Global coordination using local information in fire ant pontoon bridges

Fire ants (S. invicta) are known to collectively entangle to create free-floating rafts as a survival mechanism upon nest flooding. Here we demonstrate that they can also create structures which allow foraging 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 can reach the food by self-assembling into a floating pontoon bridge consisting of approximately 500 ants. Multiple proto-bridges initiate from the boundary; worker ants lengthen proto-bridges by directly entering the water. Ants also use the water meniscus at the bowl rim to propel themselves into the water, aggregating into floating rafts that later attach to proto-bridges. Within two hours, most proto-bridges retract and a single final bridge forms, suggesting global coordination. However, our agent-based model suggests that a local behavior, the tendency for individual ants to join proto-bridges more often in areas with stronger food scent, can cause the colony to form a single bridge. This implies that the formation of single bridges can be modulated by 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 coordinate over long distances using only local information to create functional structures.