|Distribution of the water rat|
Hydromys chrysogaster, commonly known as rakali, rabe or water-rat, is an Australian native rodent first described in 1804. The change to the aboriginal name Rakali was intended to foster a positive public attitude by Environment Australia. It is the only member of the genus Hydromys with a range extending beyond Papua New Guinea and Indonesian West Papua. Having effectively adapted and colonised a unique niche of semiaquatic and nocturnal lifestyle, this species lives in burrows on the banks of rivers, lakes and estuaries and feeds on aquatic insects, fish, crustaceans, mussels, snails, frogs, birds' eggs and water birds. Rakali have a body 231–370 millimetres (9.1–14.6 in) in length, weigh, 340–1,275 grams (0.750–2.811 lb) and have a thick tail measuring around 242–345 millimetres (9.5–13.6 in). Females are generally smaller than males but tail lengths are normally the same.They have partially webbed hind legs, waterproof fur, a flattened head, a long blunt nose, many whiskers and small ears and eyes. The body is streamlined with a skull that is large, flat and elongated, with two molars on the upper and lower jaw, similar to the False water rat Xeromys meroides. They are black to brown in colour with an orange to white belly, and dark tail with a white tip.
Hunted for their soft fur and considered a nuisance animal, numbers were under threat until a protection order in 1938. They were still under destruction permits from 1938 to 1957 due to alleged destruction of irrigation banks and destruction of fishing nets. Additionally from 1957 to 1967 a number of licensed seasons were also held for this reason.
Until the 1980s, this species was commonly known as "water-rat", but during the 1990s there was a push for such descriptive English common names to be replaced with indigenous names. In 1995 the Australian Nature Conservation Agency released a document in which the following indigenous names were recorded for H. chrysogaster. They recommended that "rakali" be adopted as the common name, and the Australian Department of Environment and Heritage has taken up this suggestion. Both common names are now widespread.
|Indigenous name||Linguistic group or area|
|Minha watha||Kugu Nganhcara|
|Ngwir-ri-gin||King George Sound|
Taxonomy and description
H. chrysogaster is the most specialised of the Hydromyini rodent group for aquatics. They feature a flattened head, partly webbed hind feet and water repellent fur that also offers insulation. Their waterproof thick coat varies from extremely dark fur, black to slate grey on their back and white to orange underneath. The thick dark tail is an identifying feature with its white tip. Pelage changes through the process of moulting varies according to sex. Males and females moult in autumn and summer, and females additionally in spring. Moulting is conditional to temperature, reproductive condition, adrenal weight, health and social interaction. The hind feet are wide and partially webbed, the broad face, flattened head and long whiskers are all distinguishing features. Their body grows to a length of 39 cm excluding the tail.
This well adapted semi-aquatic and territorial species may become very aggressive in high density populations as they are mostly solitary. Fighting is common, leading to scarred tails from being bitten during fighting amongst both adult males and females. Animals often exhibit ulcerated wounds, subcutaneous cysts and tumours mostly in the dorsal and posterior regions.
Distribution, Range and Habitat
This widespread species can be found in permanent water systems in Australia, New Guinea and offshore islands. Water rats live near permanent water in a diverse range of habitat that varies from fresh slow moving streams, brackish inland lakes and creeks to wetlands, rivers, estuaries and beaches on coastlines. Found in all states and territories, this adaptable species has resumed resettlement populations in both Australia and New Guinea. The map above shows their distribution throughout Australia and New Guinea.
Widely distributed through eastern Australia, west of the Kimberly, along the Northern coastline, throughout New South Wales, Victoria and Tasmania, their habitat varies from freshwater streams to brackish wetlands, including shallow lakes (up to 2m in depth) and farm dams, deep water storages, slow moving rivers, creeks and streams, estuaries, coastal lakes and sheltered marine bays.
Their home range typically comprises 1–4 km of waterways and they can travel a distance of between 200m to a kilometre in a night when foraging. They prefer low banks with flat, densely vegetated water edges for protection and ease of stalking prey, spending time when not foraging resting in hollow logs and burrows.
Diet and foraging
Water rats are a predominately carnivorous species feeding on a variety of aquatic animals including fish, crustaceans, shellfish, small birds, eggs, mammals, frogs, reptiles. In winter, when resources are limited, they will also take plants. A mostly nocturnal species, they source their food on dusk, partly by sight when in the water and hunting prey on the waters edge returning to a favourite feeding place where evidence of previous catches can be seen. They can be found swimming and feeding in the early morning and evening. The animal does not tolerate low temperatures well, so they prefer terrestrial prey in winter over the aquatic species. They also spend longer periods in the warmth of the burrow.
Their diet makes this species susceptible to infestations of nematodes, with large numbers of Cosmocephalus australiensis recorded in 1959 in water rats burrowed into their stomach wall. Other worms namely Cestodes and trematodes occur in high numbers in the small intestines. Sparganosis also afflicts water rats, these parasites coil under the skin surface in the shoulder and back region. The extent of infestation and the varieties of Helminth that are present reflects the region which they inhabit.
The water rat is not well adapted to hot and cold extremes as it has poor insulation qualities. Aquatic mammals need to maintain relatively constant body temperature to prevent or compensate for their thermally hostile environment. One of the few diurnal Australian rodents, they cannot maintain their body temperature below 15° and risk hyperthermia in temperatures that exceed 35°. Their vascular system has network adaptions for heat loss, but no major heat conserving vascular retia. This causes poor thermoregulation and poor insulative capacity in wet fur. They control body temperature by remaining in the burrow in the heat of the day in summer and by foraging during day in winter.
Expending energy in this aquatic species is documented as lower, using less oxygen when moving on land than locomotion in water. Water rats have the capacity to run at twice their maximum swimming velocity. Their metabolic rate increases during running at a rate of 13-40% greater than when swimming.
Predators and predation
Humans have been their greatest predator, with water rats requiring protection by legislation in 1938. They were hunted for their soft fur and considered a pest species. Populations are considered to have recovered and in all states considered of 'least concern' with the exception of Western Australia. Considered a pest in irrigation systems due to burrowing into banks and creating leaks, in a positive sense they have also been attributed to reduction of bank damage as they predate on freshwater crayfish (yabbies).
Predators of the water rat range from large fish and snakes when they are young and in the water, to birds of prey, foxes and cats. The water rat has the unusual ability to be able to kill cane toads without being poisoned.
Behaviour and life history.
The breeding behaviours are influenced by the seasonal conditions. Under good seasonal conditions the solitary water rat comes together with their mate and may produce several litters (if young are removed will reproduce again within two months), but separate on the birth of the young. Limited information is available on home range, but suggests localised movement around the nesting sites and foraging areas. Water rats construct burrows dug into river banks but have also been documented building nests within sunken logs and reeds, in areas surrounded by roots and dense riparian vegetation for cover from predators.
There is increasing evidence of altered nesting habitat with water rats using artificial shelters of drainage pipes, exhaust pipes in moored boats, and rubber tyres in human-modified areas. This occurs more frequently in high population density situations.
Breeding and growth
Females are able to reproduce from one year old or 425g. Mating commences from late winter to early spring with a gestation period of 34 days. Water rats have four mammae with nipples located in the abdominal inguinal area enabling litters of an average of four to five are born from September to February and are suckled for four weeks. Sexual maturity develops at around twelve months but has been documented to commence at 4 months and breeding in the season of their birth. They are able to continue breeding until females are three and a half years old, and have a life expectancy of 3–4 years.
A territorial species, they are mostly solitary excluding the periods of mating and rearing their young. Their social organisation suggests adults are intra-sexually aggressive with their limited home range and overlapping sex and age classes. In captivity, only the dominant female in the hierarchy will successfully reproduce. Fecundity is lower than that of other murid species producing only four to five litters, but fertility is very high which can create high population growth in a period of favourable breeding conditions. This can be also influenced by the rapid growth in juveniles who can reach maturity and size within their first year.
H. chryogaster in comparison to the Rattus species mature late and have long reproductive lifecycles. They have lower reproductive output, delayed implantation, lactation anoestrus, winter anoestrus, longer oestrous and longer gestation cycle. Their development occurs in stages associated with the eruption of incisors, hearing and eye opening, eating solid foods, puberty and full reproductive maturity.
Although nationwide the populations appear stable there are individual populations facing significant threat. Near threatened in Western Australia's wheat belt, urbanisation, salinisation, waterway degradation and pollution have all attributed to this status change. In Northern Territory, Queensland, Tasmania and New South Wales they are classified as 'least concerned'. In the Victorian Lake Wellington, Lake Victoria and Lake Tyers region numbers have declined in the past fifty years attributed to commercial eel-fishing, predation, loss of habitat due to commercial development and poisoning from baiting programs placing this species at risk.
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