Ontogeny looks at the development of an organism, from its DNA to the different stages of its development. In the case of ant-fungus mutualism, the discussion of ontogeny encompasses the growth and maintenance of the fungus within the colony, and its transmission to other colonies. The polymorphic caste system has a strong genetic basis, but the environmental aspect of cannot be ignored.

Ontogeny of Ant Colonies

nest structure
A. Vegetation is cut from nearby trees by workers following pheromone trails. Research has shown that ants do not memorize the location of food sources but depend on following scent trails left by scout ants. A classic example of emergence, the scent trails produces a cascade effect: the more ants that follow a given path, the more compelling it scent, attracting greater numbers of ants.

B. Leaves are carried back to the nest along the same trails, reinforcing them with additional pheromone markers.

C. Leaf sections are taken to the nest where they are masticated by smaller minims specialized for preparing leaves as fungal substrate.

D. The processed leaves are used to mulch the fungal garden. Fungus digest the leaves and use the nutrients to fuel their own growth as well as the production of nutrient-rich gongylidia for ant consumption.

E. The queen lives and lays her eggs within the fungus garden. After the larvae hatch, they feed on the gongylidia while being cared for by nurse workers.

F. Waste material is meticulously removed from the environment and carried to dump chambers (which also serve as a disposal for dead ants and fungus) or removed from the nest entirely.

Figure 3. Overview of leaf cutter nest structure. [Image courtesy of the Universities of Copenhagen and Aarhus in Denmark]

Development of the Colony

A young winged queen, having mated with enough males to supply her with enough sperm to last 20 years, leaves her home colony and finds a suitable site to create a new nest. She breaks off her wings and begins to excavate a tunnel, then creates a small chamber off of the main shaft. She deposits fungal spores from her mouth and harvests leaf matter as substrate throughout the 70 days it takes the first clutch of eggs to hatch (11). As more workers mature and become able to gather leaves, the colony expands, digging further chambers to house additional fungus gardens. The queen then takes up more permanent residence in the main fungus chamber and devotes her energies to her brood. After two to three years, the colony generally reaches a size of several hundreds of thousands of individuals and begins producing winged forms to create new colonies (11). At this point, the nest is considered "mature", though it will continue to expand as long as additional resources and space is availabe.

3d casts of nests
Figure 4. Cement casts of Acromyrmex rugosus rugosus nests in Brazil. (a) A nest 3.75 meters deep, the deepest reported in the study; (b) A smaller nest with a mound area of 0.27 square meters; (c) A nest from a cow pasture; and (d) A nest with 26 chambers and a mound area of 9.89 square meters, the largest in the study in both regards.[Image courtesy of S. S. Verza, et al (2007)]

Initially, the colony is very sensitive to environmental pressures, depending entirely upon the survival of the queen. A mature colony, though, can grow to cover a huge underground area. One study has found that colonies are most prevalent in areas where excavation is difficult, such as areas on inclines or near buildings or trees (N). Roots, however, are sometimes integrated as parts of tunnels or chambers. Nests of this size and complexity are a huge investment, but by allowing for the creation of microclimates optimized for fungal and larval growth, they are well worth the effort.

Learning About the Garden

Ants appear sensitive to chemical cues from their cultivars indicating unsuitable species of leaves and learn to reject injurious substances. A allomone secreted by the fungus is disseminated among the worker ants through tactile contact, leading to the avoidance of the toxin for weeks at a time (3). Like some species of bees, leaf cutter ants ants are thought to learn to odour of a food fragment brought by a successful scout and use this as a criterion for decision-making while searching for food (8). It is uncertain the degree to which ants actually learn which species of plant are acceptable and unacceptable for maintaining fungal gardens, but evidence supports the idea that familiar plants are preferable to unfamiliar ones(8). It is likely that the decision-making process is heavily influenced by chemical signals, but the nuances of this system and the significance of learning in foraging behavior are currently uncertain.