Costs of trail construction and maintenance in the leaf-cutting ant Atta columbica

Abstract

Leaf-cutting ants of the genus Atta use trunk trails during foraging which may persist for months or years. The time and energy costs of trail construction and maintenance were estimated for colonies of Atta columbica on Barro Colorado Island, Panama, to deter- mine if these costs are likely to constrain new trail con- struction and promote persistence of existing trails. Large workers 2.2-2.9 mm in headwidth participated in trail-clearing significantly more frequently than typical leaf-carriers, indicating that they may form a distinctive task group within the foraging force. Small litter items were carried off trails, while large ones were cut up be- fore removal, greatly increasing the costs of removing large litter items. The average time cost of removing a kilogram of litter was estimated at 3,359 ant-hours, and energy costs at 4.6 kJ. Colonies maintained trail systems 267 m in length and 16.5 m2 in area, and built an esti- mated 2.7 km of trail with an area of 134 m2 during a year. Based on litter standing crop and estimates of litter- fall rates, total costs to colonies averaged 11,000 ant- days of work and the energy equivalent of 8,000 leaf burdens. These costs are small relative to the number of available workers and rates of mass harvest, suggesting that costs do not significantly constrain trail construc- tion. Instead, trails may persist because they provide ac- cess to high-quality resources or because only a few trails are required to fully exploit the foraging territory.