stylistic:tone, linguistic:voice_change, structural:summarization, structural:expansion
Description

The fur is usually brown or dark grey, while the underparts are lighter grey or brown. The brown rat is a rather large murid and can weigh twice as much as a black rat (Rattus rattus) and many times more than a house mouse (Mus musculus). The head and body length ranges from 15 to 28 cm (5.9 to 11.0 in) while the tail ranges in length from 10.5 to 24 cm (4.1 to 9.4 in), therefore being shorter than the head and body. Adult weight ranges from 140 to 500 g (4.9 to 17.6 oz). Large individuals can reach 800 g (28 oz) but are not expected outside of domestic specimens. Stories of rats attaining sizes as big as cats are exaggerations, or misidentifications of larger rodents, such as the coypu and muskrat. It is common for breeding wild brown rats to weigh (sometimes considerably) less than 300 g (11 oz). The heaviest live Rattus norvegicus on record is 822 g (29 oz) and they can reach a maximum length of 48.5 cm (19 in).

Brown rats have acute hearing, are sensitive to ultrasound, and possess a very highly developed olfactory sense. Their average heart rate is 300 to 400 beats per minute, with a respiratory rate of around 100 per minute. 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 dichromats which perceive colors rather like a human with red-green colorblindness, and their colour saturation may be quite faint. Their blue perception, however, also has UV receptors, allowing them to see ultraviolet lights that humans and some other species cannot.