Leafcutter Ant

Leafcutter ants are remarkably strong carriers. Loads of up to 50 times their own weight are often reported, though some sources give far lower figures for typical loads. Powerful jaw muscles make their mandibles vibrate about 1,000 times per second, slicing leaves into fragments. Long columns march home with pieces held overhead like tiny green sails, earning them the nickname “parasol ants.” One mature Atta colombica colony harvests roughly 85 to 470 kg (190 to 1,040 lb) of dry plant material a year. Across neotropical rainforests, leafcutters consume an estimated 12 to 17 percent of all leaf production, making them among the region’s biggest herbivores.

Leafcutter ants do not eat the leaves they carry. Instead, they use them to grow a fungus, Leucoagaricus gongylophorus, in underground garden chambers, one of the few true examples of agriculture among animals. Workers chew leaves into pulp, add it to the garden, and tend the fungus constantly, weeding out contaminants and keeping it healthy. In return, the fungus produces gongylidia, nutritious swollen tips that grow in bundles called staphylae and feed the colony. The partnership is obligatory: the ants depend on the fungus for food, and the fungus depends on the ants for care. It is a genuine evolutionary partnership refined over millions of years.

Leafcutter colonies divide labor among workers of very different sizes, a system called polyethism. The smallest workers, minims, tend the fungus garden and care for the brood. Medium-sized workers forage, cut leaves, and carry them home. The largest workers, called majors or soldiers, defend the colony with big heads and powerful jaws. In Atta sexdens, soldiers have heads about 3 mm (0.12 in) wide, versus about 1 mm (0.04 in) for garden-tending nurses. Some sources count as many as seven castes. Older workers usually take the riskier foraging jobs. Because each caste specializes, the colony works far more efficiently than if every ant tried every task.

A leafcutter colony is built around a single reproductive queen. On her nuptial flight she mates with several males, an estimated 3 to 8 in Atta sexdens, and stores sperm to use for the rest of her life. She can live 10 to 20 years and may produce 150 to 200 million offspring. Deep in the nest, workers feed and groom her while she focuses on laying eggs. Because the colony generally cannot replace her, it declines once she dies. A mature colony can hold 5 to 8 million workers, so this entire society depends on one founding individual.

Leafcutter nests are among the most complex structures built by any insect. A mature Atta sexdens colony can hold up to 2,000 chambers, and nests of larger species may reach about 9 m (30 ft) deep. Over its lifetime, one colony may move up to 40,000 kg (88,000 lb) of soil. Central mounds have been reported more than 30 m (98 ft) across, though these figures are estimates. The largest recorded nest covered 67 m² (720 sq ft) and held 1,920 chambers, 238 of them fungus gardens. Chambers for fungus, brood, and waste are linked by tunnels, all built by coordinated workers with no central planning.

Leafcutter ants coordinate through chemical signals called pheromones. Foragers that find good leaves lay scent trails on the way home, and other workers follow them, so no ant needs to search alone. Trails can stretch over 30 m (100 ft) and run up tree trunks into the canopy. Alarm signals alert nestmates to danger, and workers also communicate through vibrations. This lets a colony of millions act as a coordinated whole, which is why leafcutter societies are a frequent subject of research on complex social systems.