Leaf cutter ant castes

Title Info
Common name Ants, Leaf Cutter
Scientific name Atta
Taxonomic group Formicidae
Source Dan L. Perlman
Ecosystems Forests
Forests Tropical rainforest
Ecological interactions Mutualism; Herbivory
Mutualism Ants and fungi
Behavior Social
Organisms Animals
Animals Insects
Lessons Mutualisms; Leaf Cutter Ants
Location La Selva Biological Station,Costa Rica,North America
Leaf cutter ant major and minor workers, Costa Rica
Related materials: Leaf cutter ants
Leaf cutter ants are an extraordinary example of mutualism; click the Related materials link to see a number of images illustrating their life cycle and ecology.

Leaf-cutter ant major and minor workers, Costa Rica. Leaf-cutter colonies are especially efficient because workers come in many different sizes and each size is particularly suited to certain tasks. This image shows a major worker, one of the defenders of the colony, near some minor workers. The huge head of the major is filled largely with muscles that activate the ant's mandibles (jaws). The mandibles are so sharp and the muscles so powerful that leaf-cutter majors are reputed to be able to cut through leather.

Atta ants are the dominant herbivores in many parts of the New World Tropics, where large mammalian herbivores are relatively sparse. Leafcutter ants consume prodigious quantities of vegetation and some tropical ecologists estimate that Atta colonies may cut 12 - 17% of the total leaf production of a tropical rainforest. However, the ants do not themselves eat the leaves they cut; instead, they grow a special fungus on these leaf fragments.

This is a remarkably tight mutualism: leafcutter ant colonies cannot survive without their Leucocoprinus fungus and the fungus is found nowhere but in these colonies. Worker ants transport fragments of fresh leaves, flowers, and other vegetation back to the nest, where the plant material is chewed into ever-smaller pieces until it becomes a pulp. Tiny colony-minding workers then apply fecal droplets containing digestive enzymes to the mass, which becomes the substrate for the fungus gardens. The fungus essentially functions as a giant digestive system for these leafcutter ant colonies. Workers pluck nutrient-rich swellings known as gongylidia and feed them to the colony's larvae, while the workers themselves mostly feed directly on plant sap.

Atta colonies have the impressive ability to maintain fungal monocultures in their nest chambers. Workers keep fungal growths pure by constantly removing any introduced alien fungi and by frequently transferring their fungus onto fresh plant substrate. They also use an apparently mutualistic bacterium that protects the fungus garden against an especially virulent parasitic fungus in the genus Escovopsis. Thus the ants and fungi are actually part of a three-way mutualism with a filamentous bacterium that ensures the health of the fungus in Atta gardens.

Colonies can be huge, with some estimated to contain five million workers, and some of the galleries they excavate may be 20 feet (6 meters) underground. Moreover, the colony just has a single queen laying eggs; one leafcutter queen living in a lab lived more than 14 years. These ants and their trails of moving leaf fragments are among the most spectacular sights in the tropical rainforest.