Courtesy of: The Australian Platypus Conservancy
Australian Water-Rat (Hydromys chrysogaster). Return to Platypus Page
Like the platypus, water-rats are most often glimpsed swimming on the surface of lakes or rivers in the early morning or evening. Although generally similar in terms of size and colour, the two species can be distinguished by examining either end of their body - the water-rat lacks a bill, and has a distinctive white tip to its tail.
Unlike platypus, water-rats often emerge from the water to eat (sitting up and holding their meal in their forepaws) or run along the bank searching for food. On land they strongly resemble a miniature otter, with a thick coat of soft fur; densely bewhiskered, blunt muzzle; broad, partly webbed hind feet; and furry, tapering tail.
Radio-tracking studies undertaken by Australian Platypus Conservancy staff in the upper Yarra River catchment have shown that platypus and water-rats will use the same burrows, though not at the same time. On one occasion, an adult female platypus occupied a burrow a few weeks after it served as a nursery for a female water-rat with a litter of young. Such behaviour is not especially surprising - platypus and water-rats are about the same size and both animals make use of many different burrows over time. However, it remains unknown whether the two species are equally likely to dig a new burrow in the first place.
Platypus and water-rats both function as top predators in Australian freshwater systems and probably compete to some extent for food. Both are known to eat aquatic insects, spiders, crayfish (yabbies), freshwater mussels, shrimps, and frogs. However, the size of prey that can be consumed by an adult platypus is limited by the fact that its bill is equipped only with rough grinding pads to help process food. In contrast, a water-rat has a formidable set of teeth which can be used to kill and eat fish, tortoises and water birds - sometimes up to the size of ducks. Interestingly, the grinding surfaces of water-rat molar teeth are quite smooth. Like the grinding pads of the platypus, this adaptation may be particularly effective at dealing with the hard, encased bodies of many aquatic invertebrates.
Although water-rats are widely distributed in Australia, the animals appear to be relatively uncommon along many waterways. For example, in the course of platypus surveys recently undertaken by the Conservancy in collaboration with Melbourne Water, water-rats were encountered at just 13 of 82 sites sampled in the middle Yarra catchment. Similarly, the results of earlier studies in the upper Yarra catchment indicated that only a single pair of adult water-rats occupied 3 kilometres of relatively pristine stream. However, almost nothing is known of the factors which limit the number of water-rats occupying various habitats.
Additional Information: Courtesy of Naturalist Guide Damon Ramsey
Water-rat: Hydromys chrysogaster
This is Australia's largest rodent (Cronin 2000). It's most distinguishing feature is the long tail which is pale for it's last third. The colour of the body varies from blackish to brown to grey, with paler underparts, and sometimes the fur has a golden tinge. The fur is dense and water-repellent (Olsen 1998), and always seems sleek and wet from it's swimming habit. This habit has also resulted in the animal evolving large, partially webbed back feet. As the common name suggests, it is usually found in water, including creeks, wetlands and estuaries throughout eastern and tropical Australia and New Guinea (Olsen 1998). It is often mistaken for a platypus in freshwater creeks, for it shares similar habitat and habits; it is the only other aquatic mammal in Australia, it can often be seen hunting during daylight hours, and it also often lives in a burrow dug into the side of a creek bank (Cronin 2000). In the water and on the edge, it feeds on various prey, including invertebrates, fish, frogs and waterbirds (Cronin 2000). The food is often taken to 'feeding platforms' (Triggs 1996). The resulting scat is often full of scales and bones and has a fishy smell (Triggs 1996). But it is also hunted as well as being a hunter; by birds of prey, snakes, cats, and by humans for it's pelt (Olsen 1998).
Rats and Mice: Family Muridae
This is the biggest family of all mammals, with over 1100 species (Emmons and Feer 1990). It includes the familiar house mouse and the pest rats, but there are also many native, harmless species all over the world. Because of the large size of the family, the group is divided into many subfamilies, which are then further divided into tribes.
'Rats' and 'Mice' in Australia
The huge family that contains the true rats and mice has been quite successful in Australia, with several unique species having subsequently evolved here. Currently, there are at least 60 species in Australia, which is almost a quarter of the mammals of the continent (Strahan 1998). However, some of our native rats and mice have suffered just as much as the marsupials since European settlement of the country, and several have gone extinct (Strahan 1998).
This is a sub-family basically restricted to Australia (Strahan 1998). It comprises a group of rats that reached Australia millions of years ago and has subsequently evolved into a distinctive group of species in the region. It includes the sometimes quite large 'tree rats', the rare 'stick-nest' rats, the 'pebble-mound' mice, the cute 'hopping-mice', the water rat', and the melomys, amongst other interesting rodents.
This is the group of rodents that includes the familiar house mouse and the two pest rats. It also includes over 400 other species found throughout the world (Strahan 1998).
Subfamily Murinae in Australia
In Australia, this group of rats is known as the 'new endemics' as they are thought to be a second, very distinct group of rodents that 'invaded" the continent much later than the other subfamily of the so-called 'old endemic" rodents (Straban 1998). The group includes the three introduced 'pest' species that are familiar all over the world, but also includes several native species.
Rodents: Order Rodentia
In many ways, the rodents can be considered the most successful groups of mammals alive today. With about 2000 species, some authors calculate that there are close to more types of rodents than all the other mammals combined (Emmons and Feer 1990, Archer et a12000). Rodents have also been successful in colonising every practically every corner of the plant, and within these areas have seemingly evolved to occupy every niche. Several specific species are the most widespread of all animals, having accompanied humans all over the world.
Rodents are perhaps most distinguished by their teeth. Their two large pairs of incisors, one upper and one lower, are long, tough, designed to wear out as a chisel edge, and grow continuously throughout their life.
Rodents in Australia
Rodents are relative late comers to the Australian continent. Estimates of when they 'invaded' Australia vary between 6 about 15 million years ago (Egerton 1997, Strahan 1998) with fossil evidence favouring the younger estimate (Archer et a12000). So far, only the family Muridae, the true rats and mice, has crossed into the continent from their Southeast Asian origins.