Leaf processing behavior in Atta leafcutter ants: 90% of leaf cutting takes place inside the nest, and ants select pieces that require less cutting

<p>Leafcutter ants cut trimmings from plants, carry them to their underground nests, and cut them into smaller pieces before inoculating them with a fungus that serves as a primary food source for the colony. Cutting is energetically costly, so the amount of cutting is important in understanding foraging energetics. Estimates of the cutting density, meters of cutting per square meter of leaf, were made from samples of transported leaf cuttings and of fungal substrate from field colonies of <em>Atta cephalotes</em> and <em>Atta colombica</em>. To investigate cutting inside the nest, we made leaf-processing observations of our laboratory colony, <em>A. cephalotes</em>. We did not observe the commonly reported reduction of the leaf fragments into a pulp, which would greatly increase the energy cost of processing. Video clips of processing behaviors, including behaviors that have not previously been described, are linked. An estimated 2.9 (+0.3) kilometers of cutting with mandibles was required to reduce a square meter of leaf to fungal substrate. Only about 12% (+ 1%) of this cutting took place outside of the nest. The cutting density and energy cost is lower for leaf material with higher ratios of perimeter to area, so we tested for, and found that the laboratory ants had a preference for leaves that were pre-cut into smaller pieces. Estimates suggest that the energy required to transport and cut up the leaf material is comparable to the metabolic energy available from the fungus grown on the leaves, and so conservation of energy is likely a particularly strong selective pressure for leafcutter ants.