Sequential load transport in Anoplolepis gracilipes (Hymenoptera: Formicidae) : a novel case of non-cooperation

Social insects show sequential cooperation in the foragingprocess, in which workers form a transport chain and transfer a load from one to another. A "bucket brigade" is one form of sequential load transport that uses only direct transfer between workers with no predetermined locations. It is suggested that this cooperative strategy can considerably reduce the time required for transporting materials to the nest. In this study, sequential load transport in the ant, Anoplolepis gracilipes, was analyzed by comparison with the bucket brigades (BBs) hypothesis in the laboratory. The results showed that ∼30% of food obtained in A. gracilipes was involved in food transfer when fruit flies were provided. The laden worker tended to offer resistance to relinquishing its load, but a larger worker could successfully rob a food item from a smaller one. Furthermore, worker size and the retrieval speed increased with each transfer. These results support the BBs hypothesis of sequential cooperation. However, the retrieval time for the transport chain was significantly longer than that of individual foraging. Moreover, distances of both the transport chain and individual foraging were significantly longer than that of the shortest linear route, and the distance covered by larger workers seemed to be shorter, thus the laden worker probably showed avoidance behavior in order to avert being robbed. Food transfer might not be a cooperative strategy in A. gracilipes. Hypothesis of non-cooperative load transport on the basis of nepotism is proposed and discussed.