The Attini

There are many different species of fungus-growing ants. All are members of the tribe Attini, and all have a mutualistic association with a fungus, but they vary in other aspects, such as colony size and the foods that they collect for their fungus. Recent genetic and morphological work by Ted Schultz, Rudolf Meier and James Wetterer (Schultz & Meier 1995; Wetterer et al. 1998) has enabled a phylogeny to be produced for the fungus-growing ants. This is like a family tree showing how the fungus-growing ants have evolved. It should be emphasised, however, that this phylogeny should be treated as provisional, as recent research suggests that some of the relationships may be rather different.

The nearer to the root of the tree a particular fungus-growing ant branches off, the lower it is in the phylogeny. The phylogenetic tree below is simplified to just show the genera (groups of species) of fungus-growing ants. The root of the tree is to the left. Not included in the tree is the genus Pseudoatta, which is a social parasite of Acromyrmex colonies that does not have any workers and does not culture its own fungus.

The lower attines include most of the fungus-growing ant genera.

These ants have most features in common with the non-fungus-growing ants from which the fungus-growing ants evolved

The higher attines include the leaf-cutter ants, Sericomyrmex, Trachymyrmex and some other small genera
Atta and Acromyrmex are highest in the phylogeny. These are the leaf-cutter ants.
There are several features of the fungus-growing ants that correlate with how low or high they are in the phylogeny. Generally, the lower in the phylogeny a fungus-growing ant is, the more similar it is presumed to be to its non-fungus-growing ancestors.

The lower attines

The lower attines generally have quite small nests, with a few hundred workers. They culture their fungus on a variety of materials, including grasses, leaf-litter, the fæces of other insects, and dead insects themselves.

The nests they make are usually small and quite simply constructed, with a single small fungus garden in the soil. The genus Apterostigma often builds its fungus garden under rocks or logs, although the picture to the left shows an Apterostigma nest in soil.

All the worker ants within a lower attine colony are usually similar in size and appearance.

The higher attines
The higher attines, which include the leaf-cutter ants, culture their fungus on plant material, sometimes in the form of insect fæces. Their nests are larger, with thousands to millions of workers and may be quite complex. They are normally constructed as a series of excavated tunnels around one to several chambers with fungus gardens. The picture to the right shows the funnel-like structure built around the entrance to the nest of one species of Trachymyrmex.

There are often several castes of workers in a nest. Typically this means that there are a range of sizes of workers which specialise on particular tasks.

The leaf-cutter ants

The leaf-cutter ants are the genera Acromyrmex and Atta. They show the most extreme versions of the traits possessed by the higher attines. They culture their fungus on leaf fragments cut from living plants, and will often collect material over a long distance.

The genus Acromyrmex

Acromyrmex ants have mature nests with thousands to tens of thousands of workers. The fungus garden usually occupies a volume of a few litres, and is housed in either one large chamber or many small chambers.

Workers cut leaves individually and bring them back to the nest, although in particularly large nests trail systems may become established. There are several different castes of workers - minor, media and major - but no soldier caste.

Acromyrmex species have quite variable social structure, with several species having multiple queens. It is also the only genus of fungus-growing ants known to suffer from exploitation by social parasites. Most of the research at the Universities of Copenhagen and Aarhus on fungus-growing ants is carried out on Acromyrmex.

The genus Atta

Ants in the genus Atta are what most people think of as typical leaf-cutter ants. There are several different species which vary in colony size and social structure.

Some Atta colonies reach sizes of several millions or tens of millions of workers, and have nests occupying several cubic metres (and necessitating the excavation of several tons of earth). These large colonies form trails through the forest that may stretch for 100-200m. They will often defoliate a tree before establishing a trail to a new source of leaves.

There are several different worker castes, and a specialised soldier caste.

The picture to the left shows the heads of (from left to right) a soldier, a major worker, a media worker and a minor worker of Atta cephalotes. minim workers are even smaller. These heads were collected from the dump chamber of a laboratory colony. The soldier head is about 6 mm across.