City of millions underground

They live together with millions of other ants in a confined space, they operate agriculture and the biochemical industry in a perfect division of labour: the leaf-cutter ants.

The road in the Brazilian jungle is in perfect condition. No branch lies on it. How can it be? It is cleaned every day. Hundreds of thousands of leaf-cutter ants stream back to the nest on the hand wide mini motorway. They carry with them pieces of leaf on which tiny pygmy ants from the same colony sit. They are the air defence against humpback flies that want to throw themselves at the leaf bearers in order to lay eggs there: near the neck, from where the larvae then penetrate the ant body.

The path leads the caravan more than 100 metres through the forest and ends abruptly at the gates of a city with over a million inhabitants. Countless leaf-cutter ants of the Atta genus cavort there in the shade of a two-metre-high columnar dome of loosely thrown earth. At the entrance, workers hand over the fresh harvest to smaller nestlings, who take it to underground gardens to use it as fertilizer for mushroom cultures. At the same time other ants are busy building a ventilation shaft, while soldiers, who are eight times taller than their colleagues from the workers’ caste, keep watch and protect the nest from intruders.

Urban lifestyle, agriculture and perfect division of labour in the style of a highly developed society: it is only slowly that scientists begin to understand the fascinating social structures of these tropical ants, which researchers call “superorganisms”. In order to make the megapolis architecture of the leaf-cutter ants visible, a team led by Brazilian scientist Luiz Forti developed a process for casting the inside of the nest. A mixture of eight cubic meters of water and over six tons of cement was poured into the entrance. What could be made visible in this way amazed the scientists. They were amazed by a nest of more than 67 square metres, consisting of 1920 chambers, 238 of which were covered with mushroom gardens. They found a huge underground system of horizontal and vertical tunnels and canals that reached down to eight metres. The loose earth that the ants had transported to the surface during the construction of their nests and stored there weighed about 40 tons. Up to eight million ants live in such nests.

A symbiosis with various fungi, which began about 50 to 60 million years ago, is decisive for the nutrition of such giant peoples. In short, the animals cultivate and maintain mushroom cultures inside their nests and then feed on them. In addition to leaf-cutter ants, other social insects, such as some termite species, have also made the transition from being a hunter and gatherer to farming. Leaf-cutter ants of the genus Atta and Acromyrmex, however, are so advanced in their cultivation that they have achieved what the insect researcher Bert Hölldobler calls “ecological dominance”, an obvious mastery of the environment.

While most of the 230 mushroom-growing ant species native to South and Central America use rotten leaf pieces and dead organic material as fertilizers, 45 species, including Atta and Acromyrmex, started feeding the mushroom fresh leaves about ten million years ago. As a result, the fungus and ants were able to obtain a much higher nutrient density, which led to their colonies growing to several million individuals in the course of an evolutionary push. “This leap in development is quite comparable to that of humans when they started cooking food after the fire was tamed, thus making a much wider range of vitamins and minerals available to them,” says biologist Morten Schiøtt from the Centre for Social Evolution at the University of Copenhagen.

Together with his colleague Henrik De Fine Licht, Schiøtt has deciphered some parts of the complex chemical processes that take place when fresh leaves are fed to the fungus: The fungus grows like bread mould on the substrate from leaf remains, which are traversed by tubes and resemble a bath sponge in their structure. The leaf pieces dragged in by the ants would actually be inedible for the fungus: the phenols contained in the leaves and responsible for the defence against predators would damage the fungus and eventually cause it to perish. If there were not enzymes, which the fungus carries even in itself, unfortunately only not there, where it needs them. The enzyme-rich fungal cells are found in the tips of the hyphae, thread-like cells that grow inside the substrate.

The ants then pass through the tube ducts into the interior of the substrate, eat these cells, absorb the enzyme and excrete it on the freshly cut leaf material on the surface. In this way they make the leaves edible for the fungus.

Perfect waste management
This is a fascinating process that the researchers compare with bioreactors in which humans use enzymes to decompose organic material and produce ethanol, for example. Most Atta species also build special waste chambers for the emaciated substrate, which is left behind by the fungus. The Atta colombica have therefore started to dispose of the material outside the nest. The leaf-cutter ants are therefore also proficient in waste management.

Text: Elmar Jung, Bilfinger Magazin 2/2013 

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