These trenches are continually deepened and lengthened as the burrow progresses, the angular segment of earth between them, scratched away, until by degrees it has been entirely conveyed off, and in its place is the one deep great unsymmetrical mouth I have already described. There are soils that will not admit of the animals working in this manner. Where there are large cakes of "tosca" near the surface, as in many localities on the southern pampas, the vizcacha makes its burrow as best he can, and without the regular trenches. In earths that crumble much, sand or gravel, he also works under great disadvantages.
The burrows are made best in the black and red moulds of the pampas; but even in such soils the entrances of many burrows are made differently. In some the central trench is wanting, or is so short that there appear but two passages converging directly into the burrow; or these two trenches
Biography of the Vizcacha. 295
may be so curved inwards as to form the segment of a circle. Many other forms may also be noticed, but usually they appear to be only modifications of the most common Y-shaped system.
As I have remarked that its manner of burrowing has peculiarly adapted the vizcacha to the pampas, it may be asked what particular advantage a species that makes a wide-mouthed burrow possesses over those that excavate in the usual way. On a declivity, or at the base of rocks or trees, there would be none; but on the perfectly level and shelterless pampas, the durability of the burrow, a circumstance favourable to the animal's preservation, is owing altogether to its being made in this way, and to several barrows being made together. The two outer trenches diverge so widely from the mouth that half the earth brought out is cast behind instead of before it, thus creating a mound of equal height about the entrance, by which it is secured from water during great rainfalls, while the cattle avoid treading over the great pit-like entrances. But the burrows of the dolichotis, armadillo, and other species, when made on perfectly level ground, are soon trod on and broken in by cattle; in summer they are choked up with dust and rubbish; and, the loose earth having all been thrown up together in a heap on one side, there is no barrier to the water which in every great rainfall flows in and obliterates the kennel, drowning or driving out the tenant.
I have been minute in describing the habitations of the vizcacha, as I esteem the subject of prime importance in considering the zoology of this
296 The Naturalist in La Plata.
portion of America. The vizcacha does not benefit himself alone by his perhaps unique style of burrowing; but this habit has proved advantageous to several other species, and has been so favourable to two of our birds that they are among the most common species found here, whereas without these burrows they would have been exceedingly rare, since the natural banks in which they breed are scarcely found anywhere on the pampas. I refer to the Minera (Geositta cunicularia), which makes its breeding-holes in the bank-like sides of the vizcacha's burrow, and to the little swallow (Atticora cyanoleuca) which breeds in these excavations when forsaken by the Minera. Few old vizcacheras are seen without some of these little parasitical burrows in them.