Information transfer in head-on encounters between leaf-cutting ant workers: food, trail condition or orientation cues?

Living in large societies involves costs associated with high density of individuals, but being near others includes the benefit of access to conspecifics' information. High densities of workers in ant colonies impose traffic congestion costs on foraging trails. It has been postulated that crowding also increases foraging efficiency by facilitating information transfer between workers in head-on encounters. However, this hypothesis remains untested. Here we assessed, in 24 field nests of the leaf-cutting ant Atta cephalotes, whether head-on encounters between workers facilitate information transfer about trail condition, orientation and food. Several experimental manipulations failed to fit predictions of certain types of communication. (1) Trail disturbance (and thus potential need for information transfer) did not affect the rate of head-on encounters, (2) head-on encounters did not decrease the time required for laden ants to properly orient when entering a trail, and (3) ants that had been experimentally disoriented did not increase the number of head-on encounters when they returned to the trail. Nevertheless, one experiment strongly suggested information acquisition: (4) outbound ants were more likely to find and collect food after a head-on encounter with an ant carrying the same kind of food. These results do not support the hypotheses that workers exchange information about trail condition and orientation in head-on encounters, but suggest that workers acquire food information. The information transferred in head-on encounters could thus increase foraging efficiency under crowded conditions. The cost of the reduced speed due to worker collisions might be outweighed by the benefits of information acquisition, and could explain why leaf-cutting ants do not form distinct lanes of outbound and returning workers. Our results reinforce the key role of information use in the adaptive behaviour of animals and in the maintenance of group living.