Identification
Well adapted to aquatic life with its webbed hind feet and waterproof coat, the Water-rat can be identified by its large size and long tail with a white tip. The main characteristics that help distinguish the Water-rat from other rodents include:
- Front teeth: One pair of distinctive chisel shaped incisors with hard yellow enamel on front surfaces.
- Head: Flattened head, long blunt nose, with abundant whiskers, small eyes.
- Ears: Notably small ears.
- Colouring: Variable. Near-black, grey to brown, with white to orange belly. Thick soft waterproof fur.
- Main feature: webbed hind-feet.
- Tail: Thick, white-tipped.
Size Range
Body 231 mm - 370 mm, tail 242 mm - 345 mm, weight 340 g - 1275 g.
Distribution
The Water-rat is found in Queensland, New South Wales, Victoria, Tasmania, South Australia, Western Australia (south-west and north), Northern Territory.
As the weather warms, chances are you may see a water rat, one of Australia's shyest and least known native mammals.
Water Rat/ Ralaki
You may be forgiven for thinking the quietly receding shape paddling along the surface of a lake or creek is a platypus, but it is probably a water rat, one of Australia's shyest and least known native mammals.
It's an easy mistake to make; platypus and rats are known to have similar diets, overlapping territories, and have even been known to use the same burrows, one at a time of course. With the warmer weather, water rats are abandoning their usually solitary dispositions for brief, frenzied mating encounters, before returning to their peaceful riverine existence.
Water rats, Hydromys chrysogaster are large, highly-specialised rodents, with broad, partially-webbed hind feet, water-repellent fur and streamlined bodies, and with their distinctive white-tipped tail and heavily whiskered face they strongly resemble otters.
Nocturnal animals, they spend the days protected from predators and temperature extremes in metre-long burrows dug into the creek bank. Normally they move around from burrow to burrow, sleeping in the first available shelter. They do little to maintain or improve their overnight accomodation, unless about to give birth, when a pregnant female spends time enlarging and making her nest chamber more comfortable.
Colour of aggression
Although solitary by nature, and generally considered a very mild-mannered animal, water rats are known to fight among themselves. Surprisingly, observations made a number of years ago by Dr Penny Olsen from the Australian National University found that the key to predicting a water rat's aggression level lies in the colour of its belly fur.
Like many aquatic animals they are generally dark on top and light -coloured underneath, with belly fur ranging in colour from golden (the species name "chrysogaster" means "golden bellied) to cream and all shades in between. The study found that paler the rat's underside, the less aggressive it was likely to be: golden-bellied, black-backed cold country rats (from Tasmania, or around Canberra) were consistently more aggressive than paler, warm country individuals.
Rafts and landbridges
According to a biologist with a Australian Platypus Conservancy, the earliest Hydromys fossils found in Australia are around one million years old, although much older fossils have been found in elsewhere. She suggests they crossed from PNG to Australia via the landbridge that once joined the two landmasses, and then later dispersed from the Australian mainland to other offshore islands. And because they are often found living around salt water beaches, or dependent on marine environments like the coastal mangrove swamps of New Guinea (and can therefore tolerate salt), biologists postulate that they might easily have rafted, or even swum to conquer new lands.
In Australia, they live primarily in a wide variety of freshwater habitats, from sub-alpine streams and other inland waterways to lakes, swamps, farm dams and irrigation channels and are thought to be one of the few native species to have at least partially benefited from human encroachment.
Mainly carnivorous, with aquatic insects, crustaceans and fish forming the basis of their diet, water rats have also been occasionally observed eating birds, small mammals, frogs, reptiles, mussels, spiders and plants, although the latter is only a last resort during winter or times of drought.
They tend to forage close to the shoreline, limiting their activities to shallow water, where they either wade in search of prey, or dive to up to two metres in search of food. Unlike platypus, which eat underwater, water rats carry their prey to a favourite feeding platform, such as a log or rock, on the bank. Holding it firmly in their forepaws, they eat their hapless prey with apparent relish, sitting up and crunching noisily. And it's not hard to tell which is the rat's favourite picnic spot-half-chewed remains litter the feeding platform long after they've gone back into the water for seconds.
Cane Toad eaters
A member of the public in the Northern Territory said she was fed up with the local water rats littering her pool surround with the remains of cane toads that they had killed and eaten.
"This is particularly interesting because we know that once cane toads move into an area, native carnivores such as the northern quoll, start disappearing shortly afterwards, owing to their habit of catching prey by grabbing them around the neck, right where the cane toad's poison sacs are located.
"Somehow water rats are either immune to cane toad poison, or they have figured a way to avoid coming into contact with the poison glands."
These animals are highly intelligent, and there is more anecdotal evidence of mothers spending time to teach their young the most efficient way of eating mussels (by bringing them up and leaving them in the sun, they open by themselves). it is possible that one water rat may have stumbled on an effective cane toad killing technique and passed it onto her young, and likens its subsequent spread through the population to a form of cultural evolution occasionally seen among intelligent animals.
"It is this intelligence that makes a biologists' jobs so hard, "Once caught in a trap they know the signs and are unlikely to be caught again."
Live hard, die young
Platypus live up to ten years in the wild (and 20 in captivity), water rats live only two to three years in the wild, and not much longer in captivity.
In general, they breed in the warmer months, from September to March, although they can be found breeding throughout the year in Central Australia, if there is plentiful food and water available. They have litters of between three and five young, and may have up to three litters a year, although the number of young born in a year depends very much on the availability of food.
The staff at a Australian Platypus Conservancy are concerned because with their short lifespan, and so many bad breeding years caused by the drought, Water rats have disappeared from some parts of their range.
"They have been dealt a double blow: their lifespan has been foreshortened by the drought, and their numbers are declining because they can't produce enough viable young in the limited life available to them."
Again, we are relying heavily on feedback from the public about whether water rats have disappeared from their local area,They are charming little animals and ecologically very important." Even drought stricken farmers who in the past lamented the water rats' habit of mining their irrigation ditch walls, are coming to recognise how important the rodents are at keeping the numbers of freshwater crayfish, which also dig tunnels in the irrigation channel walls and are blamed for weakening them structurally, to a manageable level.
Conservation Status
During the depression in the 1930s, a ban was placed on the import of furred skins (mostly American Muskrat). The Water-rat was seen as a perfect substitute and the price of a Water-rat pelt increased from four shillings in 1931 to 10 shillings in 1941. The species was heavily hunted during this time until protective legislation was introduced. Populations seem to have made a recovery.
The main threats to the Water-rat today are habitat alteration as a result of flood mitigation and swamp drainage, and predation by introduced animals such as cats and foxes.