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

Leafcutting ants (Atta and Acromyrmex) sometimes form piles or caches of le aves on foraging trails. Laboratory experiments have shown that leaf cachin g at the nest entrance by Atta cephalotes and Atta colombica is adaptive be cause it occurs when a colony's leaf delivery rate exceeds its leaf-process ing rate and serves to increase the probability that a dropped leaf is even tually recovered. We examined the occurrence and adaptive value of leaf cac hing on foraging trails in field colonies of A. colombica in Panama. The pr obability of leaf caching was positively related to trail traffic with cach e 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-in duced nest entrance caching in the laboratory. Leaves were recovered, more rapidly from caches but because leaves were not selected according to forag er size they were transported back to the nest more slowly than normally fo raged leaves. Thus, whilst caching provides benefits, by increasing the lik elihood of leaf recovery, it imposes a cost through mismatching forager and load size. Leaf caching is an example of an organizational shift (from dir ect foraging to foraging with task partitioning) and we provide a minimum e stimate 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 co sts 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 thei r leaf. (C) 2001 The Association for the Study of Animal Behaviour.