Unlike the black rat (R. rattus), which commonly climbs, the brown rat is largely a ground-dwelling species (1) (4) (6) and is a strong swimmer (2) (3) (5) (6), capable of crossing quite large stretches of open water (2) (4). This species is mainly nocturnal and has quite poor vision, but it makes up for this with its acute senses of smell, touch, taste and hearing (2).
The brown rat typically lives in underground burrows that have one or more exits and rooms for nesting and for food storage (2) (4) (6). There is usually a well-travelled system of pathways around the burrow, along which the rats regularly forage and which they can use to escape danger. These pathways are scent-marked with urine (2) (3).
The brown rat has a varied and opportunistic diet, feeding on almost anything edible. As well as grains, seeds, nuts, fruits and other plant material, this species also eats insects and other invertebrates, birds and their eggs, fish, and other small animals such as mice and young rabbits (2) (3) (4) (6). The brown rat has been known to attack larger animals such as poultry and young lambs (2) (3) (6), and will also eat carrion (2) and even substances such as soap, paper and beeswax (6). Food is sometimes carried back to the burrow to be stored (4) (6).
A sociable species, the brown rat tends to live in social groups which occupy a territory and can be aggressive to individuals outside of the group (2) (3) (6). The group has a dominance hierarchy, with dominant males typically occupying the prime areas in the territory and mating with several females. Females within the group often collectively nurse their young, although individuals may maintain their own nesting burrows (2) (6).
The brown rat is a prolific breeder and is capable of breeding year-round in some areas (2) (3) (5) (6), although there are usually peaks at certain times of year (2) (6). The female brown rat builds a nest in which to give birth (3), and usually has about 6 to 9 young per litter (2), although litters of up to 22 have been recorded (2) (6). The young rats are born after a gestation period of 21 to 24 days (2) (3) (4) and are naked and blind at birth, developing fur at about 1 week old and opening their eyes after around 2 weeks (2) (6). Young brown rats are weaned and leave the nest at about a month old (2) (4).
Female brown rats can become pregnant again soon after giving birth, and can raise as many as 12 or 13 litters a year (2) (5) (6). The brown rat can reach sexual maturity at just two to three months old, further contributing to this species’ rapid reproductive rate (2) (3) (5) (6). When the brown rat population becomes particularly high, large migrations of individuals away from an area may occur (3) (4) (6).
Young brown rats face a wide variety of predators, including owls, foxes, weasels, stoats, and domestic cats and dogs (2) (3) (4) (5), and most do not live for more than a year (2) (3). Many rats are also deliberately killed by humans (2). However, healthy, full-grown brown rats are less vulnerable to predators as they are relatively large and aggressive (3) (5). In captivity, this species can potentially live for up to five years (2) (3).