The Australian Water-rat – Australia’s “otter”
Australia supports a diverse array of native rodents that have been a part of the local landscape for at least half a million years and in some cases much longer.
The largest of these is the Australian water-rat (also known as rakali), a very attractive animal weighing up to 1.3 kilograms – as big as a medium-sized platypus. The water-rat’s ancestors are believed to have arrived in Australia around 5-10 million years, after swimming (or possibly rafting) from New Guinea.
The Australian water-rat is an aquatic predator which resembles a small otter in many ways:
- Muzzle is blunt and furnished with a dense set of whiskers
- Hind feet are broad, partly webbed and paddle-like (below)
- Tail is well-furred and thick to help serve as a rudder when swimming (below)
- Body is elongated and streamlined
- Ears are small and can be folded flat against the head for a streamlined profile
- Fur is soft and lustrous, drying quickly and helping to keep the animal warm in the water
Even if your local creek or lake doesn’t have platypus, there may well be a population of these fascinating rodents, which are equally deserving of protection as native wildlife.
The scientific name of the species (Hydromys chrysogaster) translates as “golden-bellied water mouse”.
Early European settlers sometimes called this animal a beaver rat, even though it is actually much more like an otter than a beaver in its behaviour.
In the early 1990s the Australian federal government proposed changing the common name of the Australian water-rat to rakali, which is one of the aboriginal terms for the animal. Although rakali has gained some public acceptance as the common name for Hydromys chrysogaster , these animals are still more widely known as water-rats.
Distribution and status
Australian water-rats occupy a wide variety of natural and manmade freshwater habitats, including swamps, ponds, lakes, rivers, creeks and irrigation channels. They also inhabit brackish estuaries and sheltered ocean beaches. Water-rats are widely distributed on both the mainland and Tasmania and also occur on many offshore islands.
Although very little is known about the current status of water-rats in most parts of their range, capture rates in most areas tend to be quite low.
In part, this reflects the fact that water-rats are highly intelligent animals that are naturally wary of entering metal cage traps and also spend a large proportion of their time feeding in the water rather than on land. Water-rats are also very good at escaping from nylon mesh survey nets set in the water, using their sharp teeth to snip holes through the netting and escape.
In addition, water-rats are relatively aggressive animals that do their best to defend a territory through scent-marking and aggressive behaviour towards other individuals. In turn, this will tend to ensure that relatively low numbers of adult water-rats occur in most habitats.
Anecdotal evidence also suggest that water-rat numbers have declined in many places in southeastern Australia particularly since the mid-1990s, probably due to the combined impacts of drought and habitat degradation. More work is needed to map where water-rats occur and determine how the species’ distribution may have changed in recent decades. In turn, this information will provide a factual basis for longer-term population monitoring.
Size and appearance
Adult water-rats measure up to 35 centimetres in length from their nose to rump, with a slightly shorter tail. Adult males typically weigh 0.8 kilograms (up to 1.3 kg) and adult females typically weigh 0.6 kilograms (up to 1.0 kg).
Depending on location, water-rats can vary considerably in colour. The head and back may be nearly black (with golden-yellow belly fur) or some shade of brown or grey (with fawn- to cream-coloured belly fur). However, apart from occasional individuals that have lost the end of their tail through fighting, water-rats are characterised by a distinctive white tip to the tail across their entire geographic range.
Water-rat fur is moulted twice a year, being coarser and denser in winter. As in the case of platypus fur, it consists of a dense, fine layer of underfur covered by coarser guard hairs. Although water-rat fur is reasonably waterproof, it is much less effective than platypus fur at keeping its owner warm – water-rats are unable to maintain their body temperature in water temperatures below 25°C and consequently need to exit cold water at regular intervals in order to warm up in a burrow or other sheltered site.
Related species and subspecies
Hydromys chrysogaster has no close relatives in Australia, although several other species of Hydromys are found in the New Guinea region. By the same token, the Australian water-rat is not closely related to either the European water vole (a.k.a. water-rat) or the American muskrat.
Several subspecies of H. chrysogaster have been proposed over time, generally based on variation in fur colour. Confirmation that valid subspecies occur will probably depend on the findings of future genetic studies.
Foraging behaviour and diet
Water-rats mainly consume aquatic prey (including fish, frogs, turtles, crayfish, crabs, large aquatic insects, mussels and clams), but the remains of terrestrial prey (such as mice and bats) have also been discovered in water-rat faeces. It has been suggested that the proportion of foraging on land may increase in winter when water temperature declines.
Large water-rats have occasionally been documented to kill reasonably large water birds, such as ducks and coots. They will also eat carrion and scavenge for human food scraps. Based on anecdotal reports, water-rats will travel several hundred metres across dry land to dine on delicacies, such as pet food left out regularly on a back porch.
Water-rats also appear to have the rare ability to be able to kill the introduced cane toads found in Australia’s tropical north. By flipping the toads over before biting them, they avoid the poisonous parotid glands found on the back of the toad’s neck.
After catching their prey, water-rats typically carry it in their mouth to a favourite feeding spot on a log or rock located at the water’s edge or in the channel. Large piles of clam shells, crayfish claws or fish bones and scales can accumulate at such platforms — the remains of many water-rat meals.
Home range size and movements
Water-rats are highly territorial, marking their home ranges with a strong scent reminiscent of the odour of cat urine. Apart from females raising dependent offspring, it is presumed that adult water-rats lead solitary lives.
Relatively little is known about home range size and movements in this species. An adult male radio-tracked along a small creek in Victoria was found to have a home range extending at least 3.9 kilometres, whereas three males living in Queensland had home ranges that extended at least 0.9-2.2 kilometres. An overland movement of at least 3 kilometres has also been recorded.
In places where populations are dense there is considerable fighting, as evidenced by a high frequency of bite marks on tails and hind feet. This suggests that juveniles have to disperse from their mother’s home range fairly soon after becoming independent; however, nothing is known about this process.
Reproduction and life history
Water-rats can potentially breed throughout the year if conditions are favourable, but mating most typically occurs in late winter to early summer, with juveniles appearing from September to February. The gestation period is around five weeks long. Females generally first breed at the age of about a year and raise two or three litters of young in a good year. A female water-rat only has four nipples and typically raises just two to four babies in a given litter, suckling her young for about a month. After weaning, juveniles remain with their mother for a few more weeks before leaving home for good.
It is believed that water-rats normally survive for a maximum of about 3-4 years in the wild.
Burrows and activity patterns
Water-rats occupy burrows located in creek and river banks, or shelter in large hollow logs lying near the water. Radio-tracking studies undertaken by Australian Platypus Conservancy staff have shown that platypus and water-rats will use the same burrows, though probably not at the same time. On one occasion, an adult female platypus occupied a burrow a few weeks after it served as a nursery for a female water-rat with a litter of young. Such behaviour is not especially surprising given that platypus and water-rats are about the same size and both are known to make use of many different burrows over time. It remains unknown whether the two species are equally likely to dig a new burrow in the first place.
Water-rats are subject to predation by many different species, including snakes, large predatory fish, birds of prey, and cats, dogs and foxes. However, there is no reason to believe that water-rats have ever vanished from any area solely due to predation. By the same token, there is no evidence that any diseases have an important impact on water-rat numbers.
Because water-rats are warm-blooded carnivores which require a lot of food to fuel their energetic lifestyle, the main problem facing the species is most likely to be habitat degradation, if this in turn reduces the animals’ aquatic food supply.
Given that water-rats have a fairly short natural lifespan (in most cases living no more than 3-4 years), local populations may decline in size and even disappear if females fail to reproduce successfully for several years in a row – for example, due to the combined effects of poor habitat quality and ongoing drought.
Water-rats were once widely trapped for their fur and sometimes culled when they were perceived to be a nuisance in irrigation districts. However, they are now fully protected by law as native wildlife.
Unfortunately, many continue to drown in opera house traps and other enclosed nets designed to capture yabbies and freshwater crayfish. These nets are also known to kill large numbers of platypus and freshwater turtles. Recreational anglers are therefore strongly advised to consider using lift-style hoop nets or old-fashioned baited lines (without hooks) as wildlife-friendly alternative methods for procuring a meal of yabbies or crays.
Australian water-rats can sometimes come into conflict with humans when they raid fish farms or chicken yards, kill free-ranging guinea pigs in gardens, steal bait from anglers, leave piles of food debris on the decks of moored yachts or on verandahs, or deposit chewed up cane-toads around the edge of swimming pools!
However, killing or relocating problem water-rats is illegal and subject to substantial fines. In any case, such action is likely to be totally ineffective because dispersing juveniles are likely to recolonise the area in a relatively short space of time. A much more sustainable solution is to learn to live with water-rats by rat-proofing problematic areas and not leaving food around that will attract them.
Co-existence of water-rats and platypus
Platypus and water-rats both function as top predators in Australian freshwater systems and probably compete to some extent for food. However, the size of prey that can be consumed by an adult platypus is limited by the fact that its bill is equipped only with rough grinding pads to help process food. In contrast, a water-rat has a formidable set of sharp incisors to help kill and dismember prey. Interestingly, the grinding surfaces of water-rat molars are quite smooth. Like the grinding pads of the platypus, this adaptation may be particularly effective at dealing with the hard, encased bodies of many aquatic invertebrates.
In practice, very little is known about the ecological and behavioural interactions between platypus and water-rats. The two species are found living together in many places, so water-rats clearly do not automatically exclude platypus from freshwater environments (and vice versa).
However, there are also waterways where only one of the two species is commonly found. In general terms, water-rats are much more likely to persist in badly degraded aquatic habitats than the platypus. This may reflect the fact that the water-rat is able to forage on land and also prey on introduced fish species.
It has been suggested that water-rats may sometimes prey on young platypus but there appears to be no actual documented evidence to support this idea.
How to report water-rat sightings
Recent sightings of water-rats (including details of when and where the animal was seen) can be reported to the Australian Platypus Conservancy via the Reporting Form. The information will be added to a secure data base, thereby contributing to an improved understanding of the species’ distribution and status in the wild
Reading list (Australian Water-rat)
Barrow, G. (1964). Hydromys chrysogaster – some observations. Queensland Naturalist 17: 43-4.
Dawson, T.J. and Fanning, F.D. (1981). Thermal and energetic problems of semiaquatic mammals: a study of the Australian water-rat, including comparisons with the platypus. Physiological Zoology 54: 285-296.
Earthcare St Kilda (2004). Rakali Watch 1999 Results (www.earthcare.stkilda.net.au).
Fanning, F.D. and Dawson, T.J. (1980). Body temperature variability in the Australian water-rat, Hydromys chrysogaster, in air and in water. Australian Journal of Zoology 28: 229-238.
Fleay, D. (1990). The shy, water-loving aristocRAT. Wildlife Australia (summer): 12-15.
Gardner, J.L. and Serena, M. (1995). Observations on activity patterns, population and den characteristics of the water rat Hydromys chrysogaster (Muridae: Hydromyinae) along Badger Creek, Victoria. Australian Mammalogy 18: 71-75.
Harris, W.F. (1978). An ecological study of the Australian water-rat (Hydromys chrysogaster Geoffroy) in southeast Queensland. M.Sc. Thesis: University of Queensland.
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. (1982). Reproductive biology and development of the water rat, Hydromys chrysogaster, in captivity. Australian Wildlife Research 9: 39-53.
Smales, L.R. (1984). A survey of Hydromys chrysogaster the Australian water rat in central Gippsland. Victorian Naturalist 101: 115-118.
Vernes, K. (1998). Observation of a long-range overland movement event by an adult common water rat Hydromys chrysogaster. Australian Mammalogy 20: 409-410.
Watts, C.H.S. and Aslin, H.J. (1981). The Rodents of Australia. Angus & Robertson, Sydney.
Williams, G.A. and Serena, M. (2004). Distribution and status of Australian water-rats (Hydromys chrysogaster) in the Melbourne metropolitan region. Information obtained from platypus surveys, 1995-2003. (Report to Melbourne Water). Australian Platypus Conservancy, Whittlesea.
Woollard, P., Vestjens, W.J.M. and Maclean, L. (1978). The ecology of the eastern water rat Hydromys chrysogaster at Griffith, N.S.W.: food and feeding habits. Australian Wildlife Research 5: 59-73.