What Is a Burrow System? Architecture Built Without Plans

The elaborate examples share a set of functional components. A nest chamber, usually lined and located where temperature is most stable, houses the sleeping and breeding animals. Storage chambers hold food in species that cache. Separate latrine chambers keep waste away from living space, and some species seal and abandon them when full. Multiple entrances provide escape routes, which is the difference between a burrow and a trap. Blind side tunnels serve as turning places in systems too narrow to reverse in, and as boltholes. Depth varies through the system, with parts placed below frost and others near the surface for warmth. In colonial species the whole arrangement is multiplied, with separate family areas connected by shared tunnels, and the largest recorded systems cover areas measured in hectares and have held very large numbers of animals.

Building underground creates difficulties that the structures address:

• •Ventilation, since a sealed tunnel accumulates carbon dioxide, solved by entrances at different heights that drive airflow by pressure difference
• •Flooding, addressed by sloped tunnels, drainage sumps and chambers above the tunnel floor
• •Collapse, managed by tunnel diameter, by digging in soils with enough cohesion and by the shape of the cross section
• •Temperature, exploited by placing chambers at depths where daily and seasonal variation is damped
• •Predator access, limited by entrance size, by bends and by plugging tunnels behind
• •Spoil disposal, which is a real problem for a long tunnel and produces the characteristic mounds that mark the systems above ground

The interesting question is how coordinated architecture arises without any animal holding a plan. The answer in most studied cases is that simple local rules produce the structure, with each digging decision made in response to immediate conditions rather than to an overall design. An animal digs until a tunnel reaches a certain length and then branches, turns when the soil resistance changes, and enlarges a space when it needs to turn around, and those rules applied repeatedly generate branching systems with appropriate dimensions. In social species the material itself coordinates behaviour, since the presence of a partially built structure triggers the next stage of work in whoever encounters it, which allows many individuals to build something none of them is directing. That mechanism, where the work in progress communicates what to do next, is one of the better-understood results in the study of collective behaviour.

A handful of species build structures well beyond the ordinary. Prairie dog colonies in North America once covered areas measured in thousands of square kilometres and held populations in the hundreds of millions, with the largest recorded system among the biggest structures built by any animal. Termite mounds in Africa and Australia reach several metres above ground and function as ventilation chimneys for a nest below, with the internal airflow driven by temperature differences that the structure is shaped to exploit. Naked mole rats maintain extensive systems with a social organisation resembling that of insects and a physiology adapted to the low oxygen and high carbon dioxide of a sealed environment. Some ocean floor invertebrates build burrow networks that reach metres into sediment. In each case the structure supports a way of life that would be impossible without it, which is why the digging is worth the enormous energetic cost.

The structures outlast their builders and support communities that never dig. Abandoned burrows are occupied by other mammals, by reptiles, by amphibians seeking moisture, by birds that nest underground and by very large numbers of invertebrates, and in several ecosystems the digging species is classified as an ecosystem engineer because removing it removes habitat for everything else. Tortoise burrows in the southeastern United States are a documented case, with hundreds of species recorded using them. The digging also alters the soil itself, mixing layers, increasing water infiltration and changing where nutrients sit, which changes what plants grow above. Where the digging species has been removed or persecuted as a pest, the consequences have been broader than expected and have prompted reintroductions. The same structures appear in the fossil record as trace fossils, so the practice and its ecological importance are ancient rather than recent.

Elaborate systems contain nest chambers, stores, latrines, escape routes and turning places, arranged to solve ventilation, flooding, collapse and temperature. Entrances at different heights drive airflow by pressure difference. The architecture arises from simple local rules and from the partially built structure triggering the next stage, with no animal holding a plan. Abandoned burrows house hundreds of other species.