FORAGER SIZE AND ECOLOGY OF ACROMYRMEX CORONATUS AND OTHER LEAF-CUTTING ANTS IN COSTA-RICA

I compare forager size and foraging ecology of the leaf-cutting ant Ac romyrmex coronatus (Fabricius) with published data on three other leaf -cutter species in Costa Rica, Atta cephalotes (L.), Acromyrmex octosp inosus (Reich), and Acromyrmex volcanus Wheeler. Intra-and interspecif ic differences in forager size in these leafcutting ants appear to ref lect the economics of harvesting different preferred resources. Ac. co ronatus colonies have relatively small foragers (mean mass=3.4+/-1.4 m g) that cut almost exclusively the thin, soft leaves and other parts o f small herbaceous plants. Similarly, small A. cephalotes colonies hav e small foragers (3.3+/-1.0 mg) that attack the leaves of small herbac eous plants. In contrast, mature A. cephalotes colonies have a wider s ize-range of foragers (7.3+/-4.1 mg) that primarily attack the leaves of trees, with lar er foragers cutting thicker, tougher leaves. In A. cephalotes, the match of forager size to leaf type (both ontogenetical ly and behaviorally) increases foraging efficiency. Extreme forager po lymorphism in mature A. cephalotes colonies appears to broaden the div ersity of tree species that they can exploit efficiently. Ac, octospin osus and Ac, volcanus both have large, relatively monomorphic foragers (13.3+/-4.2 mg and 30.6+/-4.3 mg, respectively) that typically scaven ge for pieces of fallen vegetation, such as dead leaves, fruit, and fl owers, in addition to cutting herbs. The large foragers of Ac. octospi nosus and Ac. volcanus appear to be well suited as generalist foragers , able to cut or collect any desirable vegetation encountered. Ac. cor onatus is similar to A. cephalotes in other ways. Both Ac. coronatus a nd A. cephalotes establish and maintain cleared trunk trails for forag ing, and both have minima workers that ''hitchhike'' on the loads carr ied by foragers, apparently serving to protect the larger foragers fro m attack by phorid flies. Trunk trails and hitchhikers are not known f or Ac. octospinosus and Ac, volcanus. That A. coronatus and A. cephalo tes show little overlap in geographic distribution within Costa Rica m ay relate both to differences in habitat requirements and to interspec ific competition.