Hints on Spotting Platypus and Water-rats in the Wild
When and where to look
Platypus (Ornithorynchus anatinus ) and Australian water-rats (Hydromys chrysogaster) are most likely to be observed early in the morning or late in the evening, though both animals may also be active in the middle of the day.
Both platypus and water-rats occupy weir pools, irrigation channels and man-made dams or reservoirs as well as natural lakes, rivers, creeks, backwaters and billabongs. They are generally most readily spotted in places where the water surface is fairly calm, making it easier for observers to identify the ripples formed on the water surface as the animals swim and dive. Both species occur over a wide altitudinal range, from sea level up to at least 1500 metres. However, platypus are not commonly seen (and never abundant) in the salty water of bays and estuaries. In contrast, water-rats are known to inhabit ocean beaches and are found on many islands surrounded by sea water.
Size and appearance
At a distance of more than 20 metres, even an experienced observer may find it difficult to reliably distinguish a platypus from a water-rat, especially in poor light.
Platypus are dark brown in colour, with lighter underparts and a small white patch located next to each eye. Similarly, water-rat fur usually looks dark brown when the animals are wet. When water-rats are dry and seen at close range, their fur may (depending on the area) be chocolate brown, reddish brown, mouse grey or even mottled grey-brown, with underparts that vary in colour from cream to light brown to golden yellow.
Both species typically float low in the water, with just the top of the head and back (and sometimes a bit of tail) visible as they swim on the surface.
A platypus seen from a distance of about 25 metres (above)
and a water-rat seen from a distance of about 15 metres (below)
Platypus and water-rats are also quite similar in size, with very large adult males of both species measuring up to about 60 centimetres in length (including the tail). Juveniles are of course smaller than most adults when they first enter the water. Juvenile platypus first emerge from nesting burrows in late January to early March in Victoria and New South Wales, with Queensland juveniles emerging a few weeks earlier and Tasmanian juveniles up to around two months later. By comparison, young water-rats are seen over a much longer period of time, from early spring to at least early autumn.
The best way to distinguish a water-rat from a platypus in the water is to look carefully at the tail: the water-rat has a long, narrow tail with a conspicuous white tip, whereas the platypus has a flat, uniformly dark, paddle-like tail.
Platypus are also very rarely seen on land, though they may occasionally rest on a log or rock, usually while grooming.
In contrast, water-rats are much more likely to be seen on land, either consuming their prey or running along the bank (as shown above).
Feeding habits and diet
The platypus diet mainly consists of aquatic insects such as mayfly and caddis-fly larvae, along with other invertebrates such as worms, freshwater shrimps and yabbies. Although a platypus may sometimes glean prey from the water’s surface, the animals mainly find their food by diving, with around 75 dives typically completed per hour. Prey items are stored in cheek pouches and then chewed up and swallowed after a platypus returns to the surface to breathe (as shown below).
In contrast to a platypus, a water-rat is equipped with a sharp set of teeth and front paws that are good at grasping and holding things. Hence, while water-rats eat some of the same insects and other items regularly eaten by a platypus, they also dine on fish, large mussels and crabs, frogs (including cane toads) and occasionally even waterfowl such as ducks. All prey is captured and carried using the jaws, and then normally eaten out of the water – most typically on a conveniently placed log, rock or elevated clump of reeds which often becomes densely littered with yabby claws or mussel shells.
A platypus diving sequence typically starts with the animal arching its back as it neatly propels itself forward and down into the water (as shown below), leaving an expanding ring of ripples on the surface. There is generally little or no associated sound unless the platypus has been startled, causing the animal to push itself downwards more forcefully than usual, which in turn creates an audible splash.
A platypus will normally remain underwater for less than one minute while feeding (though occasionally a bit longer, particularly if the water is more than about 3 metres deep). When the animal’s oxygen supply starts to run low it returns to the surface, most typically popping up within 10-20 metres of the point where it dived. However, if a platypus is alarmed by a bird flying overhead or some other perceived threat, it may hide underwater for 10 minutes or more, conserving oxygen by wedging itself under a handy log or the roots of an undercut tree at the waters edge. Alternatively, it may retire to a burrow or a protected location under an overhanging shrub until the danger has passed.
A platypus will hold its position by paddling gently with its front feet even while it floats on the surface, creating a distinctive ‘bullseye’ pattern of ripples which becomes stronger as the platypus dives, then gradually fades away. These strong ripples are very distinctive when the water surface is calm (as shown below) and are often the first sign that a platypus is active in the area.
However, platypus ripples usually can be detected even in fairly choppy conditions (as shown below). The path of a platypus underwater is sometimes marked by a stream of small bubbles rising to the surface, as air is squeezed from the fur.
The ripples formed when a water-rat dives are generally weaker and less well defined than those created by a platypus (as shown below). Water-rats typically stay underwater for fairly short periods of time and often come to the surface relatively far from where they dived, reflecting the fact that they have been actively chasing fish or other prey underwater.
When a platypus wants to swim rapidly and directly from point A to point B, (for example, to return to a burrow to sleep at the end of a feeding session, or to chase another platypus), he or she will travel mainly on the surface to breathe and paddle at the same time. This type of movement typically creates a long, narrow wake in the water (as shown below), often visible from a distance as a distinctive silvery streak in calm water.
The maximum rate of travel recorded for a platypus swimming in a direct manner at the surface is around 3.6 kilometres per hour, though the animals usually progress at a more leisurely rate of around 1.5 to 2.5 kilometres per hour.
Platypus use only their front legs to paddle, resulting in quite a strong bow wave but relatively narrow trailing wake (below).
By comparison, water-rats mainly use their hind legs when swimming, which typically produces a wider trailing wake (below).
When resting on the water surface, a platypus may spend quite a lot of time scratching or combing its fur with a hind foot and can often look quite contorted (or ecstatic) while involved in this activity.
A platypus grooming in the water
Water-rats may occasionally give themselves a quick scratch while in the water but prefer to carry out most grooming activities on land.
Other animals can sometimes be mistakenly identified as a platypus or water-rat
It is possible to confuse diving birds (such as ducks, grebes, cormorants and darters) with both the platypus and water-rat. This is especially true of musk ducks (Biziura lobata): large, dark birds which often swim alone and produce a platypus-like pattern of ripples when they dive. Particularly in autumn, a male musk duck will sometimes swim with his head and neck stretched out straight in front of him near the water surface, producing a very platypus-like outline.
To distinguish waterbirds from platypus and water-rats, keep in mind that the silhouette of a platypus or water-rat is much flatter than that of even relatively small birds such as grebes and coots. If the profile of a swimming animal projects well above the water surface, it is unlikely to be that of a platypus or water-rat.
Freshwater tortoises and some of the larger fish (such as carp, eels and freshwater catfish) can sometimes be mistaken for a platypus or water-rat, particularly if glimpsed for only a few seconds in dim light. Carp in particular may spend long periods of time with their backs well out of the water as they feed near the shoreline. However, differences in the appearance and behaviour of these animals and the two mammals usually become quite obvious upon more careful observation.