The black rat

Species and description

The best-known rat species are the black rat (Rattus rattus) and the brown rat (Rattus norvegicus). The group is generally known as the Old World rats or true rats, and originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but seldom weigh over 500 grams (1.1 lb) in the wild.

The term "rat" is also used in the names of other small mammals which are not true rats. Examples include the North American pack rats, a number of species loosely called kangaroo rats, and others. Rats such as the bandicoot rat (Bandicota bengalensis) are murine rodents related to true rats, but are not members of the genus Rattus. Male rats are called bucks, unmated females are called does, pregnant or parent females are called dams, and infants are called kittens or pups. A group of rats is either referred to as a pack or a mischief.

A rat in a city street

Statue of a rat in Patan (Nepal)

The common species are opportunistic survivors and often live with and near humans; therefore, they are known as commensals. They may cause substantial food losses, especially in developing countries.[1] However, the widely distributed and problematic commensal species of rats are a minority in this diverse genus. Many species of rats are island endemics and some have become endangered due to habitat loss or competition with the brown, black or Polynesian rat.

Wild rodents, including rats, can carry many different zoonotic pathogens, such as Leptospira, Toxoplasma gondii, and Campylobacter.[2] The Black Death is traditionally believed to have been caused by the micro-organism Yersinia pestis, carried by the tropical rat flea (Xenopsylla cheopis) which preyed on black rats living in European cities during the epidemic outbreaks of the Middle Ages; these rats were used as transport hosts. Other zoonotic diseases linked to pest rodents include classical swine fever and foot-and-mouth disease.

The average lifespan of any given rat depends on which species is being discussed, but many only live about a year due to predation.

The black and brown rats diverged from other Old World rats during the beginning of the Pleistocene in the forests of Asia.

Pets

Main article: Fancy rat

A domesticated rat

Specially bred rats have been kept as pets at least since the late 19th century. Pet rats are typically variants of the species brown rat, but black rats and giant pouched rats are also known to be kept. Pet rats behave differently from their wild counterparts depending on how many generations they have been kept as pets.[3] Pet rats do not pose any more of a health risk than pets such as cats or dogs.[4] Tamed rats are generally friendly and can be taught to perform selected behaviors.

Subjects for scientific research

Main article: Laboratory rat

A laboratory rat strain, known as a Zucker rat, is bred to be genetically prone to diabetes, the same metabolic disorder found among humans.

In 1895, Clark University in Worcester, Massachusetts (United States) established a population of domestic albino brown rats to study the effects of diet and for other physiological studies. Over the years, rats have been used in many experimental studies, which have added to our understanding of genetics, diseases, the effects of drugs, and other topics that have provided a great benefit for the health and well-being of humankind. Laboratory rats have also proved valuable in psychological studies of learning and other mental processes (Barnett, 2002), as well as to understand group behavior and overcrowding (with the work of John B. Calhoun on behavioral sink). A 2007 study found rats to possess metacognition, a mental ability previously only documented in humans and some primates.[5][6]

Domestic rats differ from wild rats in many ways. They are calmer and less likely to bite; they can tolerate greater crowding; they breed earlier and produce more offspring; and their brains, livers, kidneys, adrenal glands, and hearts are smaller (Barnett 2002).

Brown rats are often used as model organisms for scientific research. Since the publication of the rat genome sequence,[7] and other advances, such as the creation of a rat SNP chip, and the production of knockout rats, the laboratory rat has become a useful genetic tool, although not as popular as mice. When it comes to conducting tests related to intelligence, learning, and drug abuse, rats are a popular choice due to their high intelligence, ingenuity, aggressiveness, and adaptability. Their psychology, in many ways, seems to be similar to humans. Entirely new breeds or "lines" of brown rats, such as the Wistar rat, have been bred for use in laboratories. Much of the genome of Rattus norvegicus has been sequenced.[8]

Intelligence

A Rat in a suburb of Vancouver.

Because of the ability to learn, rats were early on investigated to see whether they may exhibit general intelligence like larger or more complex animals. A 1929 study did not find a g factor,[9] nor did a 1990 work;[10] a 1935 study[11] did:

Robert Thorndike, for example, provided strong evidence for g in rats by the use of a variety of tests such as mazes, problem-solving tasks, and simple avoidance conditioning (Thorndike 1935). Performances tended to correlate across tasks, with stronger associations found between mazes and problem-solving than with simple avoidance tasks. Thorndike (1935) also reviewed a dozen earlier studies which also suggested that the highest correlations are found between more complex problem-solving tasks. However, it should be noted that there were

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