About the size of a rabbit, H. chrysogaster is well adapted for water. The toes are webbed on front and hind feet, which are broad and act as paddles. Hydromys chrysogaster has numerous whiskers at the end of a long, blunt muzzle. The head is flat with small ears and eyes. The most notable characteristic is the water rat’s thick white tipped tail. Hydromys chrysogaster varies in color from a brown black to gray, making them somewhat cryptic in their surroundings. Some are uniform in color, while others have lighter undersides. The one unifying feature is the white tipped tail. (Watts and Aslin, 1981)
Other Physical Features: endothermic ; bilateral symmetry
Average mass: 850 g.
Average basal metabolic rate: 2.97 W.
Habitat and Ecology
Hydromys chrysogaster individuals live mainly near permanent fresh water. They live on land but depend on the water for food. Also present along the coastline, H. chrysogaster do not need completely fresh water. They can also survive in areas where rivers and streams have become polluted or are brackish. (Watts and Aslin, 1981)
Habitat Regions: tropical ; saltwater or marine ; freshwater
Aquatic Biomes: lakes and ponds; rivers and streams; coastal
Wetlands: marsh ; swamp ; bog
Other Habitat Features: urban ; suburban ; agricultural ; riparian ; estuarine
Hydromys chrysogaster feeds mainly on crustaceans, mollusks, and fish, although they have been observed feeding on aquatic insects, frogs, house mice, the eggs and young of waterfowl, poultry, and turtles, and even attacking bats. Hydromys chrysogaster have also been observed eating cane toads, an introduced species that is toxic to many other predators. They often have a favorite feeding platform on which they collect piles of food before eating it. Hoarding food in the nest site is also common. Mussels are opened by their strong incisors.
Animal Foods: birds; mammals; amphibians; reptiles; fish; eggs; insects; terrestrial non-insect arthropods; mollusks; aquatic crustaceans
Foraging Behavior: stores or caches food
Primary Diet: carnivore (Piscivore , Eats non-insect arthropods, Molluscivore )
- Fanning, F., T. Dawson. 1980. Body temperature variability in the Australian water rat, *Hydromys chrysogaster*, in air and water. Australian Journal of Zoology, 28: 229-238.
- Watts, C., H. Aslin. 1981. The rodents of Australia. Sydney: Angus & Robertson Publishers.
Water rats are abundant and are an important prey base for many small to medium-sized predators. Their burrowing and foraging activities probably also help in the redistribution of nutrients in systems.
Eagles, buzzards and kites prey on water rats, as well as snakes and small mammalian carnivores. Water rats mainly escape predation by escaping to burrows or into the water. (Watts and Aslin, 1981)
This list may not be complete but is based on published studies.
Known prey organisms
This list may not be complete but is based on published studies.
Life History and Behavior
Longevity in water rats is unknown.
Status: captivity: 7.3 years.
Lifespan, longevity, and ageing
Little is known of the mating system of water rats.
Water rats breed in the spring and summer. Females have an estrous cycle of approximately eleven days. The gestation period is about 35 days. Females can enter estrus immediately after giving birth, so litters can be produced only 35 days apart. Usually, water rats have litters of four to five young. During a good breeding season, females can have two or three litters.
At birth, the young are blind. They are usually lighter in color than the adults, but already have the characteristic white tipped tail and partially webbed feet. The young grow quickly and are usually independent after about 35 days. However, after this initial growth, maturity to adulthood takes longer. Breeding does not occur until the young are at least one year old and full size is attained at about two years of age. (Watts and Aslin, 1981)
Breeding season: Breeding occurs during spring and summer.
Range number of offspring: 8 to 15.
Average gestation period: 35 days.
Average weaning age: 35 days.
Average age at sexual or reproductive maturity (female): 1 years.
Average age at sexual or reproductive maturity (male): 1 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization (Internal ); viviparous
Average birth mass: 24.39 g.
Average gestation period: 36 days.
Average number of offspring: 3.8.
Average age at sexual or reproductive maturity (male)
Sex: male: 135 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 163 days.
Young are born helpless and are cared for by their mother in her nest burrow until they are weaned, at about 35 days old.
Parental Investment: altricial ; female parental care
Molecular Biology and Genetics
Statistics of barcoding coverage: Hydromys chrysogaster
Public Records: 0
Specimens with Barcodes: 1
Species With Barcodes: 1
IUCN Red List Assessment
Red List Category
Red List Criteria
Water rats are widespread and abundant, they are not threatened.
US Migratory Bird Act: no special status
US Federal List: no special status
CITES: no special status
IUCN Red List of Threatened Species: least concern
Relevance to Humans and Ecosystems
Economic Importance for Humans: Negative
Active burrowers, H. chrysogaster individuals have damaged channel banks and water-control structures. (Watts and Aslin, 1981)
Economic Importance for Humans: Positive
Hydromys chrysogaster are able to withstand pollution in cities and even thrive there. They are often observed by humans because they are sometimes active during the day. Farmers benefit from H. chrysogaster because they often destroy yabbies, other small rodents, which destroy irrigation systems. By eating pond snails, water rats also protect livestock from the parasites that are transmitted through snails. (Watts and Aslin, 1981)
Hydromys chrysogaster, commonly known as Rakali or Water-rat, is an Australian native rodent. The 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). They have webbed hind legs, waterproof fur, a flattened head, a long blunt nose, many whiskers and small ears and eyes. They are black to brown in colour with an orange to white belly, and dark tail with a white tip.
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 the name 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|
- Aplin, K., Copley, P., Robinson, T., Burbidge, A., Morris, K., Woinarski, J., Friend, T., Ellis, M. & Menkhorst, P. (2008). Hydromys chrysogaster. In: IUCN 2008. IUCN Red List of Threatened Species. Retrieved 10 October 2008.
- Braithwaite R. W. et al. (1995). Australian names for Australian rodents. Australian Nature Conservation Agency. ISBN 0-642-21373-9.