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Rats
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Rat is the common name for about 1,000 species of rodents in 70 genera and 8 families. These species include the kangaroo rats, Dipodomys, family Heteromyidae; wood rats, Neotoma, family Cricetidae; spiny rats, Proechimys, family Echimyidae; and the typical rats, Rattus, family Muridae. Most rodents called rats have an elongated body, a moderately pointed snout, approximately equal-length legs, and a long, sparsely haired or hairless tail. Rats are generally distinguished from mice (see mouse) by their larger size.
In the narrow sense, rat refers to members of the genus Rattus, which contains from 137 to 570 species, depending on which classification is followed. The genus Rattus is of special interest because two of its species, the black rat, R. rattus, and the Norway rat, R. norvegicus, have profoundly affected human history.
The black rat, also called the roof rat, is the primary host for bubonic plague, which is transmitted to humans by direct contact or through the bites of fleas that have fed on infected rats. The black rat is believed to have come originally from southern or southeastern Asia. Although the first written record of it in Europe was not until the 1200s, archaeological evidence places the black rat in Britain well before the plagues there of the 6th and 7th centuries. It was clearly the species that served as the major reservoir for the plague in those great epidemics, called the Black Death, which killed an estimated one-fourth of the population of Europe during the 1300s.
The black rat reached North and South America in the 1500s, and it is currently distributed in the United States in a narrow belt along both coasts and throughout most of the southeastern states. The black rat ranges from 16 to 22 cm (6.3 to 8.6 in) in length, plus a 17- to 24-cm (6.7- to 9.3-in) tail (always longer than the head and body combined), and from 115 to 350 g (4 to 12 oz) in weight. Black rats are usually grayish black with lighter gray underparts, but brown varieties are common. Breeding occurs throughout the year. Gestation is about 24 days; litter size usually ranges from 6 to 12 young.
The Norway rat, also called the brown rat, probably originated in eastern Asia, possibly northern China. The first record of its appearance in Europe was 1553, and it is now found throughout the United States. It is usually grayish brown above and pale gray or brown on its underparts, but blackish varieties also occur. Norway rats range from 22 to 26 cm (8.6 to 10.2 in) in length, plus an 18- to 22-cm (7- to 8.6-in) tail (always shorter than the head and body combined), and from 200 to 485 g (7 to 17 oz) in weight. Norway rats breed throughout the year. Gestation varies from 21 to 24 days, and litter size is usually from 6 to 12 young.
Their preference for different habitats usually keeps the black and Norway rats apart, but where they do meet, the larger and more aggressive Norway rat either forces the black rat to different portions of the habitat, such as the upper levels of buildings, or drives it completely out of the area. Both black and Norway rats will eat almost anything, but black rats show a preference for plant material and Norway rats for animal food. Norway rats even become predators, and they often associate in packs of 60 or more animals, usually all closely related and often all descended from a single pair. The rat pack is one reason that the Norway rat can survive in so many diverse environments. If a nursing female is killed, for example, other nursing females of the pack will raise her young.
With all its negative characteristics regarding human health and economics, the Norway rat, in the form of the usually albino laboratory rat, has enabled scientists and medical researchers to make significant contributions in studies of nutrition, genetics, and disease. Domestic rats also make excellent pets.
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