Leaf-Cutter Ants Tend Vast Fungal Gardens

Leaf-cutter ants have the power to slice, dice, and pilfer the foliage of an entire grove of trees in a matter of days. With impressive efficiency, swarms of leaf-cutters clip and carry leafy material in vast quantities back to their subterrainean colony. There they process the clippings into compost piles on which they cultivate crops of fungi. The ants tend these fungal gardens and, in return, the fungi provide a constant source of food for the ant colony.

Leaf-cutter ants and their fungal crops are among the most impressive symbiotic pairings known in the animal kingdom. The leaf-cutter ant and fungus relationship is estimated to be between 8 and 12 million years old. But leaf-cutting ants are not the only type of ants to rely on fungus as a food source, nor are they the first to have done so.

There are, in fact, over 230 species of ants that farm fungus—together, this group of ants is referred to as the attine ants. Throughout their evolution, attine ants have developed five different ways of farming fungus:

• Lower agriculture -
• Coral fungus agriculture -
• Yeast agriculture -
• Generalized higher agriculture -
• Leaf-cutter agriculture -

The first attine ants appeared about 50 million years ago. These ants practiced lower agriculture, operating small-scale fungi gardens in which they grew parasol mushrooms and coral fungi. The symbiotic relationships in the lower agriculture systems are characterized by a looser symbiotic relationship than later evolving systems. Fungi in lower agricultural systems rely less on their ant hosts and can grow outside of the ant colony. Additionally, the ants are not as particular about the type compost they collect for their fungal garden. They don't harvest leave cuttings but instead settle for decaying material and insect feces.

The agriculture of later-evolving attine ants is more specialized though and their symbiosis with their fungal crops are more intimately intertwined. The fungal species in these "higher agriculture" systems, including the fungi grown by leaf-cutting ants, must be tended by ants to ensure their survival. Additionally, the fungi pay the ants back well for their work by sprouting nutritious nodules called "gongylidia" that serve as a food source for the ants.

Refs:

Schultz, T., & Brady, S. (2008). From the Cover: Major evolutionary transitions in ant agriculture Proceedings of the National Academy of Sciences, 105 (14), 5435-5440 DOI: 10.1073/pnas.0711024105

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