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Behavioral Ecology Advance Access originally published online on March 18, 2008
Behavioral Ecology 2008 19(4):805-809; doi:10.1093/beheco/arn032
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© The Author 2008. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

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

O. T. Lewis, M. Martin and T. J. Czaczkes

Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK

Address correspondence to O.T. Lewis. E-mail: owen.lewis{at}zoo.ox.ac.uk.


  Abstract

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.

Key words: Atta, central-place foraging, gradient, leaf-cutting ants, loading, slope.

Received 16 April 2007; revised 24 January 2008; accepted 7 February 2008.


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