Individual and collective decision-making during nest site selection by the ant Leptothorax albipennis

Social insect colonies possess remarkable abilities to select the best amon g several courses of action. In populous societies with highly efficient re cruitment behaviour, decision-making is distributed across many individuals , each acting on limited local information with appropriate decision rules. To investigate the degree to which small societies with less efficient rec ruitment can also employ distributed decision-making, we studied nest site selection in Leptothorax albipennis. Colonies were found to make sophistica ted choices, taking into account not only the intrinsic qualities of each s ite, but also its value relative to the available options. In choices betwe en two sites, individual ants were able to visit both sites. compare them a nd choose the better one. However, most ants encountered only one site in t he course of an emigration. These poorly informed ants also contributed to the colony's decision, because their probability of initiating recruitment to a site depended on its quality. This led to shorter latencies between di scovery and recruitment to a superior site, and so created greater amplific ation via positive feedback of the population at the better site. In short, these small colonies make use of a distributed mechanism of information pr ocessing, but also take advantage of direct decision-making by well-informe d individuals. The latter feature may in part stem from the limitations of their social structure, but may also reflect the stringent demand for unani mous decisions by house-hunting colonies of any size.