The black rat is nocturnal, although it may become more active in the day in undisturbed areas (4). It is an omnivore, but tends to prefer plant matter (4) such as fruits and seeds, although it will also feed on insects, carrion, refuse and faeces (2). On Lundy Island these rats feed on crabs along the shore (4). This rat lives in groups called 'packs', consisting of several males and two or more dominant females (4). They are skilled climbers and can also swim well (2). Nests are constructed from grass and twigs, often in roof spaces, a habit which earned the species the further common name of 'roof rat' (2).
Breeding takes place between March and November; three to five litters can be produced in a year, each litter containing 1 to 16 young (although the average is seven). A single female can therefore produce a huge number of offspring; 56 young were recorded on a London ship for a single female (4). At 12 to 16 weeks of age, females are capable of breeding; they are also able to conceive whilst still suckling the previous litter, which further maximises their reproductive capability (1). Maximum lifespan in the wild is less than 18 months; populations have very high mortality rates, mainly as a result of widespread pest control measures (4). The black rat is a notorious pest, and was the host of the fleas that carried bubonic plague (2). It also carries a host of other diseases and is damaging to property and food stores (1).