FORAGING ENERGETICS OF THE ANT, PARAPONERA-CLAVATA

The energy currencies used by foraging animals are expected to relate to the energy costs and benefits of resource collection. However, actu al costs of foraging are rarely measured. We measured the ratio of ene rgetic benefit relative to cost (B/C) during foraging for the giant tr opical ant, Paraponera clavata. The B/C ratio was 3.9 for nectar-forag ers and 67 for insect prey foragers. In contrast, the B/C ratio during foraging for seed harvester ants (Pogonomyrmex occidentalis) is over 1000, demonstrating that the B/C ratio can vary widely among ants. The B/C ratio was 300 times lower for nectar-foraging Paraponera than for the seed-harvesting Pogonomyrmex because of: (1) a 5-fold lower energ etic benefit per trip, (2) a 10-fold greater cost due to longer foragi ng distances, and (3) a 6-fold greater energy cost per meter due to la rger body size. For Paraponera daily colonial energy intake rates are similar to expeditures and may limit colony growth and reproduction. I n contrast, for Pogonomyrmex energy intake rates are an order of magni tude higher than estimated costs, suggesting that Pogonomyrmex colonie s are unlikely to be limited by short-term energy intake. We suggest t hat variation in individual B/C ratios may explain why the foraging be havior of Paraponera but not Pogonomyrmex appears sensitive to foragin g costs.