How Big Are Leafcutter Ant Colonies? Millions of Ants

How many ants live in a leafcutter colony?

Colony size depends on species and age, but the big Atta leafcutters are the record holders among New World ants. Mature colonies of the leafcutter ant Atta cephalotes are estimated to hold up to several million workers. For Atta sexdens of South America, estimates for the largest colonies run to roughly 5–8 million ants.

Counting a colony exactly is almost impossible, because the ants are spread through hundreds of chambers and tunnels and many are tiny. Researchers estimate numbers from samples of fungus gardens, from the number of chambers and from the volume of the nest. Figures are therefore best read as orders of magnitude: hundreds of thousands in a growing colony, millions in a mature one.

The related genus Acromyrmex, also called leafcutters, has much smaller colonies, typically numbering in the tens of thousands to a few hundred thousand.

How big are leafcutter nests?

From above, an Atta nest looks like a broad patch of bare, loose soil dotted with entrance holes, sometimes with mounds of excavated earth. In Atta cephalotes, the area of a mature nest is often tens of square meters and can approach about 200 m² (about 2,150 sq ft). Below ground, tunnels run several meters deep.

Scientists in Brazil have studied whole nests by excavating them or by pouring liquid cement into the tunnels and digging out the hardened cast. An often-cited excavation of a several-year-old Atta sexdens nest found more than 1,000 chambers, and the ants had moved about 40 tonnes (44 short tons) of soil to build it. Some chambers hold fungus gardens, others hold waste, and the whole system is designed so that air flows through and removes the carbon dioxide produced by the colony.

How does a colony start?

Every giant colony begins with one queen. Once a year, usually at the start of the rainy season, a mature colony releases thousands of winged queens and males on a mating flight. Each queen mates with several males in the air and stores hundreds of millions of sperm in her body, enough for the rest of her life.

After landing, the queen sheds her wings, digs a narrow tunnel and seals herself in a small chamber. She brings a piece of fungus from her mother colony in a pocket inside her mouth and uses it to start a tiny garden, as explained in how do leafcutter ants grow fungus? She does not leave to find food. Instead she lives on her body reserves and lays eggs, feeding some of them to her first larvae.

The first small workers appear after roughly two months. They open the nest to the outside, start cutting leaves and take over the garden, and from then on the queen does nothing but lay eggs.

How fast do leafcutter colonies grow?

Most new colonies never get going. Queens are eaten by birds, lizards, armadillos and ants on the day of the mating flight, and many others die when their small gardens fail. Only a small fraction of young queens survive to found a lasting colony.

A colony that survives grows slowly in its first year, then faster as the worker force and fungus gardens expand. After about three years or more, it reaches the size at which it can afford to raise winged queens and males of its own. A mature colony can produce thousands of them every year, though few of these will succeed in turn.

How is such a big colony organized?

With millions of members, a leafcutter colony works through division of labor rather than central control. The queen only lays eggs; she does not give orders. Workers of different sizes take on different jobs:

• The smallest workers tend the fungus gardens and the young.
• Small to medium workers patrol, cut and carry leaves.
• The largest workers, or soldiers, defend the nest against larger enemies such as army ants and mammals.

Age also matters. Young workers tend to stay deep in the nest, while older workers take riskier jobs outside or in the waste chambers. Information spreads through chemical trail signals, touch and vibrations. Because the colony behaves like a single, coordinated organism, biologists often call it a superorganism, a term also applied to honey bee colonies (see western honey bee).

How do leafcutter colonies compare with other insect societies?

Leafcutter colonies are among the largest insect societies, but they are not the only giants. Some army ant colonies of Africa contain many millions of workers, and some termite colonies are also huge. What makes leafcutters stand out is the combination of size and complexity: millions of individuals running an underground farm.

A large leafcutter colony can harvest an enormous amount of vegetation in a year. Through this harvest, its soil digging and its waste chambers, a single nest reshapes the forest floor around it, which is why ecologists describe leafcutters as ecosystem engineers.

Myths and facts

Myth: The queen rules the colony. Fact: The queen is the colony’s only egg-layer, but she does not direct the workers. Order arises from simple rules followed by millions of ants.

Myth: A colony can live forever by raising a new queen. Fact: Atta colonies do not replace their founding queen, so a colony’s life is limited to hers, about 10–20 years.

Myth: All leafcutters build giant nests. Fact: Only Atta builds the huge cities. Acromyrmex leafcutters live in much smaller colonies and nests.