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 leafcutter 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.