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The Platypus - A Monotreme

By Edited Nov 13, 2013 0 0

Australia's Monotremes

The Duck-Billed Platypus

The platypus is one of Australia's most iconic symbols. It is a monotreme - 'mono' meaning 'one hole', the cloaca, which is used for the elimination of wastes and also for reproduction.

'Platys' is Greek for 'flat or broad' and 'pous' means 'foot'. The scientific name of the platypus is Ornithorhynchus anatinus. The platypus is also the world's only venomous furred mammal. Sharp, hollow, horny spurs are found on the inside of the ankles. Once the females are a year old, they lose their spurs.

Platyrus Spur

The spurs are 15mm long and inject a powerful venom. Although the venom is not lethal to humans, it is excruciatingly painful.

A pelt of a platypus was sent to the United Kingdom in 1798 but because of its bizarre appearance, it was dismissed as a hoax. It was called by various names by early Australian settlers such as watermole, duckbill and duckmole. The fact that it laid eggs was not confirmed until 1884.

Range
The platypus is found in streams and rivers in the eastern states of Australia (mainly east of the Great Dividing Range). They are also widespread in Tasmania.

Habitat
The platypus is found in a wide range of habitats from tropical rainforest to alpine lakes. Being semi-aquatic, they forage in freshwater streams and need earth banks for their burrows. Ideally the banks will be consolidated by the roots of shady vegetation; there will be gravel-type riverbeds and a series of pools joined by waterways.

The Platypus

Description
The length of the platypus
ranges from 45 to 60cm long, including the tail. Females are usually smaller than males. Weight ranges from 1kg to 3kg. Those in the north of their range are generally somewhat smaller than those in Tasmania. They are a deep brown in colour with the ventral surfaces being either a golden colour or silky grey. The top coat is waterproof and more dense than that of an otter or polar bear with around 800 hairs per square millimeter. The undercoat is woolly. Air trapped under the fur gives excellent insulation. The fur on the tail is coarse and bristly. The tail is used as a rudder and as a reservoir for fat reserves. A healthy platypus has a plump tail.

The bill of the platypus is really an elongated 'snout' and is a very specialized instrument. It is covered with soft, moist, leathery skin. The lower part is shorter than the upper. Sensitive and pliable, the bill contains approximately 850,000 electrical and tactile receptors. This electro-receptor system allows monotremes to locate their prey by detecting electric fields generated by muscular contractions. When diving, the platypus closes its eyes, ears and nostrils. It then rocks its head from side to side to allow the electro-receptor system to locate its quarry.

Locomotion
The webbed feet are ideal for swimming but when walking, the webbing is folded back to allow the animal to walk. The strong front legs do the paddling and digging while the hind legs steer and tread water when the platypus feeds on the surface. Because the legs extend outwards from the body, the platypus has a distinct reptilian-like waddle as it walks.

Links to reptiles and birds
The platypus uses the same opening for reproduction and elimination of waste, it synthesises ascorbic acid in the kidney, it has cervical ribs, the legs extend from the sides of the body and its eggs have large yolks and rubbery shells. All these factors it shares with reptiles.

It also shares several bird-like traits. The left ovary only is functional with the other ovary being poorly developed, one of its 'Y' chromosomes shares genes with one of the sex chromosomes found in birds and it has a wishbone (interclavicle) like a bird.

The platypus is one of only two mammals belonging to the order Monotremata. Most mammals give birth to live young eg cows and dogs but monotremes lay eggs. 

The Echidna - A Monotreme

The other monotreme, the echidna, (seen above) is also unique to Australia.

Nutrition
The platypus is carnivorous, foraging on the river bed for crustaceans, worms, molluscs, and the larvae of many freshwater insects. It is most common for the platypus to leave the burrow at dusk to forage through the night before returning to the burrow (not necessarily the same one) at daybreak. They dive for 20 to 40 seconds, performing about 75 dives each hour. The water depth is generally less than 5 metres. Small invertebrate animals are caught on the riverbed, tucked into cheek pouches and eaten on the surface. The young platypus has molars which they soon lose. In place of teeth, adults have thick, horny pads of keratin to hold and grind the food.

Reproduction
Mating occurs in the water and about 28 days after fertilisation, 1 to 3 eggs are laid. The mother incubates the eggs for a further ten days by holding them against her abdomen with her tail. When born the offspring are about 18mm long, blind and naked. The limbs are also undeveloped. There are no teats but the iron-rich milk oozes from the skin and is sucked up by the babies. At 4 to 5 months old, they spend increasing periods outside the burrow and are then weaned. Sexual maturity is attained at about two years old.

Behaviour
The platypus is reclusive and solitary. Their time is divided between resting in burrows and foraging for food in the water. There are two types of burrows. Resting burrows are about 3 to 8 metres long, oval-shaped with a concealed entrance at each end.

Nesting burrows are more elaborate and are around 20 m long with multiple chambers and earth plugs. These burrows are shared by a mother and her offspring. The passage slopes up and conforms to the mother's shape. The entrance is always above water. The female lines the burrow with dead, folded, wet leaves. The end is filled with gum leaves, thin willow twigs, or crushed reeds. These are dragged into the nest by the mother curling her tail round the material.

 Threats
Threats to the platypus include loss of habitat, watercourse degradation, death by getting tangled in illegal fish nets and traps, vehicles and flooding. Natural predators include snakes, goannas, foxes, dingoes and birds of prey.

Other articles on Australian animals that you might enjoy:

The Emu - Characteristics
Embryonic Diapause - an Overview
The Kangaroo - An Overview
The Koala - An Australian Marsupial
The Echidna - A Monotreme

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