Leaf-Cutter Ant

The leaf-cutter ant lives in huge underground nests, connected by a series of tunnels. The ants cultivate a special ‘fungus garden’ deep within the nest, and are almost entirely dependent on the fungus for food. Maintaining the garden is crucial to the survival of the colony, and worker ants perform a variety of tasks, including foraging for leaves, cutting them into suitably sized fragments,  transporting leaf fragments back to the colony, and preparing a ‘mulch’ (made from the leaves), which is used to cultivate the fungus garden. Some of the smaller ants ‘hitchhike’ on leaves carried back to the colony, and are thought to protect the foraging ants from parasitic flies (Phoridae), and may also play a part in leaf preparation. It is essential that the fungus garden remains free of parasites that could cause disease, which would be devastating to the leaf-cutter ant colony. Microorganisms that have the potential to be harmful to the fungus are removed by some of the smaller garden workers as waste, which is taken to a separate waste chamber, reducing the chance that the fungus, or other ants in the colony, will become infected by harmful pathogens.

In a colony, only the queen is able to produce offspring. The queen is capable of laying thousands of eggs per day, most of which are destined to become workers, with only a small number of these developing into males and females capable of reproduction. At the beginning of the rainy season, fertile individuals leave the nest to take part in a ‘nuptial flight’, a single flight during which mating occurs, and after which the males die. This is the only time that the females mate, and the potential queens are capable of storing several hundred million sperm, which are used to fertilise the eggs in a future colony. A new colony is created by a solitary female queen, who will dig a tunnel, and, using a tiny piece of fungus brought from the old nest inside a special cavity in the mouth, will start to cultivate a new fungus garden and begin egg laying. Despite the large numbers of leaf-cutter ant queens that attempt to establish a colony, very few actually survive, with the probability that the founding queen will die before eggs hatch and the fungus garden becomes established estimated at nearly 90 percent.

## Description

*Atta cephalotes*is a leaf-cutting, fungus-growing ant, with one of the most fascinating and complex social systems known to science. Colonies of this leaf-cutter ant species contain millions of individuals, making it possibly the most dominant invertebrate in Central and South America. A colony is made up of different classes of ant, known as castes, including the queen, workers, and at certain times, males and females (queens) that are capable of reproduction. Each individual within the colony carries out a specific job depending on its size and caste, in a behaviour known as ‘task partitioning’. As in all ant species, individuals in the worker caste of the leaf-cutter ant are wingless, sterile females of different sizes, depending on the role played within the colony. ‘Soldiers’ act to protect the colony and are the largest in the worker caste. A nest of the leaf-cutter ant will also contain tiny ‘minima’ workers, which work inside the colony and in the fungus garden, and ‘media’ and ‘maxima’ workers, larger ants with powerful jaws, which cut and transport leaf fragments back to the nest. Males are bigger than the workers, whilst the queens are larger still. Within a colony, only the males and the new queens will develop wings and are able to mate.

## Range and Habitat

Widely distributed throughout Central and South America, ranging from Mexico in the north to Argentina in the south. The leaf-cutter ant is highly specialised to live in forest gaps, and colonies are most often found on farms and plantations, in rainforests, and in forest patches (usually mature or old-growth forest.

## The Lifecycle of a Leaf-Cutter ant Colony

### **Reproduction and colony founding**

Winged females and males leave their respective nests *en masse* and engage in a nuptial flight known as the *revoada*. Each female mates with multiple males to collect the 300 million sperm she needs to set up a colony. Once on the ground, the female loses her wings and searches for a suitable underground lair in which to found her colony. The success rate of these young queens is very low, and only 2.5% will go on to establish a long-lived colony. To start her own fungus garden, the queen stores bits of the parental fungus garden mycelium in her infrabuccal pocket, which is located within her oral cavity.

### **Colony Hierarchy**

In a mature leafcutter colony, ants are divided into castes, based mostly on size, that perform different functions. *Acromyrmex* and *Atta* exhibit a high degree of biological polymorphism, four castes being present in established colonies—minims, minors, mediae, and majors. Majors are also known as soldiers or dinergates. *Atta* ants are more polymorphic than *Acromyrmex*, meaning comparatively less difference occurs in size from the smallest to largest types of *Acromymex*.

* Minims are the smallest workers, and tend to the growing brood or care for the fungus gardens. Head width is less than 1 mm.
* Minors are slightly larger than minima workers, and are present in large numbers in and around foraging columns. These ants are the first line of defense and continuously patrol the surrounding terrain and vigorously attack any enemies that threaten the foraging lines. Head width is around 1.8–2.2 mm.
* Mediae are the generalized foragers, which cut leaves and bring the leaf fragments back to the nest.
* Majors, the largest worker ants, act as soldiers, defending the nest from intruders, although recent evidence indicates majors participate in other activities, such as clearing the main foraging trails of large debris and carrying bulky items back to the nest. The largest soldiers (*Atta laevigata*) may have total body lengths up to 16 mm and head widths of 7 mm.