Leaf caching in the leafcutting ant Atta colombica: organisational shift, task partitioning and making the best of a bad job.

Leafcutting ants (Atta and Acromyrmex) sometimes form piles or caches of leaves on foraging trails. Laboratory experiments have shown that leaf caching at the nest entrance by Atta cephalotes and Atta colombica is adaptive because it occurs when a colony's leaf delivery rate exceeds its leaf-processing rate and serves to increase the probability that a dropped leaf is eventually recovered. We examined the occurrence and adaptive value of leaf caching on foraging trails in field colonies of A. colombica in Panama. The probability of leaf caching was positively related to trail traffic with cache locations being frequently associated with changes in gradient or terrain along the trail. Artificially blocking foraging trails resulted in caching behaviour but only when the blockage was near the leaf source or the nest entrance. This mirrors individual leaf dropping on trails and bottleneck-induced nest entrance caching in the laboratory. Leaves were recovered more rapidly from caches but because leaves were not selected according to forager size they were transported back to the nest more slowly than normally foraged leaves. Thus, whilst caching provides benefits, by increasing the likelihood of leaf recovery, it imposes a cost through mismatching forager and load size. Leaf caching is an example of an organizational shift (from direct foraging to foraging with task partitioning) and we provide a minimum estimate of the cost of this shift. Leaf caching encompasses two behaviours, leaf dropping and cache formation. Leaf dropping, and therefore the switch to task partitioning, is not in itself adaptive and cache formation has costs as well as benefits. We propose that leaf caching is making the best of a bad job if for some reason it is necessary for foragers to put down their leaf.