Collective decision through self-assembling.

The ant genus Oecophylla is well known for forming chains that allow a gap to be bridged. Using a set-up where the ants are given the choice of building a chain on two identical sites, we show that they always end up focusing their activity on a single one. A mathematical model suggests that this result depends on probabilities of entering and leaving the chain that depend on its size. The same model allows some predictions to be made on the influence of the nest size. Thus, a critical population size is needed in order to observe the formation of at least one chain. Over this size we observe the transitory coexistence of two chains, for which the duration is positively correlated to the nest size. However, this coexistence always leads to the formation of one chain and to the break-up of the other one. Following on from these results we give similar examples in gregarious arthropods and discuss the possibility of these mechanisms being generic for a wide range of collective activities and decisions.