Minor workers have a major role in the maintenance of leafcutter ant pheromone trails

Ants, especially those species that use pheromone trails for communication and orientation, are dominant in many ecosystems. Trail- using ants form large networks of foraging trails, which require regular updating because of pheromone decay and physical interruptions to continuity. We tested the hypothesis that leaf-cutter ants, Atta spp., possess trail maintenance workers to maintain trail network connectivity. The smallest workers in leafcutter ant colonies ( minims) are always present on trails but never engage in leaf transport. Using smoked glass we visualized trails and found that minims deposited pheromone trail with a much higher frequency ( 83.3%) than nonminims ( 20%). In the field, minims were observed to perform frequent U- turns on trails. Over the course of 10 min minims made a mean frequency of 18.0 U- turns while walking on the trail, whereas nonminims not engaged in leaf transport made a mean of only 2.02 U- turns. Ants carrying leaves never made U- turns. We investigated the effect of interruptions on trail traffic composition and the subsequent process of trail re- establishment. We simulated trail interruptions with two techniques: scraping away a section of trail and covering trail with soil. We found that the frequency of minims in the interrupted regions of trails increased by 21.0% following both types of interruption treatment. In contrast the frequency of nonminim workers in the interrupted regions declined by 44.8%. In the interrupted regions the proportion of workers represented by minims increased by 118%. Our results show a key role for Atta minims in the maintenance of pheromone trails.