The prairie-dog is truly a tenement dweller, and his home is occupied not only by his own kind, but by owls and rattlesnakes. Most naturalists believe that these incongruous families live in perfect harmony; but it is a well-known fact that the snake occasionally devours the young prairie-dogs, and he must be considered by them as an intruder who procured board and lodging without their consent. The owls, on the other hand, are supposed to do no harm, although it may be that they also occasionally feast on a tender young pup.
The magnificent little animals known to scientists as vizcachas, and whose homes are on the pampas of South America, are the most skilled builders of underground cities in the animal world. Their villages or cities are called "vizcacheras" and are provided with from ten to twenty mouths or subway entrances, with one entrance often serving for several holes. If the ground is soft, it is not uncommon to find twenty to thirty burrows in a vizcachera; but if the ground is rocky and hard, only four or five burrows are found. These wide-mouthed, gaping burrows are dug close together, and the entire town usually covers from one hundred to two hundred square feet.
The vizcacheras are different from other underground animal cities; some of the burrows are large, others are small. Most of them open into a subterranean main-street at from four to six feet from the entrance; from this street other streets wind and turn in all directions, like a man-made subway, and many of them extend clear into other streets or subways, thus forming a complete network of underground passageways. All the tunnelled-out dirt is brought to the surface and forms a large mound to prevent the water from entering the cities.
According to W. H. Hudson, in _The Naturalist in La Plata_, "in some directions a person might ride five hundred miles and never advance half a mile without seeing one or more of them. In districts where, as far as the eye can see, the plains are as level and smooth as a bowling-green, especially in winter when the grass is close-cropped, and where the rough giant-thistle has not sprung up, these mounds appear like brown or dark spots on a green surface. They are the only irregularities that occur to catch the eye, and consequently form an important feature in the scenery. In some places they are so near together that a person on horseback may count a hundred of them from one point of view."