runways and habitually follows the same courses. It is found in basements, in burrows
under slabs or the floors of unexcavated buildings, in dumps or rubbish piles, in earthen
banks, in waterfront riprap and jetties, in drains and sewers, and in houses, docks,
piers, and warehouses.
b. Roof Rat.
(1) The roof rat or Alexandrine rat (Rattus rattus alexandrinus), the black or
ship rat (Rattus rattus rattus), and the fruit tree, white-bellied, or yellow-bellied rat
(Rattus rattus frugivorus) are closely related, with generally similar habits. Full-grown
specimens range in weight from 4 to 12 ounces (110 to 340 grams). The tail of the
adult rat is longer than the combined length of head and body and is uniformly colored.
The ears are noticeably larger than those of the Norway rat and lack the hairy coating of
the latter. The female has 10 mammae. The body is slender with a pointed muzzle.
(2) Sexual maturity is reached at the age of 2 to 3 months and the gestation
period averages 22 days. There is an average of six to eight offspring per litter and an
average of four to six litters per year.
(3) Three color varieties of the roof rat occur in the United States; however,
every degree of intergradation occurs between them. Rattus rattus rattus is black to
slate-colored on both the back and the belly. Rattus rattus alexandrinus has a tawny
back and a belly that is grayish white but is never clear white or lemon-colored. The
hairs on the belly are always slate-colored at the base, except occasionally on the throat
and chest where the hairs continue the same color to the base. Rattus rattus frugivorus
also has a tawny back, but its belly is white or lemon-colored. The hairs on the belly are
white or buff-colored to the base. There is usually no gray margin at the line of
demarcation between the color of the back and the whitish belly.
(4) The feeding and scouting range of these rats is more extensive than that
of the Norway rat. They are excellent climbers and commonly frequent the upper parts
of buildings. They seldom live in burrows, preferring to build their nests in attics, trees,
and shrubbery. In the absence of Norway rats, however, their nests may be found
beneath the floors of barns, feed houses, and similar buildings. Unlike the Norway rat,
the roof rat is not fond of entering water.
c. House Mouse (Mus musculus).
(1) The house mouse is readily recognized by its small size. House mice will
weigh between one-half and three-fourths ounce (14 to 20 grams) at maturity. The
color is usually grayish to brown but may be black or pale gray. The underside will vary
from white to dark gray. The head and body are small, together totaling only 2 1/2 to 3
1/2 inches (65 to 90 mm). The tail length is equal to or greater than the combined
length of head and body. The house mouse is compared with the Norway rat and the
roof rat pictorially (see figure 1-2) and descriptively in figure 1-3.