One of the most striking modifications of mammalian economy by
environment is that shown in many small mammals of our southwestern
desert region and adjacent parts of Mexico, in which such species as
the kangaroo-rats, pocket-mice, prairie-dogs, and others are able to
exist under the most arid conditions without drinking. The liquid
necessary for supplying their bodily needs is obtained through chemical
action in their digestive tracts, whereby some of the starchy parts of
their food are changed into water.
Over considerable areas in the waterless deserts on the peninsula of
Lower California periods of from three to five years sometimes pass
without a drop of rain falling. In these areas the small desert mammals
named above, as well as wood-rats, white-footed mice, cottontails, and
jack-rabbits, are numerous and successfully pass these dry periods
without inconvenience. The absolute independence of water of these
animals has been demonstrated in southern California in the case of
pocket-mice kept for months in captivity in a box and fed solely upon
thoroughly dried seeds without their showing the slightest sign of
discomfort.
Our small mammals may be roughly classified by their food habits into
three main groups: Rodents, or gnawing animals; carnivores, or flesh
eaters, and insectivores, or insect eaters.