The rakali (Hydromys chrysogaster), also known as the rabe, the "Australian otter" or water-rat, is an Australian native rodent first scientifically described in 1804. Adoption of the Ngarrindjeri name rakali is intended to foster a positive public attitude by Environment Australia.

One of four described species in the genus Hydromys, it is the only one with a range extending beyond New Guinea. Having adapted to and colonised a unique niche of a 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, cane toads, bird's eggs and small water birds.

Rakali have a body length of , weigh , and have a thick tail measuring around . 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 water mouse. They are black to brown in colour with an orange to white belly, and dark tail with a white tip. With their numbers under threat, a protection order was issued in 1938.

Common names

Until the 1980s, this species was commonly known as "water-rat" (also "common 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.

{| class="wikitable collapsible collapsed"

|Indigenous names for Hydromys chrysogaster || Nyungar

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| Ngwir-ri-gin, ngwiridjin || King George Sound, Nyungar

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| Yurru || Mangarrayi

|}

Taxonomy and description

thumb|Rakali: Breakwater St Kilda.

Hydromys 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. Their coat changes through the process of moulting, 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 about 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. including shallow lakes (up to in depth) and farm dams, deep water storages, slow-moving rivers, creeks and streams, estuaries, coastal lakes and sheltered marine bays. In winter, when resources are limited, they will also feed on plants.

thumb|Helminth: various species of worms that infest water ratsTheir diet makes this species susceptible to infestations of nematodes, with large numbers of Cosmocephalus australiensis recorded in 1959, burrowed into their stomach walls. Other worms, namely cestodes and trematodes, occur in high numbers in the small intestines. Sparganosis also afflicts rakali; these parasites coil under the skin surface around the shoulder and back region. The extent of infestation, and the varieties of helminths that are present, vary according to the region which they inhabit.

Metabolism

The rakali 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.

Expending energy in this aquatic species is documented as lower, using less oxygen when moving on land than locomotion in water. Rakali 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

thumb|Yabbies: most common food source

Humans have been their greatest predator, with rakali 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 prey on freshwater crayfish (yabbies).

Predators of the rakali range from large fish and snakes when they are young and in the water, to birds of prey, foxes and cats.

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 estrous 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.

Status

Although nationwide the populations appear stable there are individual populations facing a significant threat. Near threatened in Western Australia's Wheatbelt, urbanisation, salinisation, waterway degradation and pollution have all attributed to this status change.

References

  • Australian Museum Factsheet
  • Australian Platypus Conservancy
  • CSIRO Water for a Healthy Country
  • Water Rat - Description and Characteristics
  • Oz Animals