Energetics of trail clearing in the leaf-cutter ant Atta

Few ant species construct cleared trails. Among those that do, leaf-cutting Atta ants build the most prominent networks, with single colonies clearing debris and obstructions from hundreds of meters of trails annually. Workers on cleared paths move at higher speed than they do over uncleared litter, and one measurement of the time and energetic costs of trail clearance suggests that benefits of trail usage far outweigh the investment costs of trail clearing. The ecological basis of trail clearing remains uncertain, however, because no full account has been made of benefits and costs in common units that allow comparison. We make such an account using a scalable, integrative model of trail investment and foraging energetics. Contrary to assumptions in previous work, we find that trail clearing needs not always be energetically profitable for leaf-cutting ants. Profitability depends on the workforce composition, specifically, on how many ants in a traffic stream act as maintenance workforce to respond to sudden and unpredictable obstructions, such as leaf fall. Such maintenance patrols have not previously been recognized as a cost of trail building. If the patrolling workforce is not too large, the energetic savings from foraging over cleared trails offset the investment and maintenance costs within a few days. Under some conditions, however, amortization can take weeks or months, or trail clearing can become unprofitable altogether. This suggests that Atta colonies must have a mechanism to regulate the intensity of their trail clearing behavior. We explore possible mechanisms and make testable predictions for future research. Leaf-cutter ants build prominent, cleared trails of up to 200 m length through rainforest undergrowth. Construction of such trails appears costly, yet little is known about the energetics of cleared trails. No research exists on the benefits of their use, and only a single case study investigated parts of the construction costs. While this case study concludes that trail clearing is “relatively inexpensive,” we argue that it failed to include a deciding factor in the cost/benefit analysis: the cost of providing a standby clearing workforce, which is distinct from the foraging workers. We construct a full, scalable cost-benefit model from our own empirical measurements and literature. Contrary to previous results, we find that trail clearing is not always profitable, but profitability depends on the foraging conditions. This prediction offers a new perspective on the occurrence of uncleared trails in the field.