f. Burrowing. The rodents differ considerably in their tendency to burrow. This
habit is most highly developed in the Norway rat and lesser bandicoot; and as an
adaptation to burrowing, the ears of this species are small, and hairs in the ear
openings keep dirt out. The roof rat and Polynesian rat are more adapted to a life of
climbing, and burrows only in areas where Norway rats are absent. Its burrow system is
seldom extensive. House mice burrow where other harborage is not available. In and
around buildings, mice seldom have trouble finding cover, but in open fields they burrow
extensively. Rats normally burrow no more than about 18 inches downward and about
3 feet horizontally. There are reports, however, of rats digging to a depth of 5 or 6 feet
and of tunnels under sidewalks running the entire length of a city block. These longer
tunnels are usually made when the rats are searching for food. Their burrows will
usually contain, in addition to the entrance, one or more escape exits that may be lightly
covered with dirt or loose vegetation.
g. Gnawing. Nature seldom has provided an animal with a more effective
cutting tool than the rodent's front (incisor) teeth. Young rats and mice begin to gnaw as
early as the second week of life. Throughout their lives, the teeth keep growing rapidly.
In adult laboratory rats, the average growth for upper incisors is 4 1/2 inches a year, and
the lower incisors grow 5 3/4 inches. This fast growth allows continuous gnawing
without wearing out the cutting edge of the teeth. Rats and mice will gnaw almost
anything. Some of this gnawing may be only to keep the teeth short; it seems to serve
no other purpose. To get to food, rats and mice gnaw any material with a gnawing edge
that is softer than the enamel of their teeth. This includes such things as wood,
paperboard, cloth sacks, lead pipes, cinder blocks, asbestos, and aluminum. Roof rats
are even better at gnawing than are Norway rats.
h. Food Habits.
(1) Foods eaten. The Norway rat and the house mouse originally came from
grain-producing regions in Asia, where their diets probably consisted largely of this sort
of food. Knowledge concerning the type of country in which the roof rat originated is
more obscure. Nevertheless, all rodent species have become adapted to a very wide
range of foods. The choice of food is determined largely by the environment where the
rat or mouse is living. In instances where food was difficult to obtain from man, reports
tell of Norway rats stealing eggs, roof rats becoming serious pests in citrus groves, and
of house mice feeding extensively on insects.
(2) Foods and growth. Differences in the nutritive value of available foods
produce obvious differences in the sizes of rats and mice. Studies comparing farm rats
and city rats have shown the city rats to be much bigger. The farm rats ate only corn,
commercial horse feed, and fresh manure; the city rats, on the other hand, had a well-
balanced diet of garbage. When farm and city rats were given the same diet, they grew
at the same rate, indicating that most of the difference in size was due to difference in