Sequential load transport in grass-cutting ants (Atta vollenweideri): Maximization of plant delivery rate or improved information transfer?

Abstract

Sequential transport of plant fragments was studied in the grass-cutting ant Atta vollenweideri. Two competing hypotheses concerning its occurrence were tested. Based on the economic-transport hypothesis," sequential transport occurs because of a mismatch between load size and ant body size, and it is therefore considered a way to improve size-matching and so the plant delivery rate. Alternatively, the information-transfer hypothesis" states that sequential transport improves the information flow during foraging. By transferring its load, a worker may return earlier to the foraging site so as to intensify chemical recruitment. To distinguish between these two competing hypotheses, standardized paper fragments that differed either in size or in quality were presented to workers of a field colony, and sequential transport was quantified. Neither an increase in fragment mass nor in fragment length influenced the occurrence of transport chains. Sequential transport took longer than transport by a single carrier. However, the occurrence of sequential transport increased with increasing fragment quality. High-quality fragments were transferred more frequently and after shorter distances than less-attractive fragments. Taken together, these results strongly support the hypothesis that sequential load transport has been favoured during evolution because of an improvement in the information flow during foraging. © 2011 Jacqueline Rschard and Flavio Roces.