The ability of individual animals to create functional structures by joining together is rare and confined to the social insects.
Army ants (Eciton) form collective assemblages out of their own bodies to perform a variety of functions that benefit the entire colony. Here
we examine ‟bridges” of linked individuals that are constructed to span gaps in the colony’s foraging trail. How these living
structures adjust themselves to varied and changing conditions remains poorly understood. Our field experiments show that
the ants continuously modify their bridges, such that these structures lengthen, widen, and change position in response to
traffic levels and environmental geometry. Ants initiate bridges where their path deviates from their incoming direction and
move the bridges over time to create shortcuts over large gaps. The final position of the structure depended on the intensity
of the traffic and the extent of path deviation and was influenced by a cost–benefit trade-off at the colony level, where
the benefit of increased foraging trail efficiency was balanced by the cost of removing workers from the foraging pool to
form the structure. To examine this trade-off, we quantified the geometric relationship between costs and benefits revealed
by our experiments. We then constructed a model to determine the bridge location that maximized foraging rate, which qualitatively
matched the observed movement of bridges. Our results highlight how animal self-assemblages can be dynamically modified in
response to a group-level cost–benefit trade-off, without any individual unit’s having information on global benefits or costs.