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 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 eveiy great rainfall flows in and
obliterates the kennel, drowning or driving out the tenant.
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