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However, as the rats age, the tendency to chirp appears to decline.
Brown rats also produce communicative noises capable of being heard by humans.
It is frequently called, in books and otherwise, the 'Norway rat', and it is said to have been imported into this country in a ship-load of timber from Norway.
Against this hypothesis stands the fact that when the brown rat had become common in this country, it was unknown in Norway, although there was a small animal like a rat, but really a lemming, which made its home there." "The brown rat is the species common in England, and best known throughout the world.
The vocalization, described as a distinct "chirping", has been likened to laughter, and is interpreted as an expectation of something rewarding.
Like most rat vocalizations, the chirping is too high in pitch for humans to hear without special equipment.
The vision of a pigmented rat is poor, around 20/600, while a non-pigmented (albino) with no melanin in its eyes has both around 20/1200 vision and a terrible scattering of light within its vision.
Brown rats are dichromates which perceive colors rather like a human with red-green colorblindness, and their colour saturation may be quite faint.
"Now there is a mystery about the native country of the best known species of rat, the common brown rat.The most commonly heard in domestic rats is bruxing, or teeth-grinding, which is most usually triggered by happiness, but can also be 'self-comforting' in stressful situations, such as a visit to the vet.The noise is best described as either a quick clicking or 'burring' sound, varying from animal to animal.Thought to have originated in northern China, this rodent has now spread to all continents except Antarctica, and is the dominant rat in Europe and much of North America—making it by at least this particular definition the most successful mammal on the planet alongside humans.With rare exceptions, the brown rat lives wherever humans live, particularly in urban areas.