Community context and specialization influence coevolution between a slavemaking ant and its hosts

The dynamics of host-parasite coevolution are thought to be influenced not only by inherent parameters, but also by their community context. Here we report strong differences in the degree of specialization of a social parasite at different geographic sites. Furthermore, we provide the first empirical evidence for a deceleration of the coevolutionary arms race caused by the inclusion of a second host species into a parasite-host association. We compare two communities, each including the North American slavemaking ant Protomognathus americanus and two Leptothorax host species, with similar parasite and host densities. At the first site, the minority host species occurred at frequencies too low to sustain the parasite, whereas at the second location, both hosts constituted an exploitable resource. Thus, only in the latter community does the parasite have the options to expand its niche or to alternate between the available host species. During slave raids, which represent the crucial parasite-host encounter, hosts and slavemakers from the second site were less efficient at defending and raiding, respectively. Thus, we demonstrate a higher degree of reciprocal adaptation at the location where the parasite specializes on a single host, indicating a more advanced stage of the arms race.