Australia supports a diverse array of native rodents that have been a part of the local landscape for several million years. The largest of these is the Australian water-rat (or 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 originally dispersed to Australia from New Guinea, where several closely related species are found today.
The Australian water-rat resembles a small otter in many ways:
Its body is elongated and streamlined, and its tail is thick and muscular to help serve as a rudder when swimming. The hind feet are partly webbed and have a broad palm for efficient paddling.
The ears are small and can be folded flat against the head, and the muzzle is blunt and furnished with a dense set of long whiskers.
The fur repels water, drying quickly once animals exit the water.
Even if your local creek or lake doesn’t support a platypus population, it may well support these fascinating and intelligent native animals, which are equally deserving of care and protection.
Photos courtesy of Carolyn Hall (top and bottom left), Ann Killeen (middle) and Lissa Ryan (bottom right)
Names and distribution
The scientific name of the Australian water-rat is Hydromys chrysogaster, which translates as “golden-bellied water mouse”. Early European settlers sometimes referred to this animal as a beaver rat, though it’s actually much more like an otter than a beaver in both its appearance and behaviour. Since the early 1990s the water-rat has also been referred to as rakali – the name used by the Ngarrindjeri aboriginal people in the lower Murray River and Coorong region of South Australia.
Rakali occupy a wide variety of natural and man-made freshwater habitats, including swamps, ponds, lakes, rivers, creeks and irrigation channels. They also inhabit brackish estuaries and sheltered ocean beaches, and may populate ephemeral rivers and lakes in inland Australia when these fill with water after periods of unusually heavy rain. They tend to be most active in places where thick grass, low-growing shrubs, reed beds or large rocks provide plenty of cover on or near the banks. As shown below, water-rats are widely distributed on both the Australian mainland and Tasmania and also inhabit many offshore islands.
Map courtesy of R. Strahan. (1995). The Mammals of Australia, 2nd edition. (Reed Books: Chatswood NSW)
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). Animals living in different places often vary in colour. Most commonly, the head and back will be dark brown (with golden-yellow belly fur) or a lighter shade of brown, reddish-brown or grey (with fawn- to cream-coloured belly fur). However, apart from animals born near Shark Bay in Western Australia, virtually all individuals have a distinctive white tail tip.
Water-rat fur is moulted twice a year, becoming thicker in winter. Like platypus fur, it consists of fine dense underfur covered by coarser guard hairs. However, water-rat fur is much less effective than platypus fur at keeping its owner warm – water-rats cannot efficiently maintain their body temperature in water below 20°C and therefore need to exit colder water periodically in order to warm up in a burrow or other sheltered site.
Photo courtesy of James Pettit
Foraging behaviour and diet
Water-rats mainly dine on fish, aquatic insects, crayfish, crabs, mussels, clams and (to a lesser extent) frogs and turtles. Small amount of particularly nutritious aquatic plants may also be eaten. They also feed opportunistically on terrestrial prey (such as house mice during a mouse plague) and will quite happily snack on pet food left out on a porch or picnic leftovers. Large individuals are also known to kill water birds, such as ducks and coots.
Rakali are also adept at killing 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 toad’s neck region.
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, representing the remains of many water-rat meals.
Photos courtesy of Carolyn Hall (top) and Louis Delamoir (bottom)
Reproduction and life history
Though water-rats can potentially breed throughout the year, mating typically occurs in the cooler southern parts of their range from late winter to early summer. Gestation lasts for about five weeks. Young females can sometimes breed at less than 6 months of age but more commonly start breeding when they’re at least 8 months old. Up to five (but more typically one or two) litters may be produced annually. A female water-rat only has four nipples and typically raises 3-4 babies at a time, suckling them 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 rakali normally survive for a maximum of 3-4 years in the wild.
Photo courtesy of David Fleay
Social organisation, home range size and shelter sites
Water-rats are highly territorial, marking their home ranges with a strong scent resembling that of cat urine. Considerable fighting occurs in places where many animals overlap, as shown by a high frequency of bite marks on tails and hind feet. Males and females are believed to live independently, so females raise their offspring without help from a mate.
Relatively little is known about home range size and movements in this species, in part because animals are very good at shedding radio-collars. However, two studies (conducted in south-eastern Queensland and Western Australia) have concluded that each individual typically inhabits a home range measuring 12 hectares or less in area. However, because home ranges typically extend along the margins of a creek or river, they may be several kilometres in length – a radio-tagged rakali has been recorded travelling 3.1 kilometres along a stream channel in just 5.5 hours.
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 burrow, though probably not at the same time. On one occasion, an adult female platypus occupied a burrow a few weeks after it had served as a nursery for a female water-rat with young. Such behaviour is not especially surprising given that platypus and rakali 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.
Photos courtesy of Con Boekel (top) and Carolyn Hall (bottom)
Conservation issues and status
The water-rat is considered to be nationally secure and has an international conservation ranking of “Least Concern”. However, it often occurs in low numbers and may be regionally threatened in some places (for example, in south-western Western Australia). Rakali is both an apex predator located near the top of the food chain, and small enough to be hunted by larger predators such as snakes, large fish, birds of prey, foxes, cats and dogs. It also mainly feeds in or near water, so numbers are likely to crash during severe droughts. The practice of lining irrigation channels with plastic as a water-saving measure is also known to have devastating consequences for this species.
Water-rats were once widely trapped for their fur and sometimes culled when they were perceived to be a nuisance in irrigation districts. Today, many drown in enclosed traps set to capture crayfish or crabs in jurisdictions such as South Australia and the Northern Territory where these traps can still be legally deployed. As a wildlife-friendly alternative, anglers can instead capture tasty crustaceans using open-topped lift nets or old-fashioned hookless baited lines.
Australian water-rats sometimes come into conflict with humans when they raid ornamental goldfish ponds or poultry yards, kill free-ranging guinea pigs in gardens, steal bait and snacks from anglers, deposit piles of fish bones on the decks of moored yachts, or leave the gory remains of cane-toads near swimming pools. However, killing or relocating water-rats is illegal and can result in hefty fines. In any case, these actions are usually ineffective as a control measure, given that dispersing juveniles are normally quick to recolonise suitable vacant habitat.
A more sustainable solution is to learn to live with these clever and attractive native animals, for example by using netting to exclude water-rats from a fish pond or not leaving out food scraps that attract them.
Photos courtesy of Ken Mival (top) and Andrew Fishman (bottom)
Co-existence of water-rats and platypus
Platypus and water-rats are both top predators in Australian freshwater systems and probably compete to some extent for food. However, the size of prey that can be killed and consumed by an adult platypus is limited by the fact that its bill contains only with rough grinding pads to process food. In contrast, a water-rat has a formidable set of sharp incisors to help dismember prey. Interestingly, the grinding surfaces of water-rat molars are relatively smooth. Like the grinding pads of the platypus, this adaptation may be particularly effective at processing aquatic invertebrates.
In practice, little is known about the ecological and behavioural interactions between platypus and rakali. Though it’s been suggested that water-rats may sometimes prey on young platypus, there appears to be no documented evidence supporting this possibility. The two species co-exist in many places, so water-rats clearly don’t automatically exclude platypus from freshwater environments (or vice versa). However, there are also waterways where only one of the two species commonly occurs. In general terms, water-rats are much more likely to occupy badly degraded or highly urbanised habitats than the platypus. This may reflect the fact that water-rats forage in part on land and can also dine on introduced fish such as carp.
Photo courtesy of B. Vickers
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 using the Reporting Form. The information will be added to a secure data base and eventually shared with the Atlas of Living Australia, thereby contributing valuable information to better understand where these animals occur and how well they’re doing in the wild.