The common water rat is generally a solitary species (2), and is unusual among Australian rodents in that it is not entirely nocturnal (5). Although it is primarily active at night, particularly at dawn and dusk (2) (7), this species is also sometimes seen during daylight hours (4) (7). The common water rat lives in a nest of shredded weeds within a riverbank burrow or hollow log (1) (4), and individuals appear to be quite territorial, with males tending to fight in areas with large populations (4).
The common water rat swims with its eyes, nose and top of the head above the water’s surface, and in many ways behaves more like an otter than a rat (6), which commonly leads to it being referred to as the Australian otter (7). It is an opportunistic feeder (13), and unlike most rodents it is a predator (7). The common water rat has an extremely varied diet, consisting of fish, large aquatic invertebrates, crustaceans, mussels, frogs, and even waterbirds and small mammals (2) (4) (6) (7) (10). When hunting, the common water rat usually swims along the bottom of streams (4), but it generally carries its prey back to a preferred resting site, such as a log or stone, before eating it (4) (6). While most foraging takes place underwater, some food is taken from waterside vegetation (1).
Although the common water rat can give birth year-round (7) (16), in Australia there are breeding peaks in spring and summer, between September and March (4) (7) (16) (17). With a gestation period of around 34 days (4), the female common water rat can produce up to 5 litters per year (1) (7). Each litter contains between one and seven young (4), with three or four being most common (1) (4) (7). The young are born blind and naked, and do not open their eyes until they are about two weeks old (4). By 35 days of age, the young common water rats are independent (16), and they reach sexual maturity at about 4 months old (4).