View Pictures of Rat Damage
	Class Mammalia Order Rodentia Family Muridae

History and Biology

The Roof rat, Rattus rattus, is a commensal rodent that is thought to have originated in the trees of Southeast Asia about four million years ago. The roof rat followed the caravan routes across India into the eastern Mediterranean region and entered Europe in the 11 th and 12 th centuries. It was the common rat in Europe during the outbreaks of the plague known as the Black Death. From there it was carried by ships to America in the 15 th century. It rapidly spread across the United States from the eastern seaports. It now occurs along the west coast and in the south central and south eastern regions of the United States . Its range is more restricted than that of the Norway rat, but it may be the most numerous rat species in large cities.

The Roof rat is also known as the black rat, ship rat, and house rat. It is a medium sized rat approximately 16 inches from the tip of the nose to the tip of the tail and weighing, on the average, about 12 ounces. It is about the same overall length as the Norway rat, but the length of the tail is longer than the body length. The Roof rat is more sleek than the Norway rat. The roof rat is not always totally black, it may be dark backed in brown or gray with a lighter colored to whitish belly.

Roof rat behavior can vary considerably and allows the species to readily adapt to its' environment. Consequently, a stereotyped set of habits should be cautiously attributed to the Norway rat. Generally, the Roof rat is considered a climber and dwells in trees and rafters, but it may frequently be found on the ground. Generally, the Roof rat has two peak activity periods; one within the hour following sunset, and a second just before dawn. However, they can be observed to be active all hours of the night, behaving primarily as a nocturnal animal. Additionally, under high population densities, a social hierarchy and competitive feeding and breeding behavior, will force some rats to seek food and mates during the daylight hours. The smaller Roof rat will coexist with the larger, stronger, and more aggressive Norway rat.

Because of its origin from arboreal ancestors, the Roof rat prefers elevated areas of treetops, palms, vines, rafters, and roofs. This habitat preference makes it less visible, less available, less dependent on ground level food and harborage, and overall more elusive. The Roof rat will cohabitate with the Norway rat in its' common sewer and storm drain harborage. Roof rats build their nests in attics, walls, and soffits of buildings, presumably because these high areas resemble the treetops this rat is familiar with. Roof rats enter buildings from trees, bushes, fences, and electrical power lines as well as from the ground, burrows, and sewers.

Roof Rats mature at about 3 months of age. They breed in the spring and fall and give birth to litters of 6 to 12 pups, 3 to 6 times per year. The gestation period is 22 days and females may mate within a day or two after giving birth. Females come into estrus every 4 to 5 days. The number of litters per year depends on age, food, climate, and shelter. A female rat normally lives for six months to one year. Roof rats are opportunistic omnivores, but they prefer vegetable matter. They prefer to feed on grains, nuts, vegetation, and seeds. But like the Norway rat, they will eat insects, slugs, snails, birds, eggs, fish, other small mammals, and anything that is available when necessary. Rats are also copraphagous; they sift through the feces of other animals, such as dogs, horses, livestock, and humans, and pick out undigested food particles. As sewer rats, they have become quite adapt at feeding on ground food waste from kitchen sink garbage disposal units. Rats ingest an amount food equal to about 10% of their body weight daily.

Roof rats must drink water on a daily basis. The amount of water required depends on age, activity, the ambient environment, and available food moisture content. On the average, the amount of free water required is approximately 1 to 2 ounces or more per day. Rats drink by lapping it up with their tongue or cupping it in their front feet. Rats will lick drops of water from various objects such as, grass, vegetation, sprinklers, fire sprinkler heads, water leaks, water bibs, garden hoses, and air conditioning condensation. They will also eat succulent greenhouse plants to obtain water.

Young rats learn a taste for locally available foods first through the taste of the mother's milk, supplemented by the smell, and taste of food residues from the mother's face and body.

Neophobia is a condition in rats where they have a fear of new objects and unfamiliar foods. When conditions are favorable for rats (i.e. plentiful food, water, and shelter), they tend to be habitually selective and avoid new materials and changes. This is called bait and/or device shyness. In unfavorable environments with limited resources, rats are more likely to attack and utilize a variety of foods and materials.

Public Health Damage:

Rat bites are common, especially in areas with high rodent infestations in close proximity to humans. Infants and the elderly are the most vulnerable to foraging rats. Rats are also a reservoir for many zoonotic diseases such as; bubonic plague, septicemic plague, pneumonic plague, sylvatic plague, murine typhus, scrub typhus, rickettsialpox, leptospirosis, rat bite fever, trichinosis, salmonellosis, hantavirus, lymphocytic choriomeningitis and rabies.

Structural Damage:

Rats inhabit the dwellings of humans, commercial buildings, industrial yards, and farming operations. Their urine odor can be inhibiting to the operations of these structures. Their scats and grease rubs can become an eyesore and odiferous. They can increase in significant numbers if not controlled and removed from anthropogenic structures. They chew and gnaw through construction materials to make dens. They tunnel through insulation and plastic membranes of commercial roofs. They burrow under concrete slabs causing settling of buildings and fractures of foundations. They chew through electrical wires, causing shorts and fires. To obtain water, rats will chew through water pipes and hoses. Rats spend 2% of their day chewing and gnawing on natural and man-made materials.

Agricultural Damage:

Rats may threaten the health of humans and livestock in the United States because they may be a reservoir for potentially transmittable disease to both humans and livestock such as; bubonic plague, septicemic plague, pneumonic plague, sylvatic plague, murine typhus, scrub typhus, rickettsialpox, leptospirosis, rat bite fever, trichinosis, salmonellosis, hantavirus, lymphocytic choriomeningitis and rabies. They also may do damage by defacement of agricultural buildings and equipment with scats, urine, and grease. Additionally, they do damage through the consumption and contamination of agricultural seed and grain products in the field, storage facilities, and wholesale and retail outlets. It is estimated that 1/5 to 1/3 of the worlds food supply never reaches the table due to contamination and consumption losses from rodents. The rat typically produces 20 to 50 scats and excretes ½ ounces of urine per day that are significant contributors to food contamination.

Environmental Damage:

Because of its commensal relationship with humans and its potential to invade structures and food supplies, the rat is a constant public health and property damage problem. In nature, rats feed on small mammals, birds, and eggs and may have a negative impact on their population richness and densities.

Economic Damage:

Economic damage from the rat is difficult to assess when you consider the intangible costs of the impact of zoonotic diseases on the public health of humans, livestock, domestic animals, and wildlife. Additionally, it is also difficult to determine the real cost of equipment and structural damages from chewing, gnawing, and defacement. Also, there is a significant cost to American consumers and business from the consumption and contamination of seed, grain, fruits, nuts, and vegetables damaged by the rat. It is estimated that 1/5 to 1/3 of the worlds food supply never reaches the table due to contamination and consumption losses from rodents.

References:

Mallis, A. 1997. Handbook of Pest Control: The behavior, Life History, and Control of Household Pests, Eighth Edition. Mallis Handbook and Technical Training Company.

Bennett, G. W., Owens, J. M., and Corrigan, R. M., 1988, Truman's Scientific Guide To Pest Control Operators, Harcourt Brace Jovanovich, Inc., One East First St. , Duluth , MN . 55802