The legionary ants are making rapid progress, hunting down their prey. Cockroaches, beetles, scorpions and tarantulas are all targets. Even lizards, snakes and small birds are attacked. Captured animals are stung, cut up and carried back to the bivouac, an aggregation of ants sometimes a hundred metres away.
In the forests of Panama, a bivouac of army ants can contain two million individuals. In order to feed them, the voracious larvae and the queen they protect, an efficient supply chain must be established. Long columns of legionnaires thus roam the ground. They consist of a caste whose sole function is to transport food, at the speed of ten body lengths per second (60 km/h in a 1.7 m human). These 5 mm creatures are the fastest of all ants, but sometimes they have to cross obstacles. When a bridge is needed, the legionnaires cluster at each end of the precipice until it is filled in, in a garland of bodies. If the column has to cross a very steep area, some ants stop in the middle of the slope and dig their claws into the substrate. In this way, the individuals become an animal ladder, a scaffold that fellow ants climb at full speed.
Is this altruistic behaviour governed by a collective organisation? In order to better understand this, colleagues (1) set up a small experiment, and a lot of mathematical models. In the path of a column of army ants, they installed a board whose inclination they could vary. The agile creatures tolerated slopes five times steeper than the steepest French departmental road, but beyond this threshold they built a scaffold in 80% of cases. This allowed them to cross even a vertical wall. The researchers believe that even if the benefit of scaffolding is collective, the decision to put it up is individual: the ant fixes itself to the slope when it notices that it or its neighbours are slipping, without waiting for an order from some headquarters. The scaffold-building legionnaires would therefore not use chemical communication, as they do when they capture a large prey. In this other case, the ants emit pheromones that attract a large number of conspecifics from the bivouac in order to take advantage of an abundant resource.
It is often assumed that coordinated animal behaviour requires thought and communication. In army ants, recent research indicates that movement in the forest remains rapid and fluid due to individual reflexes in favour of a common good. This high level of responsiveness increases the resilience of the small world of social insects; its ability to overcome obstacles and upheavals.
(1) Lutz, M. J., Reid, C. R., Lustri, C. J., Kao, A. B., Garnier, S., & Couzin, I. D. (2021). Individual error correction drives responsive self-assembly of army ant scaffolds. Proceedings of the National Academy of Sciences, 118(17).
Army ants (Ch’ien Lee/Minden Pictures/Biosphoto)