Flexibility of tandem leaders resolves traffic jams during colony relocation in an ant

Efficient traffic flow underpins biological and engineered systems. Ants, widely regarded as exemplars of collective efficiency in active matter research. We examined ants for traffic dynamics and transport efficiency during nest relocation via tandem running, that affects colony survival. Laboratory experiments, inspired by their monsoon-constrained habitats, and agent-based modeling reveal that narrow paths trigger jams 18 times more frequently than multi-lane paths, primarily driven by returning leaders (59%), followed by brood transporters (27%) and lost followers (14%). Despite a 1.3-fold transport delay, returning leaders mitigate jams through adaptive U-turns and midway recruitment, generating emergent unidirectional flow. This aligns with active matter principles of self-propelled agents navigating non-equilibrium dynamics. These findings reveal traffic jams in tandem-running ants, challenging assumptions of jam-free ant traffic, and inspire algorithms for robotic swarms and traffic optimization. The study contributes to understanding non-reciprocal interactions and emergent phenomena in biological systems.