The ecology and evolution of worker size-distribution in leaf-cutting ants (Hymenoptera: Formicidae)

Many leaf-cutting ants (Atta spp, and Acromyrmex spp.) have extremely polymorphic workers and a complex division of labor among worker size castes. I compare published data on worker size-distribution in colonies of three Atta species (A. cephalotes, A. sexdens, and A. texana) with new data from colonies of three Acromyrmex species (Ac. coronatus, Ac. octospinosus, and Ac. volcanus) in a phylogenetic context, in order to gain insight into the ecology and evolution of polymorphism in leaf-cutting ants. The leaf-cutting ants considered show striking intra- and interspecific differences in worker size-distribution that fall into three distinct and fundamentally different categories, linked with differences in basic foraging ecology: a) Large colonies of A. cephalotes, A. sexdens, and A. texana produce an extremely broad, continuous size-range of workers. In these colonies, a wide range of medium-size workers act as foragers, primarily attack the leaves and flowers of trees, with the larger foragers cutting thicker and tougher vegetation, b) Colonies of Ac. coronatus and small colonies of A. cephalotes produce a narrow range of small workers, the largest of which forage, primarily cutting the soft leaves of herbaceous plants, c) Colonies of Ac. octospinosus and Ac. volcanus produce a bimodal worker size-distribution with two distinct size castes of workers. The small workers stay inside the nest, tending the fungus garden and brood, whereas the relatively monomorphic, large workers forage for vegetation, primarily small herbs and fallen leaves, fruits, and flowers. The differences in worker size-distributions, among leaf-cutting ants appear to correspond to the ergonomics of foraging on different resources, though direct economic analyses are needed. Phylogenetic analysis suggests that a narrow worker size-distribution, much like that of Ac. coronatus, was the primitive condition among leaf-cutting ants, with the bimodal and broad worker size-distribution conditions derived from this condition independently.