Nest relocation in the slavemaking ants Formica subintegra and Formica pergandei: a response to host nest availability that increases raiding success

Social insects typically occupy spatially fixed nests which may thus constrain their mobility. Nevertheless, colony movements are a frequent component of the life cycle of many social insects, particularly ants. Nest relocation in ants may be driven by a variety of factors, including nest deterioration, seasonality, disturbances, changes in microclimate, and local depletion of resources. The colony movements of slavemaking ants have been noted anecdotally, and in recent studies such relocations were primarily attributed to nest deterioration or shifts to overwintering locations. In this study we explore nest relocations in large colonies of formicine slavemakers which occupy stable and persistent earthen nest mounds. We investigate the hypothesis that colony relocations of these slavemakers are best explained by efforts to improve raiding success by seeking areas of higher host availability. Five summers of monitoring the raiding behavior of 11–14 colonies of the slavemakers Formica subintegra and Formica pergandei revealed relatively frequent nest relocations: of 14 colonies that have been tracked for at least three of 5 years, all but one moved at least once by invading existing host nests. Movements tended to occur in the middle of the raiding season and were typically followed by continued raiding of nearby host colonies. Spatial patterns of movements suggest that their purpose is to gain access to more host colonies to raid: the distance moved is typically farther than the mean raiding distance before the move, which may indicate an effort to escape their local neighborhood. Furthermore, the mean distance of raids after relocation is shorter than the distance before relocation. For many slavemaking ant colonies, particularly those on the verge of relocating, raiding distance increased as the raiding season progressed. In addition, movements tended to be toward areas of higher local host density. Nest relocation is likely an important component of the ecology of slavemaking ants that contributes to the dynamic nature of their interaction with the host ant population.