Brown rat

(Rattus norvegicus)

Order: Rodentia

Brown rat – photo credit – Tim Preston

Description: Greyish-brown fur; prominent pointed muzzle, eyes, ears and long, almost naked, tail, about as long as the head-and-body. Much larger than any mice, but comparable with the much darker, shorter-tailed Water Vole in general size, though less ‘chubby’ in appearance.

Size: Head and body length 150 to 270mm. Tail length 105 to 240mm.

Weight: 40g at weaning, up to 600g but generally 200-300g.

Lifespan: Generally less than 1 year, but can be up to 3 years.

Breeding: Reproduction is observed all year round in human dwellings. Females can begin to breed at 3-4 months old, and if food is readily available may breed continuously, but typically have five litters a year. Litter size increases from around 6 in young females weighing 150g, to 11 in females of 500g, but the maximum recorded is 22. The young are born blind and hairless, but their eyes open at 6 days, and they are weaned at about 3 weeks. Young rats can be important food for owls, and many carnivores, including polecats, stoats and foxes, take substantial numbers of rats.

Habitat: Urban & gardens, coastal & marshland, deciduous woodland, mixed woodland, arable land

Diet: The brown rat is a true omnivore and will consume almost anything, but cereals form a substantial part of its diet. Surplus animal feed, including the fallout from bird feeders, often attracts them. Foraging behaviour is often population-specific, and varies by environment and food source. Examples have been found of rats eating birds and diving for molluscs where the food source is abundant. They will feed on many things in urban environments including food scraps from houses and restaurants.

General Ecology: Common rats are not territorial, but live in loose colonies with a hierarchy determined largely by size and age. There seem to be small family groups within the colony. Rats dig their own burrows, and entrances are usually joined by obvious well-used runs. In hedgerows, the males may have ranges averaging 600m, and females 340m, but in food stores may be as small as 65m. Reproduction is observed all year round in human dwellings.

Brown rat recorded distribution2km2 to 2018

Distribution and main sites: Native of central Asia.  Introduced to the British Isles in about 1720. Widespread throughout Shropshire and seriously under-recorded. The species map shows the main concentrations are in the most densely built up areas. Rats tend to live where there are human dwelling, also food stores and outlets which provide many of the rat’s needs.  In rural areas they will inhabit  agricultural buildings where there may be access to cereals. There are certainly more rats in the county than the species map shows, one possibility for this is that road kills may be rarely reported and that as they are a pest most people don’t record them and will generally attempt to eliminate them using traps or poisons. Rats are mainly nocturnal and being quick movers very often won’t be seen.

Local Survey history: casual records.

Conservation Status: The minimum GB pre-breeding population is 6.5 million, not including urban habitats such as sewers, rubbish tips and industrial premises where the population is reported to be continually rising. Attempts to control rat populations by poisoning may result in negative impacts on other wildlife populations.  As an alien, a major pest of stored foodstuffs, and the carrier of various human diseases (including Weil’s disease and plague), the common rat is persecuted rather than conserved.

Main recording methods: direct sightings, droppings, holes, runs

Text contributed by John Turford with additional notes on distribution by Barbara Ashton.

Photograph by Tim Preston

© Shropshire Mammal Group 2020