Performance and efficiency in leaf transport: unveiling the task allocation puzzle in Acromyrmex subterraneus

The performance of ant workers in a given task can be highly variable, generating a non-homogenous workload in the colony. However, there is no information on whether high performance workers are more efficient, or have any morphometric and physiological variation that causes them to both start earlier and continue executing the task longer. Here, we demonstrate that non-homogeneous task distribution is common in small colonies and that different performance levels occur even among Acromyrmex subterraneus workers of the same size class. High Activity workers did more work and were faster, so they were more efficient than Low Activity workers. However, their efficiency was not related to their leg length, as it was for Low Activity workers. Also, the delay of the first loaded trip of High Activity workers was shorter than that of Low Activity workers, indicating an earlier response of the former to the task. Delay variation was not affected by mass-specific metabolic rate. Considering the first five trips, we found that the first trip had a longer duration than the others, and High Activity workers were faster than Low Activity workers, suggesting that the higher efficiency of High Activity workers may be related to the reinforcement mechanism, which in turn lessens their response threshold to the task. Finally, workers had similar mandible morphometry (length of the first and second tooth, number of teeth), and body mass components (water content, lean dry weight and lipid content) despite their activity category, indicating that these variables did not explain differences in performance or efficiency among workers. The hypometric mass scaling metabolic rate showed that Non-transporters had proportionally lower energy expenditure than other categories. High Activity workers showed remarkable performance, efficiency, and faster responses to foraging stimulus. We suggest that this heightened level of individual proficiency is in line with the threshold model, explaining the operation of the task allocation mechanism within the same worker size class.