Figure 45.8: Where Does the Extra Nitrogen Come From? Leaf-cutter ants require more nitrogen than is contained in the fresh leaves that they supply to their fungal gardens. Perhaps the fungus concentrates nitrogen from the leaves into the special fungal structures that the ants eat. If so, then spent leaf material in the ants’ refuse dumps (see Figure 43.12) should be lower in nitrogen than fresh leaves—but the opposite turns out to be true. One possible explanation for this enrichment of the discarded leaf material is that the ants eat protein-rich insects as well as the fungus and fertilize the fungal garden with their feces. Another is that the fungus absorbs nitrogen from the soil. A third possibility is that the fungus absorbs nitrogen from nitrogen-fixing organisms in the nest. Pinto-Tomás and colleagues tested this third hypothesis.a