title: 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 trade-off, without any individual unit's having information on global benefits or costs.
authors.affiliation: Department of Biological Sciences, New Jersey Institute of Technology, Newark, NJ 07102; chrisreidresearch@gmail.com mlutz@princeton.edu.
authors.forename: Chris R
authors.initials: CR
authors.lastname: Reid
authors.affiliation: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544; chrisreidresearch@gmail.com mlutz@princeton.edu.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Lutz
authors.affiliation: Department of Biological Sciences, George Washington University, Washington, DC 20052;
authors.forename: Scott
authors.initials: S
authors.lastname: Powell
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138;
authors.forename: Albert B
authors.initials: AB
authors.lastname: Kao
authors.affiliation: Department of Collective Behaviour, Max Planck Institute for Ornithology, Konstanz D-78457, Germany; Chair of Biodiversity and Collective Behaviour, Department of Biology, University of Konstanz, Konstanz D-78457, Germany.
authors.forename: Iain D
authors.initials: ID
authors.lastname: Couzin
authors.affiliation: Department of Biological Sciences, New Jersey Institute of Technology, Newark, NJ 07102;
authors.forename: Simon
authors.initials: S
authors.lastname: Garnier
mesh_terms: D000818:Animals; D001000:Ants / Q000502:physiology*; D003362:Cost-Benefit Analysis; D005247:Feeding Behavior
keywords: collective behavior; optimization; self-assembly; self-organization; swarm intelligence