Domestication The "Norway" rat (also referred to as the common, brown, grey, wharf, or Hanoverian rat) is, in many parts of the world, especially the tropics, not common at all; it is normally grey-brown but sometimes black; in its domestic forms it is usually white, or white and black; it is only incidentally associated with wharves; and it does not come from Hanover or Norway. In the early days of modern Latin names it was called Mus norvegicus, which seems to signify a kind of mouse. Later, it was named Epimys instead of Mus, which suggests a superior mouse. Its modern name, finally determined by the rules of biological nomenclature, is Rattus norvegicus. During the nineteenth century, Norways became domesticated on a large scale and produced the various forms of laboratory rat. In 1894, at Clark University, wild Norways were adopted for research on the effects of diet and alcoholism. This program did not last long. Perhaps the researchers were bitten too often. In 1895, white Norways were brought in and so began the use of domestic rats as models for human physiology. In 1900, again at Clark, a psychologist, W.S. Small, founded "white rat psychology" by launching experiments on rats in mazes. Small was inspired by a famous institution at Hampton Court, in outer London. There visitors could spend a sunny afternoon getting lost in an elaborate maze made of high, well tended hedges. During the domestication, members of a species become adapted to conditions, often including close confinement, imposed by human beings. This entails selection, because many newly captive animals are infertile. The domestic stock is descended from those, sometimes only a few, that produce young. In one early case, sixteen male and twenty female Norways were trapped, but only six of the females had litters. In 1919, these gave rise to a stock which was used in laboratories for many years. The result of such inadvertent selection is unavoidably a population genetically different from the original wild type. Features not advantageous in freedom may now favor survival. One is conspicuous color, which may be welcomed by human captors but in the wild would help predators such as hawks. More important, domestic animals, when seized or restrained, rarely attack their owners, and the tendency to flee from human beings is reduced. Their social responses also change; they are tolerant of greater crowding that in nature, they mate earlier and more readily, and they breed for longer, hence may produce more young. Other differences are less obvious. Laboratory rats have smaller brains, livers, kidneys and hearts than those of their wild cousins; and their adrenal glands are also diminished. From the laboratory rat, came the domesticated pet rat. Source: the book "The Story of Rats" by S.A. Barnett