This article is about the mammal. For the genus see ambrosia beetle. For the progressive rock band see Platypus (band)

The platypus (Ornithorhynchus anatinus) is a small, half-aquatic mammal endemic to the eastern part of Australia, and one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young (the other four 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 platypus is considered to be one of the strangest specimens of the animal kingdom: a venomous, egg-laying, duck-billed mammal. Its uniqueness makes it a recognisable faunal symbol of Australia (behind the kangaroo and koala), and a platypus features on the reverse of the Australian 20 cent coin.

Taxonomy

When the platypus was first discovered by Europeans in the late 1700s, a pelt was sent back to Britain and presented for examination by Joseph Banks to 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.

The scientific name Ornithorhynchus is derived from ορνιθορυνχος (ornithorhynkhos), which literally means "bird nose" in Greek, and anatinus means "duck". The common name is Latin platypus, but 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). There is no universally agreed upon plural of platypus. Scientists generally use awkward sounding platypuses, platypoda, or simply platypus. Colloquially, platypi is used for the plural, although to follow Latin declension rules, the correct declension would be platypoda, as it is with octopus', octopoda. 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.

Physical description

The platypus's metabolic rate is remarkably low compared to other mammals, with a body temperature averaging 32°C rather than the 38°C 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.

The body and the broad, flat tail of the platypus are covered with brown fur. It has webbed feet and a large, rubbery snout that are more reminiscent of a duck's features than those of any known mammal.

Weight varies considerably between less than 1 kg to more than 2 kg; its body length ranges from 30–40 cm, and its tail length from 10–15 cm for males and 8–13 cm for females. Males are around a 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 in the middle 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.

Venom

In humans the most remarkable symptom is 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.

Electrolocation in the platypus

In the platypus, electroreceptors are located in rostro-caudal rows in the 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. This would explain the animal's characteristic side-to-side motion of its head 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, which would also allow for computation of distance from the difference in time of arrival of the two signals.

The platypus feeds by digging in the bottom of streams with the bill. The electroreceptors could also be used to distinguish animate and inanimate objects in this situation (in which 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.

Reproduction

The male platypus reproductive structure is much like that of other mammals. One exception is that their testicles are inside their body, near their kidneys. The female reproductive system, on the other hand, differs from other placental mammals. Their paired ovaries resemble those of birds and reptiles. Although the platypus has two separate ovaries, only the left one is functional. The other is primitive and underdeveloped, and does not produce eggs.

The platypus’s behavior before mating is complex but relatively undocumented. The breeding season is in the late winter to early spring, which in Australia is August to November. A male will mate with any number of females during this time, for the platypus does not form life-long mates. Few platypuses have been seen mating, but courtship has been recorded to involve the male chasing and circling the female and biting her tail. Platypus are usually solitary animals, but during courtship, bodily contact between the pair is increased. During mating, the male curls his tail beneath the female, and lays his chest on the female’s back. There have been about five different recorded courtship methods. The first involves the male resting on the water, and the female swimming over to him and resting her muzzle on his. The second included the female swimming to the male and rubbing his side. The third recorded the female swimming beneath the male upside down, while rubbing his abdomen. The fourth documented the female swimming underneath the male and surfacing behind him. Finally, the fifth recorded instance included the female and male circling one another in tight circles.

The species exhibits a single breeding season, with mating occurring in late winter or spring and young first emerging into the water after three to four months of nurture by the lactating females in the nesting burrows. Historical observation, 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.

Outside the mating season, platypuses live in simple burrows in the ground. After mating, though, the female constructs a deeper, more elaborate burrow, while the male takes no part in caring for its young, and retreats to its yearlong burrow. The new nesting burrow that females dig may be as long as 85 feet. The burrow's entrance is about one foot above the water. The female softens the ground in the burrow with dead, folded, wet leaves.

Platypuses lay small, leathery eggs similar to those of reptiles, which are slightly rounder than bird eggs. Females usually lay two eggs at a time, but sometimes they lay one egg or three, which are about 11 mm in diameter. After laying her eggs, the female curls around them. The incubation period is separated into three parts. In the first, the embryo has no functional organs and relies on the yolk sac for respiration. During the second, the fingers and toes appear, and in the last, the egg tooth appears. When the eggs hatch after an incubation period of roughly ten days, the small hairless young cling to the mother.

The newly hatched platypuses are vulnerable, blind, and nude, and are fed by the mother's milk. Although she does not have nipples, the milk is released through pores in her abdomen. There are grooves on her abdomen that form pools of milk, allowing the young platypuses to lap up the milk. After the eggs hatch, the mother only leaves the burrow for short periods of time to feed and wet her fur. When the mother leaves her young, she plugs the entrance with soil to protect her offspring. The offspring are suckled for three to four months after they have hatched. They leave the burrow when they are seventeen weeks old.

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 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 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 riverbed 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.

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, 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.

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.[1]

Conservation status

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 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. It is a protected species within 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.

The leading figure in these efforts was David Fleay who established a platypussary — a simulated stream in a tank — at the Healesville Sanctuary 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. [2] (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. [3])

See also

• Australian fauna
• List of venomous mammals

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
• PBS.org
• Tasmanian Department of Primary Industries, Water, and Environment
• Augee, Michael L. Platypus. World Book Encyclopedia. 2001 ed.
• Michael Hutch, Melissa C. McDade, eds. Grzimek's Animal Life Encyclopedia; Volume 12. Detroit: Gale, 2004

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