Global coordination using local information in fire ant pontoon bridges

Fire ants (Solenopsis invicta) can collectively entangle to create free-floating rafts which allow the colony to survive upon nest flooding. Here we demonstrate that they can also create adaptable structures which allow them to forage across a water surface. Laboratory experiments using sub-colonies of 8000 fire ants demonstrate that when the ants detect food in the center of a 14 cm diameter bowl filled with water, they self-assemble into a floating pontoon bridge consisting of approximately 500 ants to reach the food. Multiple proto-bridges initiate from the boundary; the ants add to proto-bridges by directly entering the water and/or using the water meniscus at the bowl rim to propel themselves into the water. Over time, most proto-bridges retract and a single final bridge forms, suggesting global coordination. Our agent-based model of the system suggests that the tendency for ants to join proto-bridges more often in areas with stronger food scent is sufficient for them to form a single bridge. This suggests ants’ ability to build a single bridge should be sensitive to environmental conditions. The model prediction that ants form multiple bridges in a bowl with nonuniform distance between the food and rim was verified in experiments, thus giving insights into how collectives can effectively coordinate over long distances using only local information to create adaptable functional structures.