I grew up in wheat and sheep country where there were very few native mammals around. Maybe that’s why as a kid I took great pleasure in regularly standing on a bridge over the local lagoon in anticipation of seeing the resident Water Rat working its way along the water’s edge. I recall being told how hunting had nearly wiped them out because their soft, durable pelts were much sought after during the lean depression years. The story goes that locals would make traps out of old jam tins, and once caught, the Water Rat pelts would be taken to the local tannery where they would fetch a handsome sum.
I puzzled over how this once over-hunted critter survived in water that no one ever dared swim in because it looked and smelt, well…. just a little like a cess pit. I also wondered how the sleek creature I enjoyed watching ended up with the seemingly undignified name of ‘Water Rat’. Despite the fact that there are many fascinating native rodent species in Australia, it must be said that anything with the word ‘rat’ in its title is encumbered with a bit of an image problem. Even the slickest marketing spin-doctor would struggle to win popular appeal for a rat, let alone a partially drowned rat.
My native lagoon critter looked more like the otters I had seen on nature documentaries. Ok, so I will concede that it is in fact Australia’s largest member of the rodent family and it does have large chisel shaped incisors and abundant whiskers, a bit like a very large…rat. But to this young admirer it was a glistening, streamlined native predator gliding along on its bow wave, as flash as a rat with a gold belly!
Perhaps that’s why there has been an official attempt to change its common name to ‘Rakali’, which is an indigenous name. But I think it’s safe to say that this name change hasn’t really stuck and nor has “Golden-bellied Water Mouse’ which is what its scientific name (Hydromys chrysogaster) translates as; so ‘Water Rat’ it is.
Adult Water Rats are the size of a small possum with the body measuring about 30-35 cm in length while the distinctly white–tipped tail is slightly shorter than the body. They have a flattish square head with small ears and eyes. Their colour varies considerably with some having a brown to black back and a golden belly, while in other areas they can appear slate grey with a white belly. They are well adapted to their aquatic lifestyle with dense water repellent fur. They use their large, partially webbed hind feet as paddles, while their thick tail operates like a rudder.
They occur throughout most of Queensland and much of non-arid Australia where permanent water bodies exist. Along with an additional eight closely related species (tribe Hydromyini) they also occur in New Guinea where they are thought to have originated. Apart from their obvious requirement for water they are habitat generalists capable of occupying an assortment of aquatic environments both natural and man-made, fresh, brackish and saline. They tend to avoid high energy streams, preferring slow moving or still water. As demonstrated by my lagoon creature they also appear to be able to survive in polluted waterways.
Water Rats are opportunistic predators and while they catch most of their prey in shallow waters close to the shoreline, they are also adept at hunting and scavenging on land. They are predominately carnivorous and their diet varies according to location. Prey can include crayfish, aquatic invertebrates, fish, mussels, birds (including domestic poultry), small mammals, frogs and reptiles (including small turtles). Adjacent to urban waterways they have been recorded preying on introduced Black Rats (Rattus rattus). They will also eat carrion, food scraps, the occasional plant and have been observed sneaking meals from pet bowls and camp sites.
Water Rats are intelligent animals. They remove mussels from the water and leave them in the sun to open prior to eating. There is also evidence to suggest that they can consume Cane Toads without ill effect. Researchers have found them very wary of traps and if caught they don’t make the same mistake twice. If accidentally caught in nylon cray sh traps they will often chew their way out. However like turtles and Platypus, Water Rats can and do drown if caught in an ‘opera house’ style cray fish trap.
Water Rats are generally shy and not often observed; however, one sign that indicates their presence is their habit of dining at a ‘table’. Once captured, prey is carried to a favoured feeding location such as an exposed tree root, rock or log. Discarded crayfish and mussel shells on such a ‘table’, or eaten out fish scattered around a pond, can be a good indication that a Water Rat lives nearby.
Dusk is probably the best time to catch a glimpse of them as they are usually most active after sunset, but they are unique amongst rodents for their occasional day time foraging. By day, Water Rats occupy burrows located on the banks of streams, or shelter in large hollow logs lying near the water. Burrows have a round entrance of about 15 cm diameter. They will use multiple burrows in their territory, including unoccupied Platypus burrows.
Other signs that may indicate the presence of Water Rats include well-worn runways along the water’s edge or regular crossing points. Their scats are torpedo-shaped and about 1 cm long, 8 mm wide. Males leave a distinctively pungent shy scent to mark their territory. Not only are they smelly, the males are quite aggressive and will defend their territory vigorously which can lead to fierce fights with intruders, occasionally resulting in tails being lost or injured.
Nesting occurs in a chamber at the end of the burrow and in times of plenty they can have multiple litters per year. Litters usually consist of two to four young which usually appear in the warmer months of the year. After about a month of suckling the young are weaned and have to fend for themselves. It is believed that Water Rats normally survive for a maximum of about 3-4 years in the wild and for the most part lead a solitary existence.
They are a seemingly tough and resilient species that appears to tolerate human encroachment and modification of habitat. Today there are urban populations in highly developed rat-races such as inner Sydney and Melbourne. However their relatively short lifespan leaves them vulnerable to population crashes if successive years of drought restrict successful breeding opportunities, especially in marginal habitat. Habitat alteration such as swamp drainage and flood mitigation do pose some risk for this species as does predation by introduced animals such as cats and foxes. They are also predated on by snakes and sh when they are young, and birds of prey will take adult Water Rats.
Despite the hunting frenzy in the 1930s the Water Rat’s distribution doesn’t appear to have altered much since European settlement. As urban and rural land management practices continue to improve, hopefully so too does the habitat of this poorly known Australian aquatic predator. I recently revisited my childhood lagoon and I was pleasantly surprised to see that the banks had been revegetated and the water was much cleaner. Although I didn’t see one, I was also reliably informed that it is still home to a ‘Rakali’. Who knows maybe the name change will catch on after all?
References and Further Reading
Flannery T (1995) Mammals of New Guinea. Cornell University Press.
Harris WF (1978) An ecological study of the Australian Water-rat (Hydromys chrysogaster: Geo roy) in southeast Queensland. MSc Thesis, UQ.
Lee AK (1995) The Action Plan for Australian Rodents. Dept of Environment, Australian Government.
McNally J (1960) The biology of the water rat Hydromys chrysogaster Geo roy (Muridae: Hydromyinae) in Victoria. Australian Journal of Zoology 8: 170-180.
Olsen PD (1995) Water-rat Hydromys chrysogaster. pp 628-629 in The Mammals of Australia (R. Strahan, ed.).
Van Dyck S & Janetzki H (2007) in Wildlife of Greater Brisbane, Qld Museum.
Watts CHS & Aslin HJ (1981) The Rodents of Australia. Angus & Robertson.
Woollard P, Vestjens WJM and Maclean L (1978) The ecology of the eastern water rat Hydromys chrysogaster at Gri th, NSW: food and feeding habits. Australian Wildlife Research 5: 59-73.
Article by Nick Clancy Land for Wildlife Officer Sunshine Coast Council