Defense against parasites by hitchhikers in leaf-cutting ants: a quantitative assessment

Summary A curious behavior in leaf-cutting ants in the genus Atta is the hitchhiking of small minim workers on leaf fragments carried by larger workers. Two functions of these hitchhikers have been proposed: (1) defense of leaf carriers against parasitic flies in the family Phoridae (ant protection hypothesis; Eibl-Eibesfeldt and Eibl-Eibesfeldt 1967) and (2) reduction of transport costs of small minims that collect plant fluids (energy conservation hypothesis; Stradling 1978).We studied hitchhiking behavior in colonies of Atta colombica on Barro Colorado Island, Panama, and found strong evidence in favor of the ant protection hypothesis. Females of Apocephalus attophilus (Diptera: Phoridae) attack leaf carriers of A. colombica and deposit eggs in the head capsules of these ants. Our observations indicate that parasites require leaf fragments to stand on during oviposition, and, as a result, only leaf carriers are susceptible to parasitic attack. The presence of hitchhikers reduces significantly both the time parasites spend on leaf fragments and the probability that they will land in the first place. Results of experimental introductions of parasites and a year of biweekly censuses at ten colonies indicate that leaf-cutting ants adjust the level of hitchhiking to accommodate both daily and seasonal changes in the abundance of parasites.We found little evidence in support of the energy conservation hypothesis. If it is assumed that all minim workers hitchhike back to the nest, our calculations indicate that total transport costs along a foraging trail are reduced by 10% or less. However, our observations indicate that only 50% of returning minim workers hitchhike, and therefore energy savings are actually considerably less than 10%. Leaf-cutting ants in the genera Atta and Acromyrmex are attacked by over 20 species of parasitic phorids. In the discussion we review what is known about these associations and suggest that these parasites have influenced the ecology and evolution of polyethism in leaf-cutting ants. Offprint requests to: D.H. Feener (at his present address)