Identification: Norway rats typically have coarse, brown fur with a pale gray or grayish brown underside. They have small eyes, naked ears, and a scaly tail that is shorter than the length of their head and body. Mature rats are range between 150-300 grams and about 400 mm long. The females have 12 mammae.
Rats are some of the most troublesome and damaging rodents in the United States. They consume and contaminate food, damage structures and property, and transmit parasites and diseases to other animals and humans. Rats live and thrive under a wide variety of climates and conditions; they are often found in and around homes and other buildings, farms, gardens, and open fields.
IDENTIFYING THE RAT
People do not often see rats, but signs of their presence are easy to detect (see sidebar). In Michigan the most troublesome rats are two introduced species: the roof rat and the norway rat. It is important to know which species of rat is present in order to place traps or baits in the most effective locations.
Norway Rats. Norway rats (Rattus norvegicus), sometimes called brown or sewer rats, are stocky burrowing rodents that are larger than roof rats. Their burrows are found along building foundations, beneath rubbish or woodpiles, and in moist areas in and around gardens and fields. Nests may be lined with shredded paper, cloth, or other fibrous material. When Norway rats invade buildings, they usually remain in the basement or ground floor. The Norway rat occurs throughout the 48 contiguous United States. Generally it is founds at lower elevations but may occur wherever people live.
Roof Rats. Roof rats (Rattus rattus), sometimes called black rats, are slightly smaller than Norway rats. Unlike Norway rats, their tails are longer than their heads and bodies combined. Roof rats are very agile climbers and usually live and nest above ground in shrubs, trees, and dense vegetation such as ivy. In buildings, they are most often found in enclosed or elevated spaces in attics, walls, false ceilings, and cabinets. The roof rat has a more limited geographical range than the Norway rat, preferring ocean-influenced, warmer climates. In areas where the roof rat occurs, the Norway rat may also be present. If you are unsure of the species, look for rats at night with a strong flashlight or trap a few. There are several key physical differences between the two species of rats; Table 1 summarizes identifying characteristics.
Table 1. Identifying Characteristics of Adult Rats.
color of belly
gray to white
5 to 10 ounces
7 to 18 ounces
extends at least to snout; black, fine scales
shorter than body; dark above; pale below; scales
long enough to reach eyes if folded over
do not reach eyes
While rats are much larger than the common house mouse or meadow vole, a young rat is occasionally confused with a mouse. In general, very young rats have large feet and large heads in proportion to their bodies, whereas those of adult mice are much smaller in proportion to their body size. While both rats and mice gnaw on wood, rats leave much larger tooth marks than those of a mouse. For additional information on mice, see Pest Notes:House Mouse and Pest Notes: Voles (Meadow Mice) listed in References.
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