期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：49
页码：15113-15118
DOI：10.1073/pnas.1512241112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceComplex systems, from ant colonies to stock markets, share a common property: sophisticated group-level structure emerges from simple individual-level behaviors. Using simple interaction rules, Eciton army ants construct complex bridges from their own bodies to span forest-floor gaps. These living bridges are uniquely complex in both their dynamic properties and the number of animals involved and so are of considerable interest for understanding emergent structures in complex systems. In field experiments, we show that construction interacts with traffic rate and environmental geometry, causing bridges to lengthen, widen, and migrate. Bridges provide a shortcut for foraging ants, at the cost of sequestering workers. We show that bridge location represents a cost-benefit trade-off, with potential implications for human engineered self-assembling systems. 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 colonys 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 units having information on global benefits or costs.
