(Ornithorhynchus anatinus).

The Platypus is the single surviving species of the Ornithorhtnchidae family. Ornithorhynchid teeth are known as fossils about 15 million years old. Fossils less than a million years old appear very similar to modern animals and may belong to the same species. The most significant information available from this fragmentary fossil record is that the teeth of platypuses were once better developed and retained in the adult - in the modern form they are weakly developed and lost in infancy.

Also known as a Duckbill, the Platypus' features are unmistakabe out of water. When swimming, it can be distinguished from other Australian mammals by absence of visible ears. The size of platypus' vary with locality, the significance of this variation remains unclear. Measured from the tip of the bill to the tip of the tail, males grow to between 440 mm and 550 mm, the largest being found west of the ddividing range in New South Wales. Females grow to between 410 and 470mm. The largest males can weigh up to 2 kilograms but in North Queensland reach only half this weight. Females are normally around 25% lighter than their male counterparts.

Arguably the most uniquely australian mammal and certainly the most improbable, the general appearance of the platypus is so well known that a general description is unnecessary. However, outside of several zoos, few people are privileged to see it at close hand. In the wild, it is most frequently seen in the mid-distance and often in poor light and, even when at rest, only the tip of its bill and the top of its head, back and tail are visible.

It is readily distinguished from the water-rat or any other mammal that may swim in australian rivers or streams by its smooth swimming action, characteristic bow-wave, low silhouette, absence of visible ears, and rolling dive. Swimming is executed by the webbed forefeet. Although the hindfeet are also webbed, these are folded back against the tail except when used in steering or as brakes.

The platypus is quite agile on land but, because the web of the forefoot extends beyond the toes, it supports itself on the knuckles of the hand. A covering of long flattened guard hairs confers a sleek appearance. Below this covering is an extremely dense underfur which remains dry even after long periods of immersion.

Regarded as common but vulnerable, the Platypus is distributed in Eastern Australia from the high altitudes and winter snows of Tasmania and the Australian Alps to the tropical rainforest lowlands and plateaus of Northern Queensland. The western limits of its distribution are poorly defined but, in many regions, it seems not to extend westward of the dividing range. There is some evidence that it once occurred in much of the Condamine river in Queensland and in the Murray river and its tributaries in South Australia but it is now extinct in that state, apart from an introduced population on Kangaroo Island.

A wide variety of adult and larval invertebrates make up the greater part of the diet but it occasionally eats small vertebrates. Smaller prey are sifted from bottom silt or gravel by the bill, which functions very much like that of a duck. Larger items are snapped up individually. Apart from its shape and use, however, the bill is not at all duck-like, being pliable (like soft rubber) and very sensitive. When submerged, the platypus closes its eyes, nostrils and ear apertures and most of the information it receives about its surroundings comes from touch receptors in the skin of the bill.

The infant platypus has milk teeth but these are not replaced when they are shed. Food collected from the bottom is stored in large cheek-pouches until the animal comes to the surface. It then rests, with its four legs extended, while the food is broken up (indeed 'chewed') between the tongue and an elaborate arrangement of horny grinding plates and shearing ridges on the upper and lower jaws.

When not in the water, a platypus spends most of its time in a short, simple residential burrow just above water level in the bank of a river or stream, under a tangle of tree-roots. This is usually distinguishable from other holes in the bank by its characteristically oval sections and it may be double-ended. The nesting burrow, constructed by a female prior to laying eggs, is a more elaborate structure, up to 20 m long, plugged with earth at intervals, and terminating in a chamber containing a nest of damp herbage.

The Platypus is solitary but substantial numbers may share the use of a relatively small body of water. The extent to which individuals are territorial is not known but reports of combat between individuals indicate that they may defend specific areas. There is a general tendency for foraging to be greatest around dawn and dusk but factors such as locality, human activity, day-length, air and water temperatures, and abundance of food may override this activity pattern, so that in certain places and at certain times of the year, animals may be predominantly diurnal or nocturnal.

Meticulous grooming of the fur is an important element in its behaviour. This sometimes occurs in the water but is more usually carried out on a particular log or rock. The Platypus has a range of vocalisations - a low growling sound is made by an individual when it is disturbed.

The adult male has a sharp hollow spur on each ankle, connected to a large gland in the groin. The gland, which reaches its greatest degree of development in the breeding season, produces a venom capable of causing excruciating pain and incapacity in humans and may be lethal to smaller mammals. It seems that the venom apparatus is used in conflicts between males but, if so, it is a most unusual means of competition. Juvenile females have a rudimentary spur but the structure is lost in adults.

The breeding season varies with latitude. Mating takes place (in the water) around August in Queensland; in September in New South Wales and Victoria; and October in Tasmania. Usually two eggs are laid and these tend to be stuck together, side by side. The female incubates the eggs by holding them against her belly with her tail as she lies curled up in the terminal chamber of her burrow. Eggs hatch 1-2 weeks after being laid and the young feed for 4-5 months on milk secreted from numerous ducts on the mother's abdomen. During the long period of lactation the young remain in the burrow but the female leaves it to forage.

The range of the Platypus extends along the eastern seaboard of the mainland and throughout Tasmania but its distribution is markedly discontinuous. It occurs only in suitable bodies of fresh water and these are under increasing demand for usage which is incompatible with their survival. Dams, irrigation, stream and river bank 'improvements', fish netting and trapping, and chemical pollution are among the hazards that may alienate habitat and lead to reduction or extinction of local populations, since the species has a very limited capacity for recolonisation, successive calamities - even if temporary - lead to continued reduction in the range of the species. Thus, although it is still common in much of its present range, the Platypus must be regarded as vulnerable.

Source: 'Complete Book of Australian Mammals' - Australian Museum, 1983.

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