁就足以杀死掠夺者，而掠夺者通常会在没有共生寄生蚁群的情况下消灭宿主巢穴。我们还表明，寄生客蚁的气味会阻止掠夺者侦察兵招募巢友加入宿主蚁群。我们的研究结果表明，共生真菌蚂蚁的宿主从它们昂贵的寄生客蚁身上获得了净收益，这些客蚁充当了功能性兵蚁来应对来自农业捕食者掠夺者的致命威胁。农业捕食者和寄生客蚁的基本不同的生活史似乎以负频率依赖的方式促进了它们的共存。由于一个寄生蚁群一生都要依附于一个单一的宿主蚁群，如果不保护它们继续利用的宿主，寄生客蚁就会损害自己的利益。这种条件互惠类似于慢性镰状细胞贫血增强对疟疾的抵抗力，以及在人类历史上，雇佣兵城市防御者提供净收益或造成净成本，这取决于入侵军队的威胁程度。
