Consensus decision making in the ant Myrmecina nipponica: house-hunters combine pheromone trails with quorum responses

Consensus decisions enable group-living organisms to coordinate collective tasks without the need for complex cognitive abilities or centralized leadership. Systems of nonlinear positive feedback, such as quorum responses, are thought to be integral to consensus decision processes that seek to optimize decision speed and accuracy while maintaining group cohesion, requirements typical of the house-hunting process in social insects. Studies of nest site selection in honeybees, Apis mellifera, and Temnothorax ants have revealed strong similarities in the manner in which consensus decisions are achieved, and imply a central role for quorum responses, but treatments of other species are scarce. I investigated the process of consensus decision making during colony relocation in the small colony ant Myrmecina nipponica. Colonies of individually marked ants were forced to relocate in experimental laboratory arenas, and given a choice of one or two alternative new sites. This species differs from other well-studied systems in using pheromone trails to navigate between old and new sites, and does not use obvious means of recruitment such as tandem running or adult transport. A quorum threshold apparently governed the switch to brood transport, which marked the final phase of the relocation. Furthermore, there was a strong relationship between inferred quorum threshold and colony size. A demonstration of the use of a quorum rule in M. nipponica would underline the central role of quorum responses in consensus decision making, despite interspecific variation in colony size and differing modes of recruitment and navigation.