Australia’s heaviest native rodent (2), the common water rat (Hydromys chrysogaster) is an amphibious mammal endemic to Australia and New Guinea (3).
With its sleek, streamlined appearance (4), high-set eyes and small ears (4) (5), the common water rat looks somewhat like a mink or an otter (5). Its broad, flattened head is thought to enable the common water rat to forage for food under stones on the river bed (3) (4) (5) (6), and is one of several adaptations to this species’ semi-aquatic lifestyle (3). Other adaptations include its seal-like fur (4), which is soft, dense and water-repellent (3) (5) (7), and its broad, partially webbed hind feet, which make the common water rat a strong swimmer (2) (3) (4) (5) (7) (8).
The common water rat’s scientific name is derived from the colouration of a specimen caught on Bruny Island in 1802. This particular individual had yellow-brown fur on its underside, and so was named chrysogaster, with ‘chrysos’ meaning ‘golden’, and ‘gaster’ meaning ‘belly’ (9). However, while some individuals do have beautiful golden belly fur (5), the colouration of the common water rat is actually quite variable (9). The upperparts are usually rather dark (7), ranging from brown to grey, while the underparts can be brown, golden-yellow, cream or even white (2). The common water rat’s thick, muscular tail is heavily furred, and is mostly dark except for a white tip (2) (4) (5).
The common water rat is a member of the rodent family, but while most rodents have up to 22 teeth, the common water rat has just 12 (10), and its basin-shaped molars are quite distinctive and unique among rodents (5) (8) (10).
- Also known as
- Australasian water rat, Australian otter, eastern water rat, golden-bellied beaver rat, Water rat.
- Head-body length: 29 - 39 cm (2)
- Tail length: 23 - 33 cm (2)
- 650 - 1,250 g (2)
Common water rat biology
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).
Common water rat range
The common water rat is found across much of the Australian mainland (1), as well as in Tasmania and on many smaller islands off the Australian coast (1) (4) (11). This water-dwelling species also occurs in Papua New Guinea and on islands off the coast of Indonesia, including Obi Island and the Kai and Aru Islands (1) (2) (4) (6) (11) (12).
Common water rat habitat
Aside from the platypus (Ornithorhynchus anatinus), the common water rat is the only Australian mammal specialised for an aquatic lifestyle (7). It can be found in a wide range of permanent freshwater habitats (1) (6) (7) (13) (14), including rivers, streams, swamps and even artificially irrigated sites (1) (4) (7), as well as mangroves, estuaries and salt marshes (1) (4) (6) (7) (13) (15).
On New Guinea, the common water rat occurs from sea level to elevations of 1,900 metres (1) (11).
Common water rat status
The common water rat is classified as Least Concern (LC) on the IUCN Red List (1).
Common water rat threats
The common water rat used to be trapped for its soft, dense pelt (3) (4), but trapping regulations have since been established in certain areas (4). However, although this species is generally common and is not currently thought to be facing any major threats, populations of the common water rat have declined in parts of Australia. In addition, some populations in New Guinea have become locally threatened as a result of aquatic pollution stemming from mining activities (1).
The introduction of non-native species to Australia appears to be a potential threat to the common water rat (1) (18). This species has become extinct in the Montebello Islands off Western Australia, an event which is thought to be the result of the high density of introduced black rats (Rattus rattus) and cats (1). In other parts of the country, green cabomba (Cabomba caroliniana), a type of invasive water plant, is thought to have contributed to a reduction in common water rat populations (18).
Common water rat conservation
The common water rat is common and widespread (1) (17), and so is not currently considered to be at risk of extinction (1). This species is now protected by legislation within Australia (3), which includes the establishment of trapping regulations in certain regions (4), and it is found in many protected areas within Australia and New Guinea (1).
Further studies into the taxonomy of the common water rat have been recommended (1).
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- Diverse group of animals with jointed limbs and a hard external skeleton, characterised by the possession of two pairs of antennae, one pair of mandibles (mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, woodlice and barnacles.
- A species or taxonomic group that is only found in one particular country or geographic area.
- The state of being pregnant; the period from conception to birth.
- Animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones) and echinoderms.
- Active at night.
- The science of classifying organisms, grouping together animals which share common features and are thought to have a common ancestor.
- Describes an animal, a pair of animals or a group that occupies and defends an area.
IUCN Red List (October, 2012)
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