Effects of trail gradient on leaf tissue transport and load size selection in leaf-cutter ants

Leaf-cutter ants in the genus Atta are frequent model organisms in studies of central-place foraging. Workers carry leaf fragments from the foraging site to the nest. Larger workers carry heavier loads, but it has been noted repeatedly that workers typically carry fragments lower in mass than appropriate to maximize leaf tissue transport. Here, we suggest and test a previously unconsidered explanation for this discrepancy. Previous calculations of rate-maximizing load sizes have been based on data from smooth, horizontal trails. However, foraging ants will in reality experience a variety of trail gradients, with implications for locomotion speeds of laden ants and consequently for rate-maximizing loads. In the field, we manipulated trail gradient for 389 laden Atta cephalotes workers and found that gradient had marked effects on leaf tissue transport rates, which were highest on downhill and horizontal gradients and considerably lower when ants carried loads uphill. In a captive colony where the whole length of the foraging trail was manipulated, ants apparently adjusted their loading ratios in response to gradient, carrying smaller loads relative to their mass on uphill gradients. Significantly higher loading ratios were recorded for ants carrying loads vertically downhill, perhaps because ants use gravity to reduce the energetic costs of carrying heavy loads. Our results suggest that optimal loads will differ markedly under differing trail conditions and that leaf-cutter ants show previously unsuspected plasticity in load size selection, allowing them to maintain foraging efficiency under a range of trail conditions.