How Do Prairie Dog Tunnels Work?

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The ecological design expert Sim Van der Ryn once pointed out that “in a sense, evolution
is nature’s ongoing design process.” Designers and architects had for decades been slow to
understand this concept, especially in the industrialized world. Agrarian societies, by  contrast, built structures more in keeping with the environment. In tropical Asia and Africa huts often stand on stilts to let breezes flow through and avoid moist ground, while desert dwellings consist of thick mud walls that cool interiors. It should come as no surprise that wildlife architecture, such as nests and dens, possesses the same type of melding with nature. Rodents called black-tailed prairie dogs that live on dry prairies and hot climate grasslands use a similar ingenious design with nature.

Black-tailed prairie dogs form colonies or towns of a dozen to a few dozen individuals that
cooperate in hunting, as sentries, and for building burrows. Each burrow contains a complex of tunnels and one or more mounds. The tunnels and mounds shelter adults and pups, resist predators, and regulate temperature and ventilation. Prairie dog burrows may spread out to 100 acres (0.4 km2), but, because the animal’s habitat has been dwindling in recent years, tunnels usually extend only 15 feet (4.6 m). Large or small, prairie dog colonies use the local environment to their advantage.

Prairie dog burrow entrances consist of dome mounds and crater mounds. A prairie dog
builds a dome mound by digging upward from an underground tunnel until it breaks through the surface. The prairie dog spreads the excavated soil in a layer around the entrance and then pushes it into a dome that surrounds and sometimes covers the entrance. Meanwhile, other prairie dogs build crater mounds as secondary entrances and emergency escapes. The crater begins as a ring of uprooted vegetation, humus (organic soil), and mineral soil. The prairie dog waits for the next rain to moisten the soil and then compacts it, adding bits of plants to strengthen the mound. Such volcano-shaped crater mounds can tower up to two feet (0.6 m).

Prairie dogs situate the two different mounds at opposite ends of a burrow. When an aboveground breeze blows, air enters through the lower mound and leaves through the higher mound to cause unidirectional airflow through the tunnels, not only cooling the burrow but ventilating it. But what happens inside very large burrows with intricate arrangements of thoroughfares and dead spaces? The dual mound construction creates air exchange throughout the tunnels due to a feature called a velocity gradient, which can occur in flowing air or water. A wind velocity gradient forms as wind blows over an open area. The air passing near the ground slows due to friction with the Earth, while air blowing at higher altitudes moves faster. These sheets of air supply the energy that pulls air into one burrow opening and out the other. (Because high-altitude winds move faster than lower breezes, they contain more energy. Therefore, the different-speed winds set up an energy gradient that corresponds to the velocity gradient.) The underground airflow moves at the same speed as the air blowing directly above Earth’s surface, draws stagnant air from
dead-end tunnels, and pulls in fresh air.

A prairie dog burrow with the air circulation system described here represents a zero energy
design, which means that cooling, ventilation, and other functions occur without any need for energy input. The prairie dogs expend energy to build tunnels and mounds, but once those jobs are done, Mother Nature furnishes all the energy to run the circulation system. Prairie dogs even fine-tune their burrows by adjusting the shape of an exit mound’s rim, depending on the local topography, wind exposure, humidity, and temperature. Sharp-rimmed mounds allow the most efficient airflow, so animals need these mounds for habitats with weak breezes or poor wind velocity gradients. In better wind conditions, the animals shape the mound into a blunt-style rim. Prairie dogs have therefore evolved into clever architects and likely seek places to live where the breezes are favorable for new burrows.

Humans can seldom achieve the prairie dog’s skill in combining form and function, which
means that the shape and size of a structure connects with how it works. Ecological designers can use animal dens, caves, or burrows to learn how structures work with or against local weather patterns and topography. Nature surely provides the framework for creating low-energy or zero-energy heating, cooling, ventilation, lighting, and water flow systems for new ecological buildings.