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The Brown rat (Rattus norvegicus), also referred to as the common rat, street rat, sewer rat, Hanover rat, Norway rat, Norwegian rat, or wharf rat is one of the best known and most common rats.
One of the largest muroids, it is a brown or grey rodent with a body up to 25 cm (10 in) long, and a similar tail length; the male weighs on average 350 g (12 oz) and the female 250 g (9 oz).
In addition, they commonly squeak along a range of tones from high, abrupt pain squeaks to soft, persistent 'singing' sounds during confrontations.
The brown rat is a true omnivore and will consume almost anything, but cereals form a substantial part of its diet.
Bat detectors are often used by pet owners for this purpose.
In research studies, the chirping is associated with positive emotional feelings, and social bonding occurs with the tickler, resulting in the rats becoming conditioned to seek the tickling.
Martin Schein, founder of the Animal Behavior Society in 1964, studied the diet of brown rats and came to the conclusion that the most-liked foods of brown rats include scrambled eggs, macaroni and cheese, raw carrots, and cooked corn kernels.
According to Schein, the least-liked foods were raw beets, peaches, and raw celery.
Brown rats are dichromates which perceive colors rather like a human with red-green colorblindness, and their colour saturation may be quite faint.
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.
By the early to middle part of the 19th century, British academics believed that the brown rat was not native to Norway, hypothesizing (incorrectly) that it may have come from Ireland, Gibraltar or across the English Channel with William the Conqueror.
"Now there is a mystery about the native country of the best known species of rat, the common brown rat.