International Union for the Study of Social Insects (IUSSI2018), August 5-10, 2018 in Guarujá, Brazil.

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Context-dependent mutualist warns host before lethal sting

Matthew Ryan Boot, Mazie Davis , Matthew Ryan Boot , Stefanie Neupert , Tappey H. Jones , Rachelle M. M. Adams
The Ohio State University; The Ohio State University, USA ; The Ohio State University, USA ; University of Konstanz, Germany ; Virginia Military Institute, USA ; The Ohio State University, USA
Interspecific communication occurs in species with long-term associations to enhance fitness. While some communication may only utilize a single mode or channel, complex systems can also evolve to use multiple channels. Here we investigate the relationship between a fungus-growing ant host, Sericomyrmex amabilis, and their “mercenary” social parasites, Megalomyrmex symmetochus. Mercenary-ants are long-term obligate associates, thus their fitness is reliant on the persistence of their host colony. Surprisingly, host colonies can benefit from this association as parasite workers will wield their venom weaponry against free roaming antagonists. However, because the host and parasite have mixed interests, a signaling system should evolve to deescalate conflict between the two, minimizing fitness costs to both. In the mercenary-ant system, this signal may be achieved through a threat behavior to the host by the parasite. This behavior, gaster-flagging, involves the parasite vibrating a raised abdomen with a suspended drop of alkaloid venom to signal fighting ability or aggressive intent. To understand whether this behavior is a true signal, we performed three phenotypic assays. We describe (1) the host behavior that prompts gaster-flagging, (2) the frequency of escalation resulting in stinging, and (3) potential alkaloid toxicity of parasite venom to the host. Our results indicate that a chemical signaling system has evolved between the host and parasite. If gaster-flagging does not pacify a host, the parasite will follow with lethal stinging. We argue that this series of behaviors is rooted in the fact that it does not benefit the parasite to kill host workers and therefore has evolved as a “warning shot” prompting host ants to “stand down”— benefiting both the signaler and receiver. Although we have focused on chemical signaling, this does not preclude gaster-flagging as a multimodal system of interspecific communication.