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The energy currencies used by foraging animals are expected to relate to the energy costs and benefits of resource collection. However, actual costs of foraging are rarely measured. We measured the ratio of energetic benefit relative to cost (B/C) during foraging for the giant tropical ant, Paraponera clavata. The B/C ratio was 3.9 for nectar-foragers 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 energetic benefit per trip, (2) a 10-fold greater cost due to longer foraging distances, and (3) a 6-fold greater energy cost per meter due to larger body size. For Paraponera daily colonial energy intake rates are similar to expeditures and may limit colony growth and reproduction. In contrast, for Pogonomyrmex energy intake rates are an order of magnitude higher than estimated costs, suggesting that Pogonomyrmex colonies are unlikely to be limited by short-term energy intake. We suggest that variation in individual B/C ratios may explain why the foraging behavior of Paraponera but not Pogonomyrmex appears sensitive to foraging costs.