November 10, 2015 Mammals No Comments

(Xeromys myoides)

Also known as the marine mouse or yirrkoo (Indigenous).


Water mouse (Xeromys myoides) and mound. Drawings: Angela Frost

Water mouse (Xeromys myoides)
and mound.
Drawings: Angela Frost

One of Australia’s rarest rodents, the water mouse is a nocturnal non-marsupial terrestrial carnivore that inhabits coastal marine and estuarine environments.

Until the late 1990s, when field surveys were carried out by WPSQ, very little was known of the water mouse (previously named the false water rat) and few specimens were collected. The species was first described from a single specimen collected in Mackay in 1889. Further examples were found at Mackay in 1944, at Proserpine in 1982 and across the Whitsunday to Mackay region in 1999. Van Dyck made a major ecological study on North Stradbroke Island.

An interim South-East Queensland Regional Recovery Plan for the species was submitted to the Environmental Protection Agency in 2005.


The water mouse has water-repellent, silky fur. Older males may have faint white splotching on their backs. The tail is white-haired and finely ringed. The ears are short and round.

  • Adult body length (head to tail) 90-120mm; weight 40-50g
  • Small, short-sighted eyes (similar to insectivorous bats) and ears
  • Sleek, slate-grey back and silky white underparts
  • Tail short with sparse hairs; 80-99mm, shorter than body length
  • Enlarged anal glands excrete a long-lasting pheromone that survives being covered by tides
  • Teeth contain only 2 pairs of molars – unique to this species and the water rat (Hydromys chrysogaster)


  • A smell of pilchards/mouldy leather, especially under old logs and in tree hollows
  • Mud mounds in sedgeland with clumps of couch grass on top
  • Middens of food scraps left in the hollow bases of trees made up of small crab shells and crab claws. Often the water mouse carefully removes the ventral capapace of the crab in order to get to the flesh inside. Also food scraps under logs (where slugs or snails adhere) or around the mound


Known to occur in several marine habitats ranging from sedgeland, saline grassland, marine couch grassland and tidally flooded landward mangroves to feed and nest.



Carnivorous: small crabs and shellfish, worms. Hides in tree-hollow bases and logs to eat large items but gobbles down shellfish on the run. Water mice seem to get most of their fresh water needs from their food rather than by drinking.

Did you know…?

A water mouse can take 20 minutes to eat a crab the size of a ten-cent piece.

Water mouse mounds can have a separate defecation chamber or ‘toilet’.


Mud mound about 0.5m high, sometimes with a crest of saltwater couch that rises above surrounding vegetation (possibly caused by extra nutrient load of excreta). Mounds have about 3 oval entrances top and bottom. Also makes oval-entrance burrows in banks les than 1m long. Sometimes the nest is constructed between mangrove buttress roots, particularly those of the orange mangrove (Bruguiera spp.) Chambers can be lined with leaves.


  • Mostly nocturnal but individual animals have been found out and about during the day, presumably when a night high tide prevents them from returning to nests. Their strong and long-lasting pheromone trails mean the mouse can follow habitual pathways.
  • Tides dictate feeding opportunities. The water mouse races around madly while feeding; climbs little, can swim a little.
  • The mouse forages in mangroves and nests in sedgelands. They feed out to the seaward edge of the mangroves and back into the high intertidal zone when the tide is high.
  • Home ranges are about 0.6ha and they can range up to 2km within these areas.


2-3 babies raised in dry part of mound or hole.


  • Queensland: Proserpine to Cape Palmerston, Moreton Bay and islands down to Coomera
  • Northern Territory and other: Arnhem Land and Melville Island; Papua New Guinea


Feral animals: Predation by cats and foxes. Pigs degrade the water mouse’s habitat, mostly by wallowing in and around watercourses and swamps. This destroys the native riparian vegetation and encourages erosion.

Habitat loss: Marina developments and coastal real estate developments that affect water levels; golf courses; wastewater treatment systems and bunding walls; resource extraction industry. Changes in hydrology including stormwater discharge from residential areas may affect crab populations on which the water mouse feeds.


Conservation status: Vulnerable

(IUCN Red List of Threatened Species (2000), the Federal Environment Protection and Biodiversity Conservation Act 1999 (Schedule 4) (EPBC Act) and the Queensland Nature Conservation (Wildlife) Regulation 1994, (Schedule 3) under the Queensland Nature Conservation Act 1992.)

Read more

  • Australian Nature Conservation Agency, Australian names for Australian rodents
  • Ball, Derek (2004) ‘Distribution and habitat of the false water rat, Xeromys myoides Thomas, 1889 (Rodentia: Muridae) in Intertidal areas of central eastern Queensland, Memoirs of the Queensland Museum 49(2): pp 487-94
  • Ball, Derek (2005) ‘Point discharge of stormwater runoff into a landward mangrove community: Initial investigation indicates a negative impact of keystone species (mangrove crabs: Family Grapsidae’. New Zealand Marine Science Association Conference. Wellington NZ.
  • Low, Tim & Van Dyck, Steve (2000) ‘Show me a rat…and I’ll show you a good time’, WAM 4/36 pp78 Summer 2000
  • Van Dyck, Steve (1992) ‘Parting the reeds on Myora’s Xeromys kibbutz’, WAM 1/29 pp8-11 Autumn 1992
  • Van Dyck, Steve (1996) ‘ Xeromys myoides Thomas, 1889 (Rodentia: Muridae) in mangrove communities of North Stradbroke Island, southeast Queensland. Memoirs of the Queensland Museum 42(1): pp 337-366
  • Van Dyck, Steve & Gynther, Ian (2003) ‘Nesting strategies of the Water mouse Xeromys myoides in south east Queensland. Memoirs of the Queensland Museum 49(1): pp 453-79

Written by Wildlifeqld