Rakali
The rakali, 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, 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.
Long considered a nuisance animal, rakali were hunted for their soft fur, particularly in the Depression of the 1930s, when a ban was placed on imported pelts such as the American muskrat. With their numbers under threat, a protection order was issued in 1938. However, they were still subject to destruction permits from 1938 to 1957 due to their effect on irrigation banks and alleged damage to fishing nets. Additionally, from 1957 to 1967 a number of licensed seasons were also held for this reason.
Common names
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 names for Hydromys chrysogaster | - |
| Indigenous name | Linguistic group or area |
| Ayam | Miyanmin |
| Bud-bud | Ngarigu |
| Dalgan | Djabugay |
| Dhulurryamba | East Arnhem |
| Djinnjokma | Daly River |
| Durrgin | Guugudic |
| Galiba:y | Yawaalaraay |
| Guma:y | Yawaalaraay |
| Gurrumu | Yolngu |
| Jawurrk | Kamu |
| Kal | Wik-Mungkan |
| Kathi thukathayini | Innamincka |
| Kirnrti | Kalkatungu |
| Kumbuna | Nyikina, Kayardild |
| Mamgericht | Nyungar |
| Minha watha | Kugu Nganhcara |
| Moinah | Tasmania |
| Moo'nyp | Wik-Ngathana |
| Mu-mun | Murrinh |
| Mulundarru | Upper Barcoo |
| Murit-ya | Nyungar |
| Muuruung | Kuurnkopan |
| Namurr | Yanyuwa |
| Ngaljatbil | Jawoyn |
| Ngoor-joo, Ngurju, ngoodjo | Nyungar |
| Ngwir-ri-gin, ngwiridjin | King George Sound, Nyungar |
| Njimba'rrolbumbu | Mayali |
| Nurn | Warray |
| Otol | Gnog |
| Pikun | Ngiyampaa |
| Pikunra | Kalkatungu |
| Pwampungini | Tiwi |
| Rakali | Murray River |
| Rekaldi | Ngarrindjeri |
| Renah | Tasmania |
| Riyariyangkanu | Yanyuwa |
| Thaargh | Kurtjar |
| Wambu | Wunambal |
| Wamp-wamp | Noongar |
| Wampi | Ungaringyin |
| Wapu-nu | Gupaguyngu |
| Wapun-u | Yolngu |
| Wardunda | Kayardild, Nyikina |
| Warunu | East Arnhem |
| Wath | Wik-Mungkan |
| Wirrkup | Maung |
| Wubui | Yidiny |
| Wungujang wara | Nunggubuyu |
| Wurriyangkirraburna | Groote Eylandt |
| Yanjuna | Wanyi |
| Yerr-ek | Wada wurrung |
| Yirrku | Gaagudju |
| Yirrkku | Kunwinjku |
| Yurru | Mangarrayi |
Taxonomy and description
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. Animals often exhibit ulcerated wounds, subcutaneous cysts and tumours mostly in the dorsal and posterior regions.
Ecology
Distribution, range and habitat
This widespread species can be found in permanent water systems in Australia, New Guinea and offshore islands. Rakali 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 throughout most of eastern Australia, along the coast of northern Australia, and South West WA, their habitat varies from freshwater streams to brackish wetlands, including shallow lakes and farm dams, deep water storages, slow-moving rivers, creeks and streams, estuaries, coastal lakes and sheltered marine bays.
Their home range typically comprises of waterways and they can travel a distance of between to 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
The rakali is a predominantly carnivorous species, feeding on a variety of aquatic animals including fish, crustaceans, molluscs, other shellfish, insects, small birds, eggs, mammals, frogs, and reptiles. In winter, when resources are limited, they will also feed on plants. A mostly nocturnal species, they search for their food at dusk; guided by sight while in the water, and hunting at the water's edge with the apparent memory of previous catches' locations. They can also exhibit crepuscular habits, being 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 winter periods in the warmth of their burrows. A relatively new addition to their diet is, specifically, the heart and liver of the toxic cane toad, in regions where the toads have moved into their territory. The rakali have naturally figured out how to avoid the toad's poisonous shoulder glands by flipping them onto their backs for ease of dispatch. They are, seemingly, unaffected by any poisonous secretions on the toads' skin which they may inadvertently ingest. This makes the rakali one of the very few natural defenses in the fight to contain the cane toad's population boom and spreading westward. Since the early twentieth century, the toads in Australia have multiplied from an initial 100 animals to as many as 100 million. Their poisonous skin secretions can kill a dog or cat, and have contributed to the decline of several species of Australian reptiles, mammals, fish, and birds.Their 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. They control body temperature by remaining in the burrow in the heat of the day in summer and by foraging during the day in winter.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.