Brown rat  (Rattus norvegicus)

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 8-12 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 7-9 young (4).

Maximum-known longevity is 3 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).