Army ants dynamically adjust living bridges in response to a cost–benefit trade-off

Abstract

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 tradeoff, without any individual unit's having information on global benefits or costs.collective behavior | self-assembly | swarm intelligence | self-organization | optimization

A Stochastic Approach to Shortcut Bridging in Programmable Matter

“…The weighted perimeter balances in one function (using an appropriate weight for the land and gap perimeter edges) the trade-off observed in [22] between the competing objectives of establishing a short path between the fixed endpoints while not having too many particles in the gap. We focus on gap perimeter instead of the number of particles in the gap (which is perhaps a more natural analogy to [22]) because (1) the shortcut bridges produced with this metric more closely resemble the ant structures and (2) only particles on the perimeter of a configuration can move, and thus recognize the potential risk of being in the gap.…”

“…We focus on gap perimeter instead of the number of particles in the gap (which is perhaps a more natural analogy to [22]) because (1) the shortcut bridges produced with this metric more closely resemble the ant structures and (2) only particles on the perimeter of a configuration can move, and thus recognize the potential risk of being in the gap.…”

“…In analogy to the apparatus used in [22] (see Figure 3a), we are particularly interested in the special case where L forms a V-shape, O has two objects positioned at either base of L, and σ 0 lines the interior sides of L, as in Figure 2a. However, our algorithm is not limited to this setting; for example, we show simulation results for an N-shaped land mass (Figure 2b) in Section 5.…”

“…In this paper, we consider stochastic, distributed, local, asynchronous algorithms for "shortcut bridging", in which particles self-assemble bridges over gaps that simultaneously balance minimizing the length and cost of the bridge. Army ants of the genus Eciton have been observed exhibiting a similar behavior in their foraging trails, dynamically adjusting their bridges to satisfy an efficiency tradeoff using local interactions [22]. Using techniques from Markov chain analysis, we rigorously analyze our algorithm, show it achieves a near-optimal balance between the competing factors of path length and bridge cost, and prove that it exhibits a dependence on the angle of the gap being "shortcut" similar to that of the ant bridges.…”

“…In this paper, we present an algorithm inspired by the work of Reid et al [22], who found that army ants continuously modify the shape and position of foraging bridges -constructed and maintained by their own bodies -across holes and uneven surfaces in the forest floor. Moreover, these bridges appear to stabilize in a structural formation which balances the "benefit of increased foraging trail efficiency" with the "cost of removing workers from the foraging pool to form the structure" [22].…”