A rat is any one of about 56 different species of small, omnivorous rodents belonging to the genus Rattus. The best-known rat species are the Black Rat (Rattus rattus) and the Brown Rat (R. norvegicus). The group is generally known as the Old World rats or true rats, and originated in Asia. Rats are bigger than most of their relatives, the Old World mice, but seldom weigh over 500 grams in the wild. The common term "rat" is also used in the names of other small mammals which are not true rats.


The Brown rat's fur is coarse and usually brown or dark gray, the underparts are lighter gray or brown. The length can be up to 40 cm, with the tail a further 25 cm (the same as the body length). Adult body weight averages 350 g in males and about 250 g in females, but a very large individual can reach 500 g.

Giant rats: Rats weighing over a kilogram are exceptional, and stories of rats as big as cats are exaggerations, or misidentifications of other rodents such as the Coypu and Muskrat.

Senses: Brown Rats have acute hearing and are sensitive to ultrasound, and also possess a very highly developed olfactory sense. Their average heart rate is 300 to 400 beats per minute, with a respiratory rate of around 100 per minute. Their vision is poor and they are unable to detect color and are blind to long-wave light.

The common species are opportunistic survivors and often live with and near humans. The Black Plague is traditionally believed to have been caused by the micro-organism Yersinia pestis, carried by the rat flea Xenopsylla cheopis which preyed on R. rattus living in European cities of the day; it is notable that these rats were victims of the plague themselves.

Rat infestation: Rats are frequently blamed for damaging food supplies and other goods. Their reputation has carried into common parlance: in the English language, "rat" is an insult and "to rat on someone" is to betray them by denouncing to the authorities a crime or misdeed they committed. While modern wild rats can carry Leptospirosis and some other "zoonotic" conditions (those which can be transferred across species, to humans, for example), these conditions are in fact rarely found. Wild rats living in good environments are typically healthy and robust animals. Wild rats living in cities may suffer themselves from poor diet and internal parasites but do not generally spread disease to humans.

Rat as pet: The rat makes a fine pet, known for its intelligence, playfulness and sociability. They are extremely clean. Rats can be taught entertaining tricks, in the same way as many other domesticated animals.

Brown Rat: The Brown Rat or Norway Rat (Rattus norvegicus) is one of the best-known and common rats, and also one of the largest. It is not known for certain why it is named Rattus norvegicus (Norwegian rat) since it doesn't originate from Norway, but John Berkenhour, the author of the 1769 book Outlines of the Natural History of Great Britain, is most likely responsible for the misnomer. Berkenhour gave the Brown Rat the binomial name Rattus norvegicus believing that the rat had migrated to England from Norwegian ships in 1728, although no Brown Rat had entered Norway at that time, instead coming from Denmark.

Origin of Brown Rat: Thought to have originated in northern China, this rodent has now spread to all continents and is the dominant rat in Europe and much of North America. It lives wherever humans live, particularly in urban areas. Selective breeding of Rattus norvegicus has produced the laboratory rat, an important model organism in biological research, as well as pet rats. Other names for the species include Norwegian Rat, Wharf Rat and Common Rat.

Habitat: Rats live wherever people live. It is often said that there are as many rats in cities as people, but this varies from area to area depending on climate, etc. It is probable that New York City (with a severe winter climate), for instance, has only 250,000 rats, not eight million. However, the UK official National Rodent Survey found a 2003 UK population of 60 million Brown Rats, about equal to the UK human population; winters in Britain are much warmer, making rat survival higher. Brown Rats in cities tend not to wander extensively, often staying within 20 meters (65 ft) of their nest if a suitable concentrated food supply is available, but they will range more widely where food availability is lower.

The only way to truly combat the rat problem is reduce the food supply, i.e., garbage left out on the street.

Disease Carriers: Brown Rats are supposed to carry some diseases, including Weil's disease, cryptosporidiosis, Viral hemorrhagic fever (VHF), Q fever and hantavirus pulmonary syndrome. Unlike the Black Rat, Brown Rats rarely if ever carry bubonic plague.

Brown rats in science: Selective breeding of the Brown Rat has produced the albino laboratory rat. Like mice, these rats are frequently subjects of medical, psychological and other biological experiments and constitute an important model organism. This is because they grow quickly to sexual maturity and are easy to keep and to breed in captivity. When modern biologists refer to "rats", they almost always mean Rattus norvegicus.

Scientists have bred many strains or "lines" of rats specifically for experimentation. Generally, these lines are not transgenic, however, because the easy techniques of genetic transformation that work in mice do not work for rats. This has disadvantaged many investigators, who regard many aspects of behavior and physiology in rats as more relevant to humans and easier to observe than in mice, but who wish to trace their observations to underlying genes. As a result, many have been forced to study questions in mice that might be better pursued in rats. In October 2003, however, researchers succeeded in cloning two laboratory rats by the problematic technique of nuclear transfer. So rats may begin to see more use as genetic research subjects.

The Brown Rat can breed throughout the year if conditions are suitable, a female producing up to five litters a year. The gestation period is only 21-23 days and litters can number up to fourteen, although seven is common. The maximum life span is up to three years, although most barely manage one—a mortality rate of 95% is estimated, with predators and intraspecific conflict as major causes. Brown Rats live in large hierarchical groups, either in burrows or subsurface places such as sewers and cellars. When food is in short supply, the rats lower in social order are the first to die. If a large fraction of a rat population is exterminated, the remaining rats will increase their reproductive rate, and quickly restore the old population level.

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