The brown rat (Rattus norvegicus), also known as the common rat, street rat, sewer rat, wharf rat, Hanover rat, Norway rat, Norwegian rat and Parisian rat, is a widespread species of common rat.
One of the largest muroids, it is a brown or grey rodent with a head and body length of up to 28 cm (11 in) long, and a tail slightly shorter than that.
It weighs between 140 and 500 g (5 and 17+3⁄4 oz).
With rare exceptions, the brown rat lives wherever humans live, particularly in urban areas.
Selective breeding of the brown rat has produced the fancy rat (rats kept as pets), as well as the laboratory rat (rats used as model organisms in biological research).
Both fancy rats and laboratory rats are of the domesticated subspecies Rattus norvegicus domestica.
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 (6 to 11 in) while the tail ranges in length from 10.5 to 24 cm (4+1⁄4 to 9+1⁄2 in), therefore being shorter than the head and body.
Adult weight ranges from 140 to 500 g (5 to 17+3⁄4 oz).
Exceptionally large individuals can reportedly reach 900 to 1,000 g (31+1⁄2 to 35+1⁄2 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.
In fact, it is common for breeding wild brown rats to weigh (sometimes considerably) less than 300 g (10+1⁄2 oz).
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 dichromates 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 some species cannot.
Biology and behavior
Brown rats dig well, and often excavate extensive burrow systems.
A 2007 study found brown rats to possess metacognition, a mental ability previously only found in humans and some primates, but further analysis suggested they may have been following simple operant conditioning principles.