Norway Rat — Rattus norvegicus
Roof Rat — Rattus rattus
BIOLOGY AND LIFE CYCLE OF THE RAT
Rats, like house mice, are mostly active at night. They
have poor eyesight, but they make up for this with their
keen senses of hearing, smell, taste, and touch. Rats
constantly explore and learn about their environment,
memorizing the locations of pathways, obstacles, food
and water, shelter, and other elements in their domain.
They quickly detect and tend to avoid new objects placed
into a familiar environment. Thus, objects such as traps
and baits often are avoided for several days or more
following their initial placement. While both species
exhibit this avoidance of new objects, it is usually
more pronounced in roof rats than in Norway rats.
Both Norway and roof rats may gain entry to structures
by gnawing, climbing, jumping, or swimming through sewers
and entering through the toilet or broken drains. While
Norway rats are more powerful swimmers, roof rats are
more agile and are better climbers.
Norway and roof rats do not get along. The Norway rat
is larger and the more dominant species; it will kill
a roof rat in a fight. When the two species occupy the
same building, Norway rats will dominate the basement
and ground floors, with roof rats occupying the attic
or second and third floors. Contrary to some conceptions,
the two species cannot interbreed. Both species may
share some of the same food resources but do not feed
side-by-side. Rats may grab food and carry it off to
Rats of either species, especially young rats, can
squeeze beneath a door with only a 1/2-inch gap. If
the door is made of wood, the rat may gnaw to enlarge
the gap, but this may not be necessary.
Norway Rats. Norway rats eat a wide variety of foods
but mostly prefer cereal grains, meats, fish, nuts,
and some fruits. When searching for food and water,
Norway rats usually travel an area of about 100 to 150
feet in diameter; seldom do they travel any further
than 300 feet from their burrows or nests. The average
female Norway rat has four to six litters per year and
may successfully wean 20 or more offspring annually.
Roof Rats. Like Norway rats, roof rats eat a wide variety
of foods, but their food preferences are primarily fruits,
nuts, berries, slugs, and snails. Roof rats are especially
fond of avocados and citrus and often eat fruit that
is still on the tree. When feeding on a mature orange,
they make a small hole through which they completely
remove the contents of the fruit, leaving only the hollowed
out rind hanging on the tree. The rind of a lemon is
often eaten, leaving the flesh of the sour fruit still
hanging. Their favorite habitats are attics, trees,
and overgrown shrubbery or vines. Residential or industrial
areas with mature landscaping provide good habitat,
as does riparian vegetation of riverbanks and streams.
Roof rats prefer to nest in locations off the ground
and rarely dig burrows for living quarters if off-the-ground
Roof rats routinely travel up to 300 feet for food.
They may live in the landscaping of one residence and
feed at another. They can often be seen at night running
along overhead utility lines or fence tops. They have
an excellent sense of balance and use their long tails
for balance while traveling along overhead utility lines.
They move faster than Norway rats and are very agile
climbers, which enables them to quickly escape predators.
They may live in trees or in attics and climb down to
a food source. The average number of litters a female
roof rat has per year depends on many factors, but generally
is three to five with from five to eight young in each
DAMAGE CAUSED BY RATS
Rats consume and contaminate foodstuffs and animal feed.
They also damage containers and packaging materials
in which foods and feed are stored. Both species of
rats cause problems by gnawing on electrical wires and
wooden structures (doors, ledges, in corners, and in
wall material) and tearing up insulation in walls and
ceilings for nesting.
Norway rats may undermine building foundations and
slabs with their burrowing activities. They may also
gnaw on all types of materials, including soft metals
such as copper and lead as well as plastic and wood.
If roof rats are living in the attic of a residence,
they can cause considerable damage with their gnawing
and nest-building activities. They also damage garden
crops and ornamental plantings.
Among the diseases rats may transmit to humans or livestock
are murine typhus, leptospirosis, trichinosis, salmonellosis
(food poisoning), and ratbite fever. Plague is a disease
that can be carried by both roof and Norway rats, but
in California it is more commonly associated with ground
squirrels, chipmunks, and native wood rats.