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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.Stories of rats attaining sizes as big as cats are exaggerations, or misidentifications of other rodents, such as the coypu and muskrat.In fact it is common for breeding wild brown rats to weigh (sometimes considerably) less than 300 g (11 oz).Their blue perception, however, also has UV receptors, allowing them to see ultraviolet lights that some species cannot.The brown rat is nocturnal and is a good swimmer, both on the surface and underwater, and has been observed climbing slim round metal poles several feet in order to reach garden bird feeders.
A 2007 study found brown rats to possess metacognition, a mental ability previously only found in humans and some primates, Rats may also emit short, high frequency, ultrasonic, socially induced vocalization during rough and tumble play, before receiving morphine, or mating, and when tickled.
The brown rat is a rather large true murid and can weigh twice as much as a black rat and many times more than a house mouse.
The length is commonly in the range of 20 to 25 cm (8 to 10 in), with the tail a further 18 to 25 cm (7 to 10 in), thus being roughly the same length as the body.
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.