| Platypus[1] Temporal range: 9–0 Ma Miocene to Recent | |
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Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Monotremata |
| Family: | Ornithorhynchidae |
| Genus: | Ornithorhynchus Blumenbach, 1800 |
| Species: | O. anatinus |
| Binomial name | |
| Ornithorhynchus anatinus (Shaw, 1799) | |
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| Platypus range (red — native, yellow — introduced) | |
The platypus (Ornithorhynchus anatinus), sometimes referred to as the duck-billed platypus, is a semiaquatic egg-laying mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth. The animal is the sole living representative of its family (Ornithorhynchidae) and genus (Ornithorhynchus), though a number of related species have been found in the fossil record. The first preserved platypus body was thought to have been a fake, made of several animals sewn together,[3] when it was first looked at by scientists in 1799.[4]
The unusual appearance of this egg-laying, duck-billed, beaver-tailed, otter-footed mammal baffled European naturalists when they first encountered it, with some considering it an elaborate hoax. It is one of the few species of venomous mammals: the male platypus has a spur on the hind foot that delivers a venomcapable of causing severe pain to humans. The unique features of the platypus make it an important subject in the study of evolutionary biology and a recognisable and iconic symbol of Australia; it has appeared as a mascot at national events and is featured on the reverse of its 20-cent coin. The platypus is the animal emblem of the state of New South Wales.[5]
Until the early 20th century, it was hunted for its fur, but it is now protected throughout its range. Although captive breeding programs have had only limited success and the platypus is vulnerable to the effects of pollution, it is not under any immediate threat.
Taxonomy and etymology
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.[6] British scientists' initial hunch was that the attributes were a hoax.[7]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,[8] and Robert Knox believed it might have been produced by some Asian taxidermist.[7] It was thought that 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.[9]
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Description
In David Collins's account of the new colony 1788 – 1801, he describes coming across "an amphibious, mole like" animal. His account includes a drawing of the animal.[16]
The body and the broad, flat tail of the platypus are covered with dense, brown fur that traps a layer of insulating air to keep the animal warm.[9][14] The fur is waterproof, and the texture is akin to that of a mole.[17] The platypus uses its tail for storage of fat reserves (an adaptation also found in animals such as the Tasmanian devil[18]). It has webbed feet and a large, rubbery duck-like snout. The webbing is more significant on the front feet and is folded back when walking on land.[14] Unlike a bird's beak (in which the upper and lower parts separate to reveal the mouth), the snout of the platypus is a sensory organ with the mouth on the underside. The nostrils are located on the dorsal surface of the snout, while the eyes and ears are located in a groove set just back from it; this groove is closed when swimming.[14] Platypuses have been heard to emit a low growl when disturbed and a range of other vocalisations have been reported in captive specimens.[9]
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Diet
The platypus is a carnivore: it feeds on annelid worms, insect larvae, freshwater shrimp, and freshwater yabby that it digs out of the riverbed with its snout or catches while swimming. It uses cheek-pouches to carry prey to the surface, where it is eaten.[52]The platypus needs to eat about 20% of its own weight each day, which requires it to spend an average of 12 hours daily looking for food.[54]
Reproduction
When the platypus was first encountered by European naturalists, they were divided over whether the female laid eggs. This was not confirmed until 1884, when William Hay Caldwellwas sent to Australia, where, after extensive searching assisted by a team of 150 Aborigines, he managed to discover a few eggs.[14][29] Mindful of the high cost per word, Caldwell tersely wired London, "Monotremes oviparous, ovum meroblastic." That is, monotremes lay eggs, and the eggs are similar to those of reptiles in that only part of the egg divides as it develops.
The species exhibits a single breeding season; mating occurs between June and October, with some local variation taking place between different populations across its range.[48] Historical observation, mark-and-recapturestudies, and preliminary investigations of population genetics indicate the possibility of both resident and transient members of populations, and suggest a polygynous mating system.[57] Females are thought likely to become sexually mature in their second year, with breeding confirmed still to take place in animals over nine years old.[57]
Outside the mating season, the platypus lives in a simple ground burrow, the entrance of which is about 30 cm (12 in) above the water level. After mating, the female constructs a deeper, more elaborate burrow up to 20 m (66 ft) long and blocked at intervals with plugs (which may act as a safeguard against rising waters or predators, or as a method of regulating humidity and temperature).[58] The male takes no part in caring for its young, and retreats to his year-long burrow. The female softens the ground in the burrow with dead, folded, wet leaves, and she fills the nest at the end of the tunnel with fallen leaves and reeds for bedding material. This material is dragged to the nest by tucking it underneath her curled tail.[9]
The female platypus has a pair of ovaries, but only the left one is functional.[49] The platypus' genes are a possible evolutionary link between the mammalian XY and bird/reptile ZW sex-determination systems because one of the platypus' five X chromosomes contains the DMRT1 gene, which birds possess on their Z chromosome.[59] It lays one to three (usually two) small, leathery eggs (similar to those of reptiles), about 11 mm (0.43 in) in diameter and slightly rounder than bird eggs.[60] 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).[49] After laying her eggs, the female curls around them. The incubation period is divided into three phases.[61] In the first phase, the embryo has no functional organs and relies on the yolk sac for sustenance. The yolk is absorbed by the developing young.[62] During the second phase, the digits develop, and in the last phase, the egg tooth appears.[61]
Most mammal zygotes go though holoblastic cleavage, meaning that following fertilization the ovum is split due to cell divisions into multiple, divisible daughter cells. This is in comparison to meroblastic division in birds and platypuses, which causes the ovum to split but not completely. This causes the cells at the edge of the yolk to be cytoplasmically continuous with the egg’s cytoplasm. This allows the yolk, which contains the embryo, to exchange waste and nutrients with the cytoplasm.[63]
The newly hatched young are vulnerable, blind, and hairless, and are fed by the mother's milk. Although possessing mammary glands, the platypus lacks teats. Instead, milk is released through pores in the skin. The milk pools in grooves on her abdomen, allowing the young to lap it up.[9][48] After they hatch, the offspring are suckled for three to four months. During incubation and weaning, the mother initially leaves the burrow only for short periods, to forage. When doing so, she creates 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 forces water from her fur and allows the burrow to remain dry.[64] 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.[48] A platypus is born with teeth, but these drop out at a very early age, leaving the horny plates it uses to grind food.[65]
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Cultural references
The platypus has been featured in the Dreamtime stories of indigenous Australians, who believed the animal was a hybrid of a duck and a water rat.[90]:57–60 According to one story, the major animal groups, the land animals, water animals and birds, all competed for the platypus to join their respective groups, but the platypus ultimately decided to not join any of them, feeling that he did not need to be part of a group to be speci
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