Exploring cooperative treadmilling and protrusion growth in fire ant rafts

Fire ants (Solenopsis invicta) are well-documented forming buoyant and dynamic aggregations consisting entirely of worker ants when exposed to water. Here, we observe the collective morphogenesis of fire ant rafts docked to stationary, vertical rods. These rafts consist of a condensed, floating, structural network of interconnected ants on top of which a dispersed, pedestrian layer of freely active ants walks. Under these conditions, ant rafts can change their shape substantially and continuously over the span of several hours through cooperative global treadmilling. During treadmilling, these rafts frequently sprout tether-like protrusions from their edges that fire ants can use as land bridges to escape flooded environments. Employing both experimental characterization and an agent-based, numerical model, we here unveil a local set of mechanisms that reproduce the stochastic emergence of these instabilities in the absence of long-range interactions, targeted cues, or external gradients. Furthermore, we demonstrate that simply through the modulation of free ant activity, the model reproduces oscillatory phases of high outwards expansion (exploration) and predominantly inwards contraction (dormancy). These results suggests that collective morphogenesis of this system is strongly mediated by local interactions at the constituent length scale, perhaps providing inspiration for the development of decentralized, autonomous active matter and swarm robotics.