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The brown or Norway rat, Rattus norvegicus, which most authorities agree originated in Central Asia, was supposed to have reached Western Europe through the agency of the Russian fleet with visited Copenhagen. And in 1727, Pallas observed the rats to emigrate from Russia and to swim across the Volga in vast hordes, probably in search of food. They thereupon overran Europe."

However, naturalists persist in asserting that Norway rats did not reach Europe before the 18th century, and give figures purporting to be exact dates of the first arrival of this rats in different countries, Prussia, 1750; Norway, 1762; Faroe Island, 1768; Sweden, 1790; Switzerland, 1808; as if they had actually stamped their passports. We know from Pallas that in 1727 -a 'mouse year' in the Caspian region- vast hordes crossed the Volga and swarmed into Astrakan, thence spreading westward across Russia, but the fact that while traveling in Southern Russia, a distinguished naturalist had witnessed the migration of a great rat army does not prove that the brown rat first came to Europe at that particular moment. Aelian, in his work, 'De Natura Animalium', written in the second century A.D., undoubtedly refers to the brown rat when he states that 'Caspian rats' at time migrate in countless hosts and bridge the rivers, forming live rafts, each rat holding by teeth to the tail of the rat in front. Among ancient bronze representations of rats found in Italy, while some show the large ears, sharp muzzle, slender build and long tail of the black rat and, therefore, often are confounded with mice, others portray most faithfully and unmistakably the small ears, blunt muzzle, heavy build and shorter tail of the brown rat, proving that both species were available as models to the Italian sculptor of at least 20 centuries ago."

It is evident from these quotations that serious disagreement and confusion exists over the spread of rats from Asia. The related literature is speculative, often erroneous, and widely dispersed. The most recent review (Davis, 1986) suggests that roof rats, though there may have been scattered colonies in earlier centuries, did not spread through Europe until the mid-15th century. Norway rats came along a couple of centuries later.

In the United States the brown rat has replaced the black rat everywhere in the more temperate areas except in the southern states, especially in Florida, the Gulf states and California; in some sections the black rat may actually predominate.

There is evidence that the Norway rat is a later, more highly developed species originating in or near the center of origin of the Rattus group. This comparatively late comer is adapted to the dry, grassy plains of Central Asia. It is characteristic among mammals that the most advanced species of a group are found closest to the center of origin, where they replace the more primitive forms. So it appears to be with the rats. As the more highly-developed, more aggressive Norway rat spread outward from Asia, the more primitive roof rat disappeared over much of its original range.

"The Norway rat first appeared in Europe in the 1770s. It spread so rapidly that the Europeans called it the "Wanderatte" or migratory rat. Soon after the Norway rat reached Western Europe, it was carried to the New World. Here it quickly began spreading outward from the seaports, especially along the east coast of North America."

"The present distribution of the Norway and roof rats appears related to two factors, competition between the two species and the reaction of both to different climates. When the aggressive Norway rat and the roof rat compete for the same areas, the Norway rat frequently becomes dominant, and the roof rat soon disappears. Only under special conditions do both species live in the same area. In one eastern seaport, roof rats live in the top of grain elevator and Norway rats live in the bottom. This is probably because roof rats are better climbers than Norways. It is generally only in such situations as these that roof rats are found living in Norway rat territory."

"As the spread of the Norway rat approaches tropical regions, the picture is altered by its reaction to the warmer climate. It appears that the Norway rat is definitely an animal of the temperate climates. In its original range in Asia it is restricted to temperate regions. It is found in the tropics only in seaport areas. On the other hand, the roof rat is most common today throughout the tropics. This is true both in its native area and in areas where it has been introduced. In these areas roof rats commonly inhabit regions quite far removed from man's activities."

Williams (1948) notes rats "require harborage for two major purposes, one as a protective refuge for the adults and the other as a protection for the young." According to Williams, harborage "generally means retired spaces wherein rats are at least out of sight. Good harborage consists of retired spaces wherein the rat is not only out of sight but cannot be directly reached by its enemies."

"Rats nests are made up of bits of any kind of material available. Soft materials are preferred such as bits of paper, rags, burlap, straw, string, chips, etc. These are carried into the selected harborage, packed loosely and built up around the side until there appears a crudely constructed nest resembling a bird's nest. In the nests the young are deposited and cared for by the mothers." Hamilton (1947) notes "both bank notes and corn husks are acceptable for the nest, and that more than one innocent victim has been made out of a thief, only to be pardoned when a piece of paper currency has been recovered from a rat's nest."