Prairie dogs are highly social rodents that build extensive underground burrows in the plains of North America to house their family groups. The burrows can reach 10 m (32 ft) in length, and this size means that diffusion alone is not sufficient to replace used air inside the burrow with fresh air. The way that a prairie dog builds the openings to its burrow, however, helps to harness wind energy from the windy plains and create passive ventilation through the burrow’s tunnels.
As air flows across a surface, a gradient in flow speed forms, where air moves slower the closer it is to the surface. The prairie dog is able to take advantage of this gradient by building a mound with an elevated opening upwind and a mound with a lower opening downwind. Over the elevated opening, wind velocity is faster than it is over the lower opening, creating a local region of low pressure (following Bernoulli’s principle). The result of this difference in pressure between the two openings is one-way air flow through the burrow as air gets sucked into the lower opening and flows out the elevated one. This is the mechanism behind a Venturi tube.
The mounds around the burrow openings serve additional functions for the prairie dog, like providing a perch to watch for predators. Other organisms use a similar arrangement of openings to generate passive flow, including sea sponges and limpets.Edit Summary
“Where a fluid flows across a surface, such as wind over the earth, the velocity gradient created provides a potential source of work. This gradient might be employed by one burrowing animal to induce air-flow in its long, narrow burrow. The burrow of the black-tailed prairie-dog constitutes a respiratory dead-space of extraordinary magnitude in which diffusion appears inadequate for gas exchange. But the burrow is arranged in a manner appropriate for wind-induced ventilation, typically with two openings at opposite ends and with mounds surrounding these openings of two forms (Fig. 3), with one form on each end. When a breeze crosses the mounds, air enters the burrow through the lower mound and leaves through the higher. The same unidirectional flow is evident with scale models of real mounds on a model burrow in a wind tunnel; flow inside the burrow is nearly a linear function of flow across the mounds (Fig. 4). Wind-induced ventilation in the model burrow could also be induced with model mounds differing in shape but not height. Mounds with sharp rims were more effective exits for air than mounds with rounded tops; in nature such shape differences complement the differences in height.” (Vogel et al. 1973: 1)