Central-place foraging continues beyond the nest entrance: the underground performance of leaf-cutting ants

Leaf-cutting ants (Atta spp.) are central-place foragers that use fragments of leaf tissue as substrate for symbiotic fungal gardens. A standard hypothesis of central-place foraging theory is that foragers will select load sizes that optimize their performance, but previous study has shown that Atta workers carry loads well below the size needed to maximize either the rate or the energetic efficiency of leaf tissue delivery to the nest. We propose that the underground processing of leaf fragments following delivery restricts the overall rate of resource acquisition in a way that should favour the collection of small loads. We used laboratory colonies of Atta colombica housed in transparent nests to study tissue transfer between chambers, and hoisting, cleaning and shredding of fragments on the fungal gardens. Leaf fragment size had strong effects on all these processes except hoisting fragment on to the garden. The time needed for these underground activities would often be greater than the time used to retrieve it from a vegetation source. Thus, maximization of the delivery rate may not maximize the overall rate of resource acquisition. Load selection by foragers outside the nest may have evolved to optimize the processing activities below ground in which they do not take part.