wood rat: see pack rat.

white-tailed rat: see hamster.

trade rat: see pack rat.

rat snake: see black snake.

rat, name applied to various stout-bodied rodents, usually having a pointed muzzle, long slender tail, and dexterous forepaws. It refers particularly to the two species of house rat, Rattus norvegicus, the brown, or Norway, rat and R. rattus, the black, roof, or Alexandrine, rat. Both species originated in Asia, but have spread throughout the world, mostly on board ships. The black rat was common in Europe in the Middle Ages and was responsible for the spreading of plague. It has since been largely displaced in cooler regions by the brown rat, which reached Europe early in the 18th cent. and North America by 1775.

The brown rat is the larger of the two, growing up to 10 in. (25 cm) long excluding the naked, scaley tail and sometimes weighing more than a pound (.5 kg). It is commonly brown with whitish underparts and pink ears, feet, and tail. It is a poor climber, but an excellent burrower and swimmer; it is found in the damp basements and sewers of most temperate zone cities. The laboratory white rat is an albino strain of the brown rat.

The black rat is commonly dark gray. It reaches a maximum length of 8 in. (20 cm) and has a longer tail and larger ears than the brown rat. A good climber, the black rat inhabits attics and upper floors in warm areas; it is the common rat of the Mediterranean region, the SE United States, and Central and South America.

Rats are omnivorous, aggressive, intelligent, adaptable, and extremely fecund. Females produce as many as 8 litters each year with as many as 20 young per litter. The gestation period is three weeks, and the young reach sexual maturity in about two months. Rats may live as long as four years. They are social animals but sometimes fight among themselves.

Rats live mostly in and around human settlements, where they have few natural enemies and an abundant source of food. They invade food supplies and cause widespread destruction; they also spread human diseases such as typhus and tularemia. Despite human efforts to exterminate rats, the house rat population is probably equal to the human population.

Besides the house rats, the genus Rattus contains several hundred wild-living species. In addition, many other members of several different rodent families are called rats, e.g., the bandicoot rat, the wood rat, or pack rat, the rice rat, the muskrat, and the kangaroo rat. House rats are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Muridae.

See also mouse.

pack rat, rodent of the genus Neotoma, of North and Central America, noted for its habit of collecting bright, shiny objects and leaving other objects, such as nuts or pebbles, in their place; also called trade rat or wood rat. Most common in the southern and western parts of the United States, but found as far south as Nicaragua, the pack rat stores the objects it collects to decorate its nest. The rodent may reach a length of 18 in. (45.7 cm) including tail, has soft brown fur, and resembles a squirrel with large ears. It eats nuts, berries, seeds, twigs, and roots. Its nest is a large stick structure built in a sheltered area. The desert species adorns its nest with bits of cactus, turning it into an impenetrable fortress. A litter is born after a gestation period of 33 to 39 days and contains from two to six young. Pack rats are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Cricetidae.

naked mole rat, name applied to a species (Heterocephalus glaber) of small rodents found in E Africa, whose members—the only hairless rodents—live entirely in underground communities of 80 or more individuals with a structure resembling that of social-insect colonies. Colony members are closely related and seem to work for the success of the whole community rather than individual survival. The queen is head of the colony and is the only member to breed. She has approximately ten pups per litter and about four litters per year. The mole rats divide up the specialized chores, such as collecting food, disposing of waste, and digging tunnels. The colonies have extensive tunnel systems and a large central area for nesting. Naked mole rats are classified as Phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia.

moon rat: see hedgehog.

kangaroo rat, small, jumping desert rodent, genus Dipodomys, related to the pocket mouse. There are about 20 kangaroo rat species, found throughout the arid regions of Mexico and the S and W United States. Kangaroo rats have large, mouselike heads with big eyes, external fur-lined cheek pouches for food storage, and extremely long, tufted tails. In many species the tail is longer than the combined head and body length. The total length, including the tail, is 10 to 15 in. (25-37.5 cm), depending on the species. The front limbs are very short and the back limbs extremely long and stiltlike. The animal moves by long leaps, like a kangaroo, using its tail for balance and as a rudder for turning at high speeds. Kangaroo rats have long silky fur, pale brown above and white beneath, with black and white tail tufts and black face markings. Solitary, nocturnal creatures, they live in burrows by day and forage at night for seeds, grass, and tubers. Active hoarders, they sometimes dry their food in shallow pits just below the surface of the ground, then dig it up and store it in their burrows. Like a number of other desert animals, the kangaroo rat has physiological mechanisms for conserving the water that it obtains from food or produces metabollically, so that it does not need to drink. A related genus, Microdipodops, is called the kangaroo mouse, or dwarf kangaroo rat. It is about 6 in. (15 cm) in total length and is found in the Great Basin of the W United States. Kangaroo rats are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Heteromyidae.

bandicoot rat, giant rat of southern Asia, unrelated to true bandicoots. It is an agricultural pest in the grain crops and gardens of India and Sri Lanka and is known for the piglike grunts it emits when attacked. Bandicoot rats are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Muridae.

Rat Islands: see Aleutian Islands.

Norway rat: see rat.

or pack rat

Any of 22 species (genus Neotoma, family Cricetidae) of rodents that are nocturnal vegetarians of North and Central American deserts, forests, and mountains. Wood rats are buff, gray, or reddish brown, usually with white undersides and feet. They have large ears and are 9–19 in. (23–47 cm) long, including the 3–9-in. (8–24-cm) furry tail. The nest, up to 3 ft (1 m) across and usually built of twigs or cactus, is placed in a protected spot (e.g., under a rock ledge). Wood rats are sometimes called pack rats because they collect material to deposit in their dens.

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Any of 40–55 snake species in the genus Elaphe (family Colubridae) and similar forms, found in woodlands and barnyards in North America, Europe, and Asia. Nonvenomous, they kill rats and mice by constriction, and also eat eggs and poultry. Some hunt birds in trees. These egg-laying snakes are normally slow and docile, but in self-defense they vibrate the tail, discharge a foul liquid, and strike from an upreared position. The black rat, or pilot black, snake (E. obsoleta obsoleta) of the eastern U.S. may exceed 8 ft (2.5 m).

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Any of more than 500 forms of Asian rodent (genus Rattus, family Muridae) that have been introduced worldwide. The black rat (Rattus rattus) and the Norway rat (R. norvegicus) are the aggressive, omnivorous animals commonly associated with the name. They prefer areas of human habitation, where they can easily find food. They have keen senses and can climb, jump, burrow, or gnaw their way into seemingly inaccessible places. They reproduce extremely rapidly (up to 150 offspring a year) and have few natural predators. Rats transmit numerous human diseases and have often destroyed grain supplies. The black rat is about 8 in. (20 cm) long, excluding the slightly longer tail. The Norway rat (also called the brown, barn, sewer, or wharf rat) has proportionately smaller ears and a shorter tail. Laboratory rats are strains of the Norway rat. The name rat is applied, without scientific basis, to other rodents (e.g., kangaroo rat, wood rat).

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Any of about 25 species (genus Dipodomys, family Heteromyidae) of rodents that leap about on their hind legs; found in dry regions of North America. They have large heads, large eyes, short forelimbs, long hind limbs, and fur-lined external cheek pouches that open alongside the mouth. They are 4–6.5 in. (10–16 cm) long without the long tail, which usually ends in a furry tuft. They are pale buff to brown above and white below, with a white stripe on each hip. They forage by night for seeds, leaves, and other vegetation, carrying food in their cheek pouches to store in their burrows, but seldom drink water.

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