Question: What carnivorous Australian mammal is about half a metre in
length and occupies a variety of wetland habitats, commonly sheltering in
burrows in the banks of waterways?
Answer: The platypus...or the Australian water-rat (Hydromys chrysogaster).
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.
A DINKY DI RAT?
Mention the word 'rat' to most Australians and
images of two widespread pests - the black rat and brown
rat - immediately spring to mind.
Unfortunately, because of the nefarious activities
of these introduced species, we often forget that Australia has some
extremely attractive native rodents that were a natural part of our environment
long before their Eurasian cousins arrived on the ships of the early colonists.
The Australian water-rat is one of these native
rodents. Even if your local creek lacks platypus, it may well support population
of these fascinating animals, which are equally deserving of our best efforts
to provide good quality freshwater habitats.
WHAT'S IN A NAME?
In an attempt to foster more positive public attitudes
towards Australia's native rodents, new names for these species have been
proposed by the Australian Nature Conservation Agency (now Environment Australia),
selected from aboriginal terms for the animals. The new name for the Australian
water-rat is Rakali .
DO WATER-RATS CO-EXIST PEACEFULLY WITH PLATYPUS?
Very little is known about the ecological and
behavioural interactions between the platypus and water-rats. The two species
are found living together in many places, so presumably one does not exclude
the other from freshwater environments.
However, there also waterways where only one species
is resident. In general terms, water-rats appear more likely to persist
in certain disturbed habitats where it is not currently possible for platypus
to survive. This may reflect the fact that the water-rat is able to forage
out of the water and also prey on larger animals, especially fish.
Much more research is required before we have
a true picture of platypus/water-rat relationships. As part of its commitment
to conserving freshwater ecosystems, the Australian Platypus Conservancy
plans to expand its research programs to learn more about the biology of
water-rats and their interactions with platypus.
A PLAT OR A RAT?
From a distance of 20 metres or more it is virtually
impossible, without binoculars, to reliably distinguish a platypus from
a water-rat swimming in the water.
The thin, white-tipped tail of the rat is its
best identifying mark.
In addition, water-rats swim mainly using their
back legs. This produces a relatively narrow wake of ripples behind them.
In contrast, platypus mainly use their front legs to swim, producing a
somewhat wider wake and sometimes a matching bow wave.
As compared to platypus, water-rats also tend
to swim with a more sinuous motion and are less likely to be seen floating
in one place for many consecutive seconds.
Recent reliable sightings of Australian water-rats
can be reported to the Australian Platypus Conservancy, including details
of when and where the animal was seen. The information will be added to
a computerised data base, thereby contributing to our understanding of the
ecology of both water-rats and platypus!
Further reading about Water-rats
Gardner, J.L. and Serena, M. (1996). Observations of activity and habitat
use by the water rat Hydromys chrysogaster (Muridae: Hydromyinae)
along Badger Creek, Victoria. Australian Mammalogy 18:71-75.
Watts, C.H.S. and Aslin, H.J. (1981). The rodents of Australia.
Angus and Robertson, Sydney.
