A HT to former A2 economics student Shelale Mazari for this piece on efficiency and ants. According to some research, they weigh up the costs and benefits and build bridges using their bodies to allow their fellow ants to forage for food. When it becomes inefficient for them to continue doing so – i.e. when there are not enough foragers – they withdraw. And apparently, they do all this without a lead ant.
Army ants (Eciton) form collective assemblages out of their own bodies to perform a variety of functions that benefit the entire colony. 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.