Taxon Attribute Profiles

photo: Jim Parke, Healesville Sanctuary

Hydromys chrysogaster

Water-rat

Taxonomy and Ecology

Life Form/Morphology

Hydromys chrysogaster is a distinctive rodent specialised for an aquatic existence. It is a relatively large murid (bodylength, not including tail, up to 40 cm) with broad partially-webbed hind-feet, water-repellent fur, and abundant whiskers. The body is streamlined, with small ears and eyes, and colour is variable, ranging from slate grey to nearly black dorsally and from white to orange ventrally. The tail is thick and usually tipped white. The skull of H. chrysogaster is large and elongated and there are only two molars on either side of the upper and lower jaws, a feature shared with the False Water-rat Xeromys meroides (Watts and Aslin 1981). More than fifteen synonyms are available for this highly variable species (Flannery 1995), the validity of most of which, even at the subspecific level, is questionable.

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Distribution

H. chrysogaster is found in Australia, New Guinea, and a number of adjacent islands. This species is widely distributed in eastern Australia, occurring west to the Kimberleys along the northern coast and west to Eyre Peninsula on the southern coast. Its range is more restricted inland, where is found as far west as northeastern South Australia. Isolated populations occur in southwestern Western Australia and on a number of offshore islands, including Tasmania and Barrow, Bernier, and Dorre Islands (Olsen 1983). Inland populations, often associated with temporary water, can be highly unstable; H. chrysogaster is subject to heat stress and captive animals are unable to survive high temperatures without large amounts of water (Watts and Aslin 1981).

Habitat

The Water-rat generally occurs in permanent fresh or brackish water, although it can also be found in marine environments, including coastal mangroves in New Guinea (Flannery 1995). The species occupies a wide variety of freshwater habitats, from subalpine streams and other inland waterways to lakes, swamps, and farm dams. Populations may be abundant in drainage swamps, although the Water-rat seems to be much less common along river channels proper. H. chrysogaster is able to persist in urban areas and may be one of the few native species to have benefitted, at least in some areas, from human activity.

Role in Community

H. chrysogaster is a largely carnivorous rodent: crustaceans, aquatic insects, and fish form the bulk of its diet. Among insects, water beetles (Dytiscidae) and water bugs (Hemiptera) are of primary importance, and nymphs of damselflies and dragonflies (Odonata) can be seasonally important items. Birds, mammals, frogs, reptiles, mussels, spiders, and plants are also occasionally taken, with plants more commonly consumed in winter or during periods of limited resources (Woollard et al. 1978; Harris 1978).

The Water-rat typically forages close to the shoreline, restricting its movements to shallow water (up to 2 m in depth). In sufficiently shallow areas, it wades through the water in search of aquatic prey, and it dives in areas of greater depth (Watts and Aslin 1981). Prey is often taken to a favourite feeding platform, such as a log, rock, or stump, located close to the water, where remains of its food are left.

Reproduction and Establishment

Reproduction

Although breeding can occur throughout the year, young of H. chrysogaster are typically born from September through January. A litter typically consists of 3-4 young, but litter size is variable and as many as seven have been recorded. Up to five litters may be produced in years of abundant food and water, whereas breeding is irregular and smaller litters are produced in drier years. Social factors also affect the timing of breeding and age at first breeding. Nesting occurs in burrows in banks of lakes, streams, and other bodies of water (Olsen 1995).

photo: Jim Parke, Healesville Sanctuary

Dispersability

H. chrysogaster is a generalist species and shares the good dispersal capabilities often associated with this life history. It is known to forage on land and may move considerable distances when doing so. Water-rats undertake regular movements along shorelines, where their tracks and runs may be readily seen, and also follow regular routes when crossing bodies of water (Harris 1978). H. chrysogaster is mainly nocturnal, although it differs from most Australian rodents in being partially diurnal. It is most active in the hours following sunset, but may also be found swimming or foraging during daylight in the early morning or early evening. The Water-rat is territorial and may be quite aggressive when populations are at high density.

Juvenile Period

Young are suckled for approximately four weeks and remain with the female for up to an additional four weeks. Breeding generally does not begin until eight months of age, although females can become sexually mature at four months and have been known to breed in the season of their birth (Olsen 1995).

Hydrology and Salinity

Salinity

Substantial declines of H. chrysogaster have been noted in southwestern Western Australia and along inland waterways affected by salinity and degradation (Lee 1995).

Flooding Regimes

The Water-rat prefers slower-moving waters to faster flowing river channels, and restricted flooding has some potential to affect this species.

Conservation Status

As a species H. chrysogaster is of least conservation concern, although Although water-use practices have undoubtedly removed habitat from H. chrysogaster, its current range is likely similar to that occupied prior to European settlement (Watts and Aslin 1981). The Water-rat has been considered a pest in irrigation districts (such as those along the Murray), where it burrows in channel banks and other water-control and irrigation structures, causing leakage and sometimes collapse of structures (McNally 1960). Some sources, however, regard this damage as less substantial than that caused by freshwater crayfish, whose populations H. chrysogaster helps to control (e.g., Olsen 1983). Natural predators of the Water-rat include raptorial birds and cats, and the young are vulnerable to snakes and large fish.

Use

H. chrysogaster was formerly trapped for its fur, but the Water-rat is now a protected animal in Australia and populations appear to have recovered from the effects of hunting.

Summary

The Water-rat occurs in a wide variety of aquatic habitats in the Murray Darling Basin. It is a good coloniser and can be expected to be a reasonable indicator of the presence of its largely aquatic prey and the general quality of the water bodies it typically inhabits.

References

Flannery, T. 1995. Mammals of New Guinea. Reed Books, Chatswood, NSW.

Harris, W. F. 1978. An ecological study of the Australian Water-rat (Hydromys chrysogaster: Geoffroy) in southeast Queensland. MSc thesis. University of Queensland, Brisbane.

Lee, A. K. 1995. The action plan for Australian rodents. Australian Nature Conservation Agency, Canberra.

McNally, J. 1960. The biology of the water rat Hydromys chrysogaster Geoffroy (Muridae: Hydromyinae) in Victoria. Australian Journal of Zoology 8:170-180.

Olsen, P. D. 1983. Water-rat Hydromys chrysogaster. Pp. 367-368 in The Australian Museum Complete Book of Australian Mammals (R. Strahan, ed.). Angus and Robertson, Sydney.

Olsen, P. D. 1995. Water-rat Hydromys chrysogaster. Pp. 628-629 in The Mammals of Australia (R. Strahan, ed.). Reed Books, Chatswood, NSW.

Watts, C. H. S. and H. J. Aslin. 1981. The rodents of Australia. Angus and Robertson, Sydney.

Woollard, P., Vestjens, W. J. M., and L. Maclean. 1978. The ecology of the Eastern Water Rat Hydromys chrysogaster at Griffith, NSW: food and feeding habits. Australian Wildlife Research 5:59-73.