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Hydromys chrysogastergolden-bellied water rat

By Barbara Lundrigan and Kim Pfotenhauer

Geographic Range

Hydromys chrysogaster dwells in freshwater lakes and rivers throughout Australia and Tasmania and on offshore islands. They are also found on New Guinea. (Watts and Aslin, 1981)

Habitat

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)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • coastal

Physical Description

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)

  • Average mass
    850 g
    29.96 oz
    AnAge
  • Average basal metabolic rate
    2.97 W
    AnAge

Reproduction

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 number of offspring
    3.8
    AnAge
  • Average gestation period
    35 days
  • Average gestation period
    36 days
    AnAge
  • Average weaning age
    35 days
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    163 days
    AnAge
  • Average age at sexual or reproductive maturity (male)
    1 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    135 days
    AnAge

Young are born helpless and are cared for by their mother in her nest burrow until they are weaned, at about 35 days old.

Lifespan/Longevity

Longevity in water rats is unknown.

  • Average lifespan
    Status: captivity
    7.3 years
    AnAge

Behavior

Hydromys chrysogaster live in burrows alone or with young. However, populations are concentrated in certain areas. Their burrows run parallel to the banks of lakes or rivers. The entrances are often hidden under a log or root. Inner nest chambers are about 20 cm high. These burrows may have more than one chamber and a second entrance. Water rats got their name from their ability to swim and dive. They are often mistaken for a platypus (Watts and Aslin, 1981). However, H. chrysogaster individuals are not as well equipped as platypus are for aquatic life. They do not have water resistant fur or many adaptations for the water. This becomes a problem when the temperature is low. Water rats will go into rivers for short periods of time in the winter, but they still suffer regional hypothermia from both the air and water when the temperature drops. One adaptation that may help them survive this hypothermia is an interscapular pad of brown fat. Metabolism of this pad insulates the heart and allows warm blood to flow to the heart and the spinal cord during cold weather. (Fanning and Dawson, 1980)

Communication and Perception

Food Habits

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. (Fanning and Dawson, 1980; Watts and Aslin, 1981)

  • Animal Foods
  • birds
  • mammals
  • amphibians
  • reptiles
  • fish
  • eggs
  • insects
  • terrestrial non-insect arthropods
  • mollusks
  • aquatic crustaceans

Predation

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)

Ecosystem Roles

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.

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)

Economic Importance for Humans: Negative

Active burrowers, H. chrysogaster individuals have damaged channel banks and water-control structures. (Watts and Aslin, 1981)

Conservation Status

Water rats are widespread and abundant, they are not threatened.

Contributors

Barbara Lundrigan (author), Michigan State University, Kim Pfotenhauer (author), Michigan State University.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

agricultural

living in landscapes dominated by human agriculture.

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

bog

a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

freshwater

mainly lives in water that is not salty.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

piscivore

an animal that mainly eats fish

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"

suburban

living in residential areas on the outskirts of large cities or towns.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

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.

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