Building a city – without an architect

New York, Rome, or Tokio are prominent examples of a human metropolis and seen as a demonstration of human ingenuity and culture. To reach their current size and architectural characteristics a myriad of planners, and architects have designed their buildings and roadways to provide living space and infrastructure to support human everyday life. Here, Daniela Römer explains how leaf-cutting ants create their subterranean nests.

A view compiled by Daniela Römer

Millions of years before the first humans started to congregate ‘en masse’ in one dwelling, a group of invertebrates – the social insects, bees, wasps, termites, and ants, were already displaying this form of life. These tiny animals with rather simple brains, manage to construct and maintain structures which can be thousands of times larger than the individuals themselves, very much like a human metropolis, and this without a single architect in sight!

My name is Daniela Römer, and I study the nest building behaviour of leaf-cutting ants at the University of Würzburg in Germany. Leaf-cutting ants life in the neotropics, between Argentina and the southern United states, and gained their name due to their foraging of massive amounts of life plant material, which they need for another feat normally associated with human culture and evolution – agriculture. These ants, and future generations, i.e., brood, are sustained by a symbiotic fungus which they rear in large masses in their underground nests. Therefore, these ants not only have to excavate living space for millions of individuals in the colony, but even more space for the symbiotic fungus growing in the nest chambers. When a group of scientists a few years ago mapped a leaf-cutting ant nest of the genus Atta, they needed 14 tons of wet cement to cast it and discovered thousands of nest chambers and an intricate system of tunnels. The single queen of the colony is only found in one of these chambers , so how can she guide the performance essential colony tasks, such as nest excavation?

The answer is, she doesn’t. With the exception of her digging the founding nest, a single, downward leading tunnel with an attached fist-sized chamber, all other nest chambers and tunnels are excavated by the workers, without any guidance or plan. Thousands of individual workers create nest space by removing tiny soil pellet after tiny soil pellet, each time reacting to local cues of their immediate environment to decide where to dig.

Here, abiotic cues play a big role, such as temperature, moisture or soil gas content. Evolution has shaped the worker’s preferences and avoidances of these climatic conditions in such a way, that ants dig more in places, where they find favorable conditions and less or not at all in unfavorable soil regions. The content of the nest chambers, fungus and brood, is also used as a cue for nest building, i.e., workers excavate more soil around such nest contents, leading to the emergence of chambers, while a reduced digging activity usually leads to the emergence of tunnels. When a nest becomes too small for the inhabiting colony the ensuing increased local density of the workers triggers a new bout of excavation until the resulting increase in space and spreading out of the workers makes them stop digging. Even other digging workers act as involuntary cues to other ants to start digging close by.

Ant nests offer their inhabitants all commodities found in human cities, like living quarters and child-rearing facilities, traffic roadways, and access to goods or waste removal services. However, these complicated and intricate structures with their millions of ‘citizens’ are the result of a self-organized process following simple rules and mechanisms that do not require a (human) architect.