Invertebrates
Leafcutter Ant
Atta cephalotes · also called leaf-cutter ant, parasol ant
Leafcutter ants are the farmers of the insect world. The endless files of ants carrying green flags of leaf along a forest trail are not gathering food to eat — they are gathering compost for an underground fungus crop that has fed their species for over 50 million years.
Overview
Leafcutter ants of the genus Atta live throughout the American tropics, from Mexico to northern Argentina, in rainforest, dry forest, and increasingly in plantations and gardens. A colony centres on a huge subterranean nest — a network of chambers and tunnels that can be several metres across and deep, ventilated by the ants’ architecture, and home to a single long-lived queen and up to eight million sterile female workers.
The fungus farm
Leafcutters practise agriculture:
- Foragers cut crescents of leaf from trees and shrubs with their serrated jaws and carry the pieces back to the nest, sometimes along cleared trails hundreds of metres long.
- Processing workers clean the fragments, chew them into a pulp, and press them into the growing fungus garden.
- The fungus (a species in the Agaricales) breaks down the tough plant material the ants cannot digest and, in return, produces nutritious swellings called gongylidia that the ants and their larvae eat.
- Gardener workers weed the crop, remove diseased material, and apply antibiotics produced by a bacterium they carry on their bodies to fight a specialised parasitic mould.
Neither partner can now live without the other. When a virgin queen flies off to mate and found a new colony, she carries a starter pellet of the fungus in a pouch in her mouth.
A society of specialists
The workers of a single colony vary in size more than almost any other ant — by a factor of around 200 in body mass. Each size class has a job: minute “minims” farm and guard leaves in transit against parasitic phorid flies; medium workers cut and carry; large workers open trails; and giant-headed “soldiers” defend the nest with jaws powerful enough to slice leather.
Ecological weight
Because a mature colony consumes hundreds of kilograms of vegetation a year, leafcutters are often the single biggest herbivore in a Neotropical forest, rivalling all the mammals put together. Their cutting shapes which plants thrive, their enormous nests aerate and enrich the soil and channel water, and their refuse dumps are hotspots of nutrient recycling.
Status
Leafcutter ants are Least Concern and superabundant. In agricultural areas they are significant crop pests, capable of stripping young citrus, cassava, or eucalyptus overnight, and are the target of control efforts. In natural forest they are a keystone group, and their fungus-farming symbiosis is one of the most studied examples of mutual dependence in biology.
Leafcutter Ant: common questions
Do leafcutter ants eat the leaves they carry?
No. Leafcutter ants are farmers. The leaf fragments are transported into the nest, chewed into a moist pulp, and used as a growing medium for a specific fungus that the colony has cultivated for millions of years. The ants — and especially the larvae — feed on nutrient-rich swellings the fungus produces. The relationship is so complete that neither the ant nor the fungus can now survive without the other, and a new queen carries a pellet of the fungus in her mouth when she leaves to found a colony.
How is a leafcutter colony organised?
As a rigid division of labour based on body size. The smallest workers (minims) tend the fungus garden and ride on cut leaves to fend off parasitic flies. Slightly larger workers do most of the cutting and carrying. Larger workers clear trails and haul big loads, and the biggest — soldiers with massive heads and jaws — defend the colony and can cut through leather and skin. All are sterile daughters of the single queen, who can live 10–20 years and lay tens of millions of eggs.
How much do leafcutter ants actually eat?
A mature Atta colony harvests an estimated 200–400 kilograms of fresh leaf material a year and can defoliate a whole tree in a day. Across a tropical forest, leafcutters are typically the largest single consumers of plant matter — collectively taking as much vegetation as the region's mammalian herbivores combined — which makes them major shapers of forest structure and important recyclers of nutrients into the soil.
Why do leafcutter ants glow, or carry white stuff on their bodies?
The whitish coating often visible on leafcutter workers is a bacterium (an actinomycete) that the ants cultivate on their own cuticle. It produces antibiotic compounds that suppress Escovopsis, a parasitic mould that would otherwise overrun and destroy the fungus garden. It is one of the oldest known uses of antibiotics by any animal — tens of millions of years before humans discovered them.