Science Fair Project Encyclopedia
- This article is about the mammal. For the genus see ambrosia beetle.
The platypus (Ornithorhynchus anatinus) is a small, semi-aquatic mammal endemic to the eastern part of Australia, and one of the four extant monotremes, the only mammals that lay eggs instead of giving birth to live young (the other three are echidnas). It is the sole representative of its family (Ornithorhynchidae) and genus (Ornithorhynchus), though a number of fossilised relatives have been found, some of them also in the Ornithorhynchus genus.
The scientific name Ornithorhynchus literally means "bird nose" in Greek, and anatinus means "duck". The common name is derived from the Greek words platus (flat) and pous (foot), meaning "flat foot" and was originally given to it as a Linnaean genus name, but it was discovered to already belong to the wood-boring ambrosia beetle (genus Platypus). The correct plural of platypus is "platypuses" and not "platypi", as is sometimes heard. Australian Aborigines call the platypus by many names including mallangong, boondaburra, and tambreet. Early British settlers called it by many names as well, such as watermole, duckbill, and duckmole. The name platypus is often prefixed with the adjective "duck-billed" to form duck-billed platypus, despite there being only one species of platypus.
Physiology and Anatomy
The physiology of the platypus is unique. The metabolic rate of the animal is remarkably low compared to other mammals, with a body temperature averaging 32°C (90°F) rather than the 38°C (100.4°F) typical of placental mammals. The extent to which this is a characteristic of monotremes, as opposed to an adaptation on the part of the small number of surviving species to harsh environmental conditions, is uncertain.
Size varies considerably between less than a kilogram (two pounds) and over two kilograms (just under four and one half pounds); with body length ranging from 30 to 40 cm (nearly 1' to 1'3", and tail length from 10 to 15 cm (4" to 6") for males and 8 to 13 cm (3" to 5") for females. Males are around one-third larger than females. There is substantial variation in average size from one region to another, though oddly this pattern does not seem to follow any particular climatic rule.
Modern platypus young have tribosphenic (three-cusped) molars, which are one of the hallmarks of mammals; adults are toothless. The platypus jaw is constructed somewhat differently from that of other mammals, and the jaw opening muscle is different. As in all true mammals, the tiny bones that conduct sound to the inner ear are fully incorporated into the skull, rather than lying in the jaw as in cynodonts and other pre-mammalian synapsids. However, the external opening of the ear still lies at the base of the jaw. The platypus has extra bones in the shoulder girdle, including an interclavicle , which is not found in other mammals. It also has a reptile-like gait, with legs that are on the sides of rather than underneath the body.
The male platypus has venomous ankle spurs, used in vicious territorial battles and fights over mates. The poison is not lethal to humans but produces excruciating pain and swelling that may last for several months. The venom can be lethal to dogs and smaller domestic animals.
Venom is produced in the crural glands of the male during the breeding season and is aggressively inflicted through a calcaneous spur on each hindlimb. Because the venom appears to have a different function from venoms produced by nonmammalian species, it may contain peptides or molecules whose principal effects are non-life threatening but nevertheless may seriously impair the victim. That this could be the case is evident from the symptoms of platypus envenomation.
In the human the most remarkable symptom is an immediate and excruciating pain. Edema rapidly develops around the wound and gradually spreads throughout the affected limb. Information obtained from case histories and anecdotal evidence indicates that the pain develops into a long-lasting hyperalgesia that persists for days or even months.
Ecology and behaviour
The platypus is nocturnal and semi-aquatic, inhabiting small streams and rivers over an extensive range from the cold highlands of Tasmania and the Australian Alps to the tropical rainforests of coastal Queensland as far north as the base of the Cape York Peninsula. Inland, its distribution is not well known: it is extinct in South Australia (bar an introduced population on Kangaroo Island) and is no longer found in the main part of the Murray-Darling Basin, probably because of the declining water quality brought about by extensive land clearing and irrigation schemes. Along the coastal river systems, its distribution is unpredictable; it appears to be absent from some relatively healthy rivers, and yet maintains a presence in others that are quite degraded (the lower Maribyrnong, for example).
The platypus is an excellent swimmer and spends much of its time in the water. When swimming they are distinguished from other Australian mammals by the absence of visible ears. It keeps its eyes tightly shut when swimming, relying completely on its other senses. All four feet of the platypus are webbed. When it swims, it propels itself by paddling with the front two feet. The tail and hind feet assist in steering but not propulsion.
The platypus is a carnivore. It feeds on worms and insect larvae, freshwater shrimps , and yabbies (freshwater crayfish) that it digs out of the river bed with its snout or catches while swimming. Its bill is very sensitive, allowing it to hunt its food without using sight. It is one of the few mammals known to have a sense of electroception: it locates its prey in part by detecting their body electricity. Its electroception is the most sensitive of any mammal. This is discussed in more detail below.
When not in the water, the platypus retires to a short, straight burrow of oval cross-section, nearly always in the riverbank not far above water level, and often hidden under a protective tangle of roots. For breeding, the female digs much larger and more elaborate burrows, up to 20 m long and blocked with plugs at intervals. She fills the nest at the end of the tunnel with reeds for bedding material.
As a monotreme, the platypus does not give birth to live young but instead lays eggs in its nest. The eggs are retained in the body for some time before they are laid and cared for actively by the parent. When the eggs hatch after an incubation period of roughly ten days, the small hairless babies cling to the mother. Like other mammals, the mother produces milk for the young. The platypus does not have nipples, but excretes the milk through pores in her skin. The young suckle milk off the mother's belly while she lies on her back.
Electrolocation in the platypus
In the platypus, electroreceptors are located in rostro-caudal rows in skin of the bill, while mechanoreceptors are uniformly distributed across the bill. The electrosensory area of the cerebral cortex is contained within the tactile somatosensory area, and some cortical cells receive input from both electroreceptors and mechanoreceptors, suggesting a close association between the tactile and electric senses. The platypus can determine the direction of an electric source, perhaps by comparing differences in signal strength across the sheet of electroreceptors as the animal characteristically moves its head from side to side while hunting. The cortical convergence of electrosensory and tactile inputs suggests a mechanism for determining the distance of prey items which, when they move, emit both electrical signals and mechanical pressure pulses. Distance could be computed from the difference in time of arrival of the two signals. Much of the platypus's feeding is done by digging in the bottom of streams with the bill. Perhaps the electroreceptors could also be used to distinguish animate and inanimate objects in this situation where the mechanoreceptors would be continuously stimulated. Much of this is speculation, and there is still much to be learned about electroreception in the platypus and its fellow monotreme, the echidna.
Field biology of the platypus
The field biology of the platypus was first studied by a number of expatriate biologists who visited the Australian colonies to collect specimens in the 1800s. Their work was followed in the early to mid-1900s by a group of resident natural historians and later by an increasing number of academic biologists. All of these workers contributed significantly to the current understanding of the field biology of this unique Australian species. The platypus occupies much the same general distribution as it did prior to European occupation of Australia, except for its loss from the state of South Australia. However, local changes and fragmentation of distribution due to human modification of its habitat are documented. The species currently inhabits eastern Australia from around Cooktown in the north to Tasmania in the south. Although not found in the west-flowing rivers of northern Queensland, it inhabits the upper reaches of rivers flowing to the west and north of the dividing ranges in the south of the state and in New South Wales and Victoria. Its current and historical abundance, however, is less well known and it has probably declined in numbers, although still being considered as common over most of its current range. The species was extensively hunted for its fur until around the turn of the 20th century. The platypus is mostly nocturnal in its foraging activities, being predominantly an opportunistic carnivore of benthic invertebrates. The species is endothermic, maintaining its low body temperature (32°C), even while foraging for hours in water below 5°C. Its major habitat requirements include both riverine and riparian features which maintain a supply of benthic prey species and consolidated banks into which resting and nesting burrows can be excavated. The species exhibits a single breeding season, with mating occurring in late Winter or Spring and young first emerging into the water after 3-4 months of nurture by the lactating females in the nesting burrows. Natural history observations, mark and recapture studies and preliminary investigations of population genetics indicate the possibility of resident and transient members of populations and suggest a polygynous mating system. Recent field studies have largely confirmed and extended the work of the early biologists and natural historians.
When the platypus was first discovered by Europeans in the late 1700s, a pelt was sent back to Britain for examination by the scientific community. The British scientists were at first convinced that the seemingly odd collection of physical attributes must be a hoax, produced by some Asian taxidermist. It was thought that somebody had sewn a duck's beak onto the body of a beaver-like animal. Scientists were also divided over whether the female platypus laid eggs, until this was later confirmed in Australia.
Much of the world was introduced to the platypus in 1939 when National Geographic magazine published an article on the platypus and the efforts to study and raise it in captivity. This is a very difficult task, and only a few young have been successfully raised since — notably at Healesville Sanctuary in Victoria.
Seeing a platypus in the wild is more a matter of luck and of patience than of difficulty. They tend to dislike populated areas, spend almost all their time underground or under water, and are primarily nocturnal. However, they are not especially uncommon, and in suitable areas most keen anglers or birdwatchers see a platypus feeding quietly along a riverbank every year or two.
The platypus was hunted for its fur, but now does not appear to be in immediate danger of extinction. It is variously classified as secure but faces future threat or common but vulnerable, mainly because the species is sensitive to water pollution.
The platypus in mammalian evolution
The platypus and other monotremes were very poorly understood for many years, and to this day some of the 19th century myths that grew up around them endure, particularly in the northern hemisphere. It is still sometimes thought, for example, that the monotremes are "inferior" or quasi-reptilian, and that they are the distant ancestor of the "superior" placental mammals. It is now known that modern monotremes are the survivors of an early branching of the mammal tree; a later branching is thought to have led to the marsupial and placental groups. The oldest fossils of monotremes (Teinolophos and Steropodon) are closely related to the modern platypus. A fossil relative of the platypus has been found in Argentina, indicating that monotremes may have reached South America from Australia, while the two continents were joined via Antarctica. In summary, the platypus is one of the closest relatives of ancestral mammals, but not itself a link in the chain of mammalian evolution. It is a branch quite separate from any other known one. Some people think that the platypus is more closely related to marsupials than it is to the spiny anteaters (Tachyglossidae).
In 2004 researchers at the Australian National University discovered the platypus has ten sex chromosomes, compared to two (XY) found in most other mammals. The chromosome system features characteristics found in mammals, but also those found in the WZ system of birds. This news has further pronounced the individuality of platypuses amongst the animal kingdom, and a target for further research into evolutionary links between mammals, birds and reptiles.
The platypus has proven itself extremely difficult to breed in captivity: Only four successful breeding occasions have been recorded.
The leading person in these efforts was David Fleay who established a platypussary - a simulated stream in a tank - at the Healesville Sanctuary in Victoria and had a successful breeding in 1943. Healesville repeated its successes in 1998 and in 2000 with a similar stream tank. Taronga Zoo in Sydney had success in 2003 with twins being bred.  (In 1972, David Fleay also found a dead baby about 50 days old, presumably bred, at his wildlife park at Burleigh Heads on the Gold Coast, Queensland. )
References and Links
- Burrell, H. The Platypus. Adelaide: Rigby, 1974.
- Griffiths, Mervyn. The Biology of the Monotremes. Academic Press, 1978.
- Strahan, R. The Mammals of Australia. New South Wales: Reed Books, 1995.
- Nature abstract on sex chromosome discovery
- Australian Platypus Conservancy Fact File
- Tasmanian Department of Primary Industries, Water, and Environment
Platypus was also the name of a progressive metal supergroup side-project featuring John Myung (of Dream Theater), Derek Sherinian (former Dream Theater member), Ty Tabor (of King's X), and Rod Morgenstein .
The contents of this article is licensed from www.wikipedia.org under the GNU Free Documentation License. Click here to see the transparent copy and copyright details