A rat is any of about 56 different species of small, nearly omnivorous rodent belonging to the genus Rattus. The most well-known species are the Black Rat Rattus rattus and the Brown Rat Rattus norvegicus. more...
The group is generally known as the old world rats or true rats, and originated in Asia. Rats are bigger than their relatives the 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. Examples include the North American pack rats, a number of species loosely called kangaroo rats, and a number of others. Other rats such as the Bandicoot Rat Bandicota bengalensis are murine rodents related to the true rats, but are not members of the genus Rattus. The widely distributed and problematic commensal species of rats represent 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 brown, black, or Polynesian rats.
In Western countries, many people keep domesticated rats as pets. These are of the species Rattus norvegicus, which originated in the grasslands of China and spread to Europe and eventually, in 1775, to the New World. Pet rats are descendants of Brown Rats bred for research, and may be called "fancy rats." But they are the same species as the common city "sewer" rat. Domesticated rats tend to be both more docile than their wild ancestors and more disease prone, presumably due to inbreeding.
Rats have a significant impact on food production. Estimates vary, but it is likely that anything between 1/5 and a 1/3 of the world's total output is eaten, spoiled or destroyed by rats and other rodents.
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 Xenopsilia cheopis which preyed on Rattus rattus living in European cities of the day; it is notable that these rats were also victims themselves. It should perhaps also be noted that it has recently been suggested that neither rats nor infected fleas would have spread fast enough through Europe to have been a likely culprit. Regardless, 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 a crime or misdeed they committed to the authorities. 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 largely spread disease to humans.
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