  Efficient transportation, a hot topic in nonlinear science, is essential for
modern societies and the survival of biological species. Biological evolution
has generated a rich variety of successful solutions, which have inspired
engineers to design optimized artificial systems. Foraging ants, for example,
form attractive trails that support the exploitation of initially unknown food
sources in almost the minimum possible time. However, can this strategy cope
with bottleneck situations, when interactions cause delays that reduce the
overall flow? Here, we present an experimental study of ants confronted with
two alternative routes. We find that pheromone-based attraction generates one
trail at low densities, whereas at a high level of crowding, another trail is
established before traffic volume is affected, which guarantees that an optimal
rate of food return is maintained. This bifurcation phenomenon is explained by
a nonlinear modelling approach. Surprisingly, the underlying mechanism is based
on inhibitory interactions. It implies capacity reserves, a limitation of the
density-induced speed reduction, and a sufficient pheromone concentration for
reliable trail perception. The balancing mechanism between cohesive and
dispersive forces appears to be generic in natural, urban and transportation
systems.
