Rakali/Water-rat – Australia’s “otter”

 

w rat Wendouree (Hall) feedingAustralia 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 (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 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:

w-rat hind footIts 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 paddle-like.

The ears are small and can be folded flat against the head, and the muzzle is blunt and furnished with a dense set of over-sized whiskers.

The fur repels water, drying quickly when animals exit the water.

w rat Wendouree (Hall)-3   file10-1

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 efforts to protect them.

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 originally used by the Ngarrindjeri aboriginal people in the lower Murray River and Coorong region of South Australia.

Australian water-rats 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. They most commonly occur where dense vegetation provides cover on or near the banks – thick grass, low-growing shrubs or reed beds. As shown below, water-rats are widely distributed on both the Australian mainland and Tasmania and also occur on many offshore islands.

Capture map

Map courtesy of R. Strahan. (1995). The Mammals of Australia, 2nd edition. (Reed Books: Chatswood NSW)

Size and appearance

w-rat KangLake 2018 Aug (James Pettit) K1__3067 15%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). Water-rats 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 or grey (with fawn- to cream-coloured belly fur). However, all water-rats are born with a distinctive white tail tip (though this may be lost later through fighting).

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

w rat Wendouree with fish (Hall)

Water-rats mainly dine in the water on fish, aquatic insects, crayfish, crabs, mussels, clams and (to a lesser extent) frogs and turtles. Small amount of selected 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.

w-rat kill -cane toad (Mt Archer) L Delamoir 2017 AugWater-rats also show unusual proficiency 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 — the remains of many water-rat meals.

Photos courtesy of Carolyn Hall (top) and Louis Delamoir (bottom)

Reproduction and life history

juveniles (Fleay)Water-rats can potentially breed throughout the year if conditions are favourable. However, mating typically occurs in the cooler southern parts of their range from late winter to early summer. The length of the gestation period is about five weeks. Young females can sometimes breed when they’re less than 6 months old but more commonly delay breeding until the age of at least 8 months. 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 water-rats 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 evidenced 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.

w-rat (Con Boekel) 2016 July

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-east 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 water-rat has been recorded travelling 3.1 kilometres along a stream channel in just 5.5 hours.

3A6A7153 w rat on log (Killeen)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 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.

Photos courtesy of Con Boekel (top) and Ann Killeen (bottom)

Conservation issues and status

IMG_0112 Ken Mival 10%The water-rat is considered to be nationally secure and has an international conservation ranking of “Least Concern”. However, though widely distributed, it often occurs in low numbers and there is some evidence that it may be regionally threatened (for example, in south-western Western Australia). This reflects the fact that the species is both a predator located near the top of the food chain, and small enough to be eaten by many larger predators (including snakes, large fish, birds of prey, foxes and feral and domesticated cats and dogs). It also mainly feeds in or near the water, so numbers are likely to crash during severe or extended droughts. There is also abundant anecdotal evidence that the practice of lining irrigation channels with plastic as a water-saving measure can have devastating consequences for this species.

dead w-rats OHT Gunbower 2018 (C Edge)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 continue to drown in enclosed traps (such as the opera house trap shown at left that contained 5 dead water-rats) that are set to capture crayfish or crabs. These traps can also kill large numbers of platypus and freshwater turtles. Recreational anglers should therefore always consider using either lift-style/ open-topped hoop nets or old-fashioned baited lines (without hooks) as wildlife-friendly methods to procure a meal of yabbies or crays.

water-rat scavenging bread 1 (Andrew Fishman)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 food debris on the decks of moored yachts, or leave the remains of cane-toads around the edge of a swimming pool.

However, killing or relocating water-rats is illegal and can be subject to substantial fines. In any case, these actions are usually ineffective as dispersing juveniles are likely to recolonise the area quite quickly. A more sustainable solution is to learn to live with water-rats, for example by using netting to exclude water-rats from a pond or not leaving food scraps around that will attract them.

Photos courtesy of Ken Mival (top), Cameron Edge (middle) 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 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 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 processing aquatic invertebrates.

In practice, very little is known about the ecological and behavioural interactions between platypus and water-rats. 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 are found living together 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 can forage on land and also dine on introduced fish such as carp.

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, thereby contributing valuable information to better understand the species’ distribution and status in the wild.