Division of labour within teams of New World and Old World army ants

In army ants, prey items are often retrieved by cooperative teams of workers rather than by single porters. We used experiments and randomization tests to explore the division of labour within such teams in the New World army ant Eciton burchelli, and the Old World army ant Dorylus wilverthi. We evaluated these teams in the light of a recent proposal that teams should be defined in terms of the concurrent performance of different subtasks by their members. This is a broader and more useful definition of teams than a previous one in which teams were defined by a membership necessarily involving different castes. Within army ant teams there is a front runner who initiates prey retrieval and one or more followers. Hence, there are two qualitatively different subtasks that must be performed concurrently during such teamwork. Previous work has shown that these teams are superefficient: the combined weight of the prey retrieved by the team is greater than the sum of the maximum weights the team members could carry when working singly. Here we show, for both species of army ant, that such teams have a nonrandom composition of members. The front runner is typically unusually large and the second-largest ant in a team is typically unusually small. These analyses are based on worker dry weights rather than assigning, workers to discrete caste categories. Our analysis also suggests that the behaviour of army ants is more sophisticated then previously suspected. Our data imply that if an unnecessarily large supplementary ant (follower) tries to help the front runner to move a large prey item, but finds that the remaining work is too slight to use her full efforts, she does not join the team. One or more smaller ants whose efforts become fully employed become involved instead. This suggests that army ants engaged in teamwork have both upper and lower workload thresholds. (C) 2001 The Association for the Study of Animal Behaviour.