platypus

The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or monotypic taxon of its family Ornithorhynchidae and genus ''Ornithorhynchus'', though a number of related species appear in the fossil record.

Together with the four species of echidna, it is one of the five extant species of monotremes, mammals that lay eggs instead of giving birth to live young. Like other monotremes, the platypus has a sense of electrolocation, which it uses to detect prey in cloudy water. It is one of the few species of venomous mammals, as the male platypus has a spur on the hind foot that delivers an extremely painful venom.

The unusual appearance of this egg-laying, duck-billed, beaver-tailed, otter-footed mammal at first baffled European naturalists. In 1799, the first scientists to examine a preserved platypus body judged it a fake made of several animals sewn together.

The unique features of the platypus make it important in the study of evolutionary biology, and a recognisable and iconic symbol of Australia. It is culturally significant to several Aboriginal peoples, who also used to hunt it for food. It has appeared as a national mascot, features on the reverse of the Australian twenty-cent coin, and is an emblem of the state of New South Wales.

The platypus was hunted for its fur, but it has been a legally protected species in all states where it occurs since 1912. Its population is not under severe threat, although captive-breeding programs have had slight success, and it is vulnerable to pollution. It is classified as a near-threatened species by the IUCN, but a November 2020 report has recommended that it be upgraded to threatened species under the federal '' EPBC Act'', due to habitat destruction and declining numbers in all states.

Taxonomy and naming

Australian Aboriginal people name or have named the platypus in various ways depending on Australian indigenous languages and dialects. Among the names found: ''boondaburra'', ''mallingong'', ''tambreet'', ''watjarang'' (names in Yass, Murrumbidgee, and Tumut), ''tohunbuck'' (region of Goomburra, Darling Downs), ''dulaiwarrung'' or ''dulai warrung'' ( Woiwurrung language, Wurundjeri, Victoria), ''djanbang'' ( Bundjalung, Queensland), ''djumulung'' ( Yuin language, Yuin, New South Wales), ''maluŋgaŋ'' (ngunnawal language, Ngunnawal, Australian Capital Territory), ''biladurang'', ''wamul'', ''dyiimalung'', ''oornie'', ''dungidany'' ( Wiradjuri language, Wiradjuri, Vic, NSW), ''oonah'', etc. The name chosen and approved in Palawa kani (reconstructed Tasmanian language) is ''larila''.

When the platypus was first encountered by Europeans in 1798, a pelt and sketch were sent back to Great Britain by Captain John Hunter, the second Governor of New South Wales. British scientists' initial hunch was that the attributes were a hoax. George Shaw, who produced the first description of the animal in the ''Naturalist's Miscellany'' in 1799, stated it was impossible not to entertain doubts as to its genuine nature, and Robert Knox believed it might have been produced by some Asian taxidermist. It was thought somebody had sewn a duck's beak onto the body of a beaver-like animal. Shaw even took a pair of scissors to the dried skin to check for stitches.

The common name "platypus" literally means 'flat-foot', deriving from the Greek word (), from ( 'broad, wide, flat') and ( 'foot'). Shaw initially assigned the species the Linnaean name ''Platypus anatinus'' when he described it, but the genus term was quickly discovered to already be in use as the name of the wood-boring ambrosia beetle genus '' Platypus''. It was independently described as ''Ornithorhynchus paradoxus'' by Johann Blumenbach in 1800 (from a specimen given to him by Sir Joseph Banks) and following the rules of priority of nomenclature, it was later officially recognised as ''Ornithorhynchus anatinus''.

There is no universally agreed plural form of "platypus" in the English language. Scientists generally use "platypuses" or simply "platypus". Alternatively, the term "platypi" is also used for the plural, although this is a form of pseudo-Latin; going by the word's Greek roots the plural would be "platypodes". Early British settlers called it by many names, such as "watermole", "duckbill", and "duckmole". Occasionally it is specifically called the "duck-billed platypus".

The scientific name ''Ornithorhynchus anatinus'' literally means 'duck-like bird-snout', deriving its genus name from the Greek root ( ''ornith'' or ''órnīs'' 'bird') and the word ( 'snout', 'beak'). Its species name is derived from Latin ('duck-like') from 'duck'. The platypus is the sole living representative or monotypic taxon of its family (Ornithorhynchidae).

platypus-plate.jpg, A book for children published in Germany in 1798
Platypus (Ornithorhynchus anatinus). First Description 1799.jpg, Illustration from the first scientific description in 1799
platypus-sketch.jpg, Colour print from 1863

Description

In David Collins's account of the new colony 1788–1801, he describes "an amphibious animal, of the mole species", with a drawing.

The body and the broad, flat tail of the platypus are covered with dense, brown, biofluorescent fur that traps a layer of insulating air to keep the animal warm. The fur is waterproof, and textured like that of a mole. The platypus's tail stores fat reserves, an adaptation also found in the Tasmanian devil. Webbing is more significant on the front feet, which in land walking are folded up in knuckle-walking to protect the webbing. The elongated snout and lower jaw are covered in soft skin, forming the bill. The nostrils are located on the snout's dorsal surface, while the eyes and ears are just behind the snout in a groove which closes underwater. Platypuses can give a low growl when disturbed, and a range of vocalisations have been reported in captivity.

Size varies considerably in different regions, with average weight from ; males have average length , while females are the smaller at . This variation does not seem to follow any particular climatic rule and may be due to other factors such as predation and human encroachment.

The platypus has an average body temperature of about , lower than the typical of placental mammals. Research suggests this has been a gradual adaptation to harsh environmental conditions among the few marginal surviving monotreme species, rather than a general characteristic of past monotremes.

In addition to laying eggs, the anatomy, ontogeny, and genetics of monotremes shows traces of similarity to reptiles and birds. The platypus has a reptilian gait with legs on the sides of the body, rather than underneath. The platypus's genes are a possible evolutionary link between the mammalian XY and bird/reptile ZW sex-determination systems, as one of the platypus's five X chromosomes contains the DMRT1 gene, which birds possess on their Z chromosome.

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 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 not found in other mammals. As in many other aquatic and semiaquatic vertebrates, the bones show osteosclerosis, increasing their density to provide ballast.

The platypus jaw is constructed differently from that of other mammals, and the jaw-opening muscle is different. Modern platypus young have three teeth in each of the maxillae (one premolar and two molars) and dentaries (three molars), which they lose before or just after leaving the breeding burrow; adults instead develop heavily keratinised food-grinding pads called ceratodontes. The first upper and third lower cheek teeth of platypus nestlings are small, each having one principal cusp, while the other teeth have two main cusps.

Duck-billed platypus (Ornithorhynchus anatinus) diving Scottsdale.jpg, Diving
Duck-billed platypus (Ornithorhynchus anatinus) surfacing Scottsdale.jpg, Surfacing
Duck-billed platypus (Ornithorhynchus anatinus) Scottsdale 4.jpg

Venom

While both male and female platypuses are born with back ankle spurs, only the males' deliver venom.
It is powerful enough to kill smaller animals such as dogs, and though it is not lethal to humans, it can inflict weeks of agony. Edema rapidly develops around the wound and gradually spreads through the affected limb, and it may develop into an excruciating hyperalgesia (heightened sensitivity to pain) persisting for days or even months.

The venom is composed largely of defensin-like proteins (DLPs) produced by the immune system, three of which are unique to the platypus. In other animals, defensins kill pathogenic bacteria and viruses, but in platypuses they are also collected into a venom against predators. Venom is produced in the crural glands of the male, which are kidney-shaped alveolar glands connected by a thin-walled duct to a calcaneus spur on each hind limb. The female platypus, in common with echidnas, has rudimentary spur buds that do not develop (dropping off before the end of their first year) and lack functional crural glands. Venom production rises among males during the breeding season, and it may be used to assert dominance.

Similar spurs are found on many archaic mammal groups, indicating that this was an ancient general characteristic among mammals.

Electrolocation

Monotremes are the only mammals (apart from the Guiana dolphin) known to have a sense of electroreception, and the platypus's electroreception is the most sensitive of any monotreme. Feeding by neither sight nor smell, the platypus closes its eyes, ears, and nose when it dives. Digging in the bottom of streams with its bill, its electroreceptors detect tiny electric currents generated by the muscular contractions of its prey, enabling it to distinguish between animate and inanimate objects. Experiments have shown the platypus will even react to an "artificial shrimp" if a small electric current is passed through it.

The electroreceptors are located in rostrocaudal rows in the skin of the bill, while mechanoreceptors for touch are uniformly distributed across the bill. The electrosensory area of the cerebral cortex is in the tactile somatosensory area, and some cortical cells receive input from both electroreceptors and mechanoreceptors, suggesting the platypus feels electric fields like touches. These receptors in the bill dominate the somatotopic map of the platypus brain, in the same way human hands dominate the Penfield homunculus map.

The platypus can feel the direction of an electric source, perhaps by comparing differences in signal strength across the sheet of electroreceptors, enhanced by the characteristic side-to-side motion of the animal's head while hunting. It may also be able to determine the distance of moving prey from the time lag between their electrical and mechanical pressure pulses.

Monotreme electrolocation for hunting in murky waters may be tied to their tooth loss.Masakazu Asahara; Masahiro Koizumi; Thomas E. Macrini; Suzanne J. Hand; Michael Archer (2016). "Comparative cranial morphology in living and extinct platypuses: Feeding behavior, electroreception, and loss of teeth". Science Advances. 2 (10): e1601329. . The extinct '' Obdurodon'' was electroreceptive, but unlike the modern platypus it foraged pelagically.

Eyes

In recent studies it has been suggested that the eyes of the platypus are more similar to those of Pacific hagfish or Northern Hemisphere lampreys than to those of most tetrapods. The eyes also contain double cones, unlike most mammals.

Although the platypus's eyes are small and not used under water, several features indicate that vision was important for its ancestors. The corneal surface and the adjacent surface of the lens is flat, while the posterior surface of the lens is steeply curved, similar to the eyes of other aquatic mammals such as otters and sea-lions. A temporal (ear side) concentration of retinal ganglion cells, important for binocular vision, indicates a vestigial role in predation, though the actual visual acuity is insufficient for such activities. Limited acuity is matched by low cortical magnification, a small lateral geniculate nucleus, and a large optic tectum, suggesting that the visual midbrain plays a more important role than the visual cortex, as in some rodents. These features suggest that the platypus has adapted to an aquatic and nocturnal lifestyle, developing its electrosensory system at the cost of its visual system. This contrasts with the small number of electroreceptors in the short-beaked echidna, which dwells in dry environments, while the long-beaked echidna, which lives in moist environments, is intermediate between the other two monotremes.

Biofluorescence

In 2020, research revealed that platypus fur gives a bluish-green biofluorescent glow in black light.

Distribution, ecology, and behaviour

The platypus is semiaquatic, 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 was considered extinct on the South Australian mainland, with the last sighting recorded at Renmark in 1975. In the 1980s, John Wamsley created a platypus breeding program in Warrawong Sanctuary (see below), which subsequently closed. In 2017 there were some unconfirmed sightings downstream from the sanctuary, and in October 2020 a nesting platypus was filmed inside the recently reopened sanctuary.

There is a population on Kangaroo Island introduced in the 1920s, said to stand at 150 individuals in the Rocky River region of Flinders Chase National Park. In the 2019–20 Australian bushfire season, large portions of the island burnt, decimating wildlife. However, SA Department for Environment and Water recovery teams worked to reinstate their habitat, with a number of sightings reported by April 2020.

The platypus is no longer found in the main Murray–Darling Basin, possibly due to declining water quality from land clearing and irrigation although it is found in the Goulburn River in Victoria. Along the coastal river systems, its distribution is unpredictable: absent in some relatively healthy rivers, but present in some quite degraded ones, for example the lower Maribyrnong.

In captivity, platypuses have survived to 30 years of age, and wild specimens have been recaptured when 24 years old. Mortality rates for adults in the wild appear to be low. Natural predators include snakes, water rats, goannas, hawks, owls, and eagles. Low platypus numbers in northern Australia are possibly due to predation by crocodiles. The introduction of red foxes in 1845 for sport hunting may have had some impact on its numbers on the mainland. The platypus is generally nocturnal and crepuscular, but can be active on overcast days. Its habitat bridges rivers and the riparian zone, where it finds both prey and river banks to dig resting and nesting burrows. It may have a range of up to , with a male's home range overlapping those of three or four females.

The platypus is an excellent swimmer and spends much of its time in the water foraging for food. It has a swimming style unique among mammals, propelling itself by alternate strokes of the front feet, while the webbed hind feet are held against the body and only used for steering, along with the tail. It can maintain its relatively low body temperature of about 32°C (90°F) while foraging for hours in water below 5°C (41°F). Dives normally last around 30 seconds, with an estimated aerobic limit of 40 seconds, with 10 to 20 seconds at the surface between dives.

The platypus rests in a short, straight burrow in the riverbank about above water level, its oval entrance-hole often hidden under a tangle of roots. It may sleep up to 14 hours per day, after half a day of diving.

Diet

The platypus is a carnivore, feeding on annelid worms, insect larvae, freshwater shrimp, and yabby (crayfish) that it digs out of the riverbed with its snout or catches while swimming. It carries prey to the surface in cheek-pouches before eating it. It eats about 20% of its own weight each day, which requires it to spend an average of 12 hours daily looking for food.

Reproduction

The species has a single breeding season between June and October, with some local variation. Investigations have found both resident and transient platypuses, and suggest a polygynous mating system. Females are believed to become sexually mature in their second year, with breeding observed in animals over nine years old. During copulation, the male grasps the female's tail with his bill, wraps his tail around her, then grips her neck or shoulder, everts his penis through his cloaca, and inserts it into her urogenital sinus. He takes no part in nesting, living in his year-long resting burrow. After mating, the female constructs a deep, elaborate nesting burrow up to long. She tucks fallen leaves and reeds underneath her curled tail, dragging them to the burrow to soften the tunnel floor with folded wet leaves, and to line the nest at the end with bedding.

The male platypus has penile spines and an asymmetrical glans penis, with the right side smaller than the left. The female has two ovaries, but only the left one is functional. She lays one to three (usually two) small, leathery eggs (similar to those of reptiles), about in diameter and slightly rounder than bird eggs. The eggs develop '' in utero'' for about 28 days, with only about 10 days of external incubation (in contrast to a chicken egg, which spends about one day in tract and 21 days externally). The female curls around the incubating eggs, which develop in three phases. In the first, the embryo has no functional organs and relies on the yolk sac for sustenance, until the sac is absorbed. During the second phase, the digits develop, and in the last phase, the egg tooth appears. At first, European naturalists could hardly believe that the female platypus lays eggs, but this was finally confirmed by William Hay Caldwell in 1884.

Most mammal zygotes go through holoblastic cleavage, splitting into multiple divisible daughter cells. However, monotremes like the platypus, along with reptiles and birds, undergo meroblastic cleavage, in which the ovum does not split completely. The cells at the edge of the yolk remain continuous with the egg's cytoplasm, allowing the yolk and embryo to exchange waste and nutrients with the egg through the cytoplasm.

Young platypus are called "puggles". Newly hatched platypuses are vulnerable, blind, and hairless, and are fed by the mother's milk, that provides all the requirements for growth and development. The platypus's mammary glands lack teats, with milk released through pores in the skin. The milk pools in grooves on the mother's abdomen, allowing the young to lap it up. After they hatch, the offspring are milk-fed for three to four months.

During incubation and weaning, the mother initially leaves the burrow only for short periods to forage. She leaves behind her a number of thin soil plugs along the length of the burrow, possibly to protect the young from predators; pushing past these on her return squeezes water from her fur and allows the burrow to remain dry. After about five weeks, the mother begins to spend more time away from her young, and at around four months, the young emerge from the burrow. A platypus is born with teeth, but these drop out at a very early age, leaving the horny plates it uses to grind food.

Evolution

The platypus and other monotremes were very poorly understood, and some of the 19th century myths that grew up around themfor example, that the monotremes were "inferior" or quasireptilianstill endure. In 1947, William King Gregory theorised that placental mammals and marsupials may have diverged earlier, and a subsequent branching divided the monotremes and marsupials, but later research and fossil discoveries have suggested this is incorrect. In fact, modern monotremes are the survivors of an early branching of the mammal tree, and a later branching is thought to have led to the marsupial and placental groups. Molecular clock and fossil dating suggest platypuses split from echidnas around 19–48million years ago.

The oldest discovered fossil of the modern platypus dates back to about 100,000 years ago during the Quaternary period, though a limb bone of ''Ornithorhynchus'' is known from Pliocene

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