Brown rat Rattus norvegicus
Introduced as stowaways from Russia in the eighteenth century, brown rats have replaced the once abundant Black rats (which came to Britain with the Romans), and which are now very rare. The wild cousins of the fancy rats kept as pets, Brown rats get a terrible but largely unfair press. They are intelligent, adaptable animals, and while they can carry some human diseases, the risk to most people is very small. As long as an area isn’t overrun by them, they can be engaging neighbours.
In small, well-fed groups, rats groom meticulously but as colonies become more crowded, they groom less and scuffles between individuals are more common. They have keen senses of hearing and smell and use scent and short, ultrasonic calls to communicate. They may also use ultrasound to echolocate in a similar way to bats.
Rats tend to move about along well-trodden routes and, with practice, it is possible to spot these ‘runs’. They are excellent swimmers (both on the surface and underwater) and can climb, readily getting into the upper storeys of buildings. They dig extensive burrows, often in sloping ground or the sides of ditches, or beneath flat stones or tree roots, and burrows may be used for several generations.
In urban areas, rats will make use of buildings, nesting in floor cavities or beneath floorboards. The English House Condition Survey in 2007 found that rats were present in three per cent of gardens but in only four in a thousand domestic properties.
Appearance and activity Brown rats are much larger than either mice or voles, with a body length up to 28cm and a tail nearly as long. They have grey-brown fur and a long, scaly tail, which is used to balance. In water, they can be distinguished from water voles by their more pointed face, prominent ears and longer tail. Young rats resemble adult mice, but have much broader hindfeet and relatively thicker tails.
Rats are active throughout the year and mostly nocturnal but will take advantage of opportunities to feed during daylight.
Status, population size and distribution Non-native and common; GB population 7,000,000; about 1,000,000 live in or close to urban buildings. Dave Cowan, an author of the English House Condition Survey, estimated that, taking into account rats living in and around homes, commercial properties and in sewers, inhabitants of UK cities are probably within 50m of a rat – quite a bit further than the often quoted figure of ‘six feet’.
Baker, P.J., Ansell, M.J., Dodds, P.A.A., Webber, C.E., Harris, S. (2003) Factors affecting the distribution of small mammals in an urban area. Mammal Review 33 (1): 95-100