Brown rat  (Rattus norvegicus)

The brown rat is classified as Least Concern (LC) on the IUCN Red List (1). It is not legally protected in the UK. No conservation designations (3).

The brown or Norway rat (Rattus norvegicus) is a supreme generalist; its opportunistic lifestyle, agility and prolific breeding potential have helped it to colonise practically every part of the world (2). The fur of this rat varies in colour but is generally brown or greyish-brown with some black hairs, becoming paler on the underside (2). The tail is long and largely hairless, the naked ears are prominent (1) and the eyes are relatively small (2).

The brown rat is believed to have originated from China. It reached Europe at some point in the early 18th century and was first seen in England in 1720 (4). At present it has a very wide distribution in Britain in both urban and rural areas, but is absent from a number of smaller islands. It is widespread and common in urban areas across the globe, with the exception of some tropical and subtropical areas (4).

You can view distribution information for this species at the National Biodiversity Network Gateway.

This species is associated with farm buildings in rural areas. Refuse tips, sewer systems, hedges and field margins are also suitable habitats for the brown rat (4).

Considered one of the most serious mammalian pests ever known, the brown rat is a true omnivore, eating a huge range of food including invertebrates, frogs, small mammals, birds' eggs, scavenged meat and bones, cereals and seeds (4), fruit, carrion, and any food discarded by humans (1). They have also been known to catch and eat fish (2). They are mainly nocturnal, but like many mammals they become increasingly active in the day where they are undisturbed by humans (4). They generally move around on the ground, but are also expert jumpers, climbers and swimmers (1), holding their tail aloft for balance when swimming (2). Brown rats live in 'packs' in which there is a dominant male who gains priority access to food, water and resting sites, and defends a harem of females, preventing subordinate males from mating (1).

This rat is one of the most prolific of all mammals (1). Females become sexually mature at just 8 to 12 weeks, gestation is between 21 and 23 days, and females are able to conceive whilst suckling a previous litter, often mating within 18 hours of giving birth (1). They can breed throughout the year if the weather is mild and there is plenty of food; 13 litters are possible each year, each one consisting of seven to nine young (4).

Maximum-known longevity is three years, but in the wild, life span is probably less than 18 months (4). The greatest cause of mortality is poisoning by rodenticides, but predators such as cats, foxes, dogs, mink, stoats and owls also take their toll (4).

Brown rats are controlled as they are serious pests. They carry dangerous diseases, and cause serious damage to property and crops (1). Brown rats have developed resistance to rodenticides, and many newer poisons are unfortunately highly toxic to many other vertebrates (4).

Not relevant.

View information on this species at the UNEP World Conservation Monitoring Centre.

There may be further information about this species available via the National Biodiversity Network Gateway.

For more on the brown rat:

• BBC Wildlife Finder:
  http://www.bbc.co.uk/nature/life/Brown_rat