Platypus

• 1. Introduction

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  In 1799, the first scientists to examine a preserved platypus body decided that it had to be a fake. They suspected a collector had stitched together pieces of several different animals. George Shaw, who wrote the first formal description, even used a pair of scissors to check for stitches. The platypus, Ornithorhynchus anatinus, is a semiaquatic mammal of eastern Australia, including Tasmania. It is the only living member of its family, Ornithorhynchidae. It belongs to a tiny group called the monotremes, mammals that lay eggs. That group holds just five living species today. Four are echidnas and one is the platypus. This documentary asks how an animal can hunt with its eyes, ears and nostrils sealed shut. It asks why a mammal carries venom, and where such a creature came from. It also asks how people have treated the platypus, and whether it will keep its place in Australia's rivers.

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• 2. Naming the Puzzle

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  Captain John Hunter, the second Governor of New South Wales, sent a pelt and a sketch to Great Britain after Europeans first met the animal in 1798. Robert Knox offered a theory for who might have made the trick. Because the specimen reached England by way of the Indian Ocean, he suspected Chinese sailors had built it.

  Shaw published his description in the Naturalist's Miscellany in 1799 and called the animal Platypus anatinus. That genus name already belonged to a beetle, so it could not stand. In 1800, Johann Blumenbach independently described the animal as Ornithorhynchus paradoxus, using a specimen given to him by Sir Joseph Banks. Following the rule that the earliest valid name wins, the animal was later officially recognised as Ornithorhynchus anatinus.

  The common name platypus comes from Greek and means flat-foot. It joins platus, meaning broad or flat, to pous, meaning foot. The scientific name means duck-like bird-snout. Its genus half draws on the Greek roots for bird and for snout, and its species half comes from the Latin word for duck.

  Even the plural is disputed. Dictionaries list either platypuses or simply platypus. The form platypi is also common, but it is pseudo-Latin, and the Greek roots would give platypodes. Early European settlers preferred names such as watermole, duckbill and duckmole. No official word exists for a young platypus, though platypup and puggle are used informally.

  Aboriginal peoples had names for the animal long before any of these. Boondaburra, mallingong, tambreet and watjarang were recorded around Yass, the Murrumbidgee and Tumut. Tohunbuck comes from the region of Goomburra on the Darling Downs. Dulaiwarrung belongs to the Woiwurrung language of the Wurundjeri in Victoria. Djanbang is a Bundjalung word from Queensland, and djumulung is a Yuin word from New South Wales. The Ngunnawal of the Australian Capital Territory say malungang. In Palawa kani, the reconstructed Tasmanian language, the chosen and approved name is larila.

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• 3. An Odd Body

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  Only otters have denser fur among mammals than the platypus. Short brown hairs trap a layer of air against the skin, which keeps the animal warm in the water and out of it. The coat is waterproof, made of flattened guard hairs over curly underfur. Under ultraviolet light it glows cyan and green, which may hide the animal from predators that see UV. The pigment granules in its hair, called melanosomes, are hollow, a feature no other mammal shares.

  The bill is nothing like a bird's. It is a long snout and lower jaw covered in soft skin, with nostrils near the top of the tip. The eyes and ears sit just behind it in a groove that closes underwater. Cheek pouches hold food for later. The wide, flat tail resembles a beaver's but is furry rather than scaly. It stores fat and works as a rudder while swimming.

  The legs are short and sprawl out to the sides. Webbing is larger on the front feet, so on land the animal folds them under and walks on its knuckles to protect the skin between the toes. Its shoulder girdle contains an interclavicle bone, a trait it shares with reptiles. Its bones are unusually dense, which, as in many other water-dwelling vertebrates, reduces buoyancy.

  Adult platypuses have no teeth. They grind food with tough pads of keratin instead. Young animals do have teeth, but lose them around the time they leave their natal burrow. There is also no functional stomach, because the genes for making pepsin are missing. A pouch-like section of the digestive tract helps the intestines absorb nutrients.

  Males average about 50 cm in length and 1700 g in weight. Females average 43 cm and 900 g. The species follows Bergmann's rule, so animals grow larger the farther south they live, though local variation exists. Body temperature averages about 32 C, against 37 C in placental mammals. Research suggests this was a gradual adaptation among the few surviving monotremes, not a trait of their ancestors. A single opening, the cloaca, serves both the reproductive and waste systems. In females only the left ovary works.

  The back ankle spurs tell a stranger story. Males and females are both born with them, but only males keep them into adulthood. Similar spurs appear in many archaic mammal groups, so the feature is ancient. In males the spur delivers venom made mostly of defensin-like proteins, some unique to the platypus. Kidney-shaped glands in the thighs produce it, and the glands connect to the spurs. Males use these weapons against rival males when competing for breeding. The venom causes pain strong enough to hurt a human. The affected limb swells, and it can develop an excruciating hypersensitivity to pain that lasts for months.

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• 4. Feeling Electricity

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  Researchers once passed a small electric current through an artificial shrimp, and the platypus reacted to it. That response reveals how the animal hunts. Underwater its eyes, ears and nose are closed, so it digs along the bottom of a stream with its bill and reads the electric signals given off by the muscles of its prey. Among mammals, only monotremes and the Guiana dolphin are known to sense electricity this way.

  The bill carries about 40,000 electroreceptors, arranged in rows from front to back. Touch receptors, by contrast, are spread evenly across it. In the brain, the electrical sense area lies within the touch area of the cortex, and some cells receive input from both kinds of receptor. This suggests the platypus feels an electric field as a kind of touch. Bill receptors take up so much of the brain's map of the body that they play the role human hands play in the Penfield homunculus.

  The animal can tell which way a source lies, perhaps by comparing signal strength across the array of receptors. Its habit of swinging the head from side to side while hunting probably sharpens that comparison. It may also judge the distance of moving prey from the delay between the electrical signal and the pressure wave.

  The eyes tell a different story. They are small and shut underwater, yet several features show that the ancestors relied on sight. The cornea is flattened, and the back of the lens is sharply tilted, as in other aquatic mammals. Cells in the retina that support binocular vision are concentrated on the side nearest the ear, hinting at a faded role in catching prey. The animal's actual acuity is too poor for that. Some of the eye's traits, such as scleral cartilage and double cones, are also found in lungfish and amphibians. Brain structure points to the visual midbrain doing more work than the visual cortex, as in some rodents. Electric sensing has been developed at the cost of vision.

  The short-beaked echidna lives in dry places and has few electroreceptors. The long-beaked echidna lives in wetter habitats and sits between the two extremes.

  The ears are tuned to hearing in air, not water. Like all true mammals, the platypus has three middle ear bones. Its cochlea lacks spirals but is described as well organised, with rows of inner and outer hair cells. It has more rows of inner hair cells than placental mammals. The sense of smell evolved independently from that of the echidna. The platypus olfactory bulb lacks the complex layers of the echidna's, and its piriform cortex and lamellae are simpler.

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• 5. River Life

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  A range of freshwater habitats suits the platypus, from tropical rainforest to cold alpine country, as long as permanent water is present. It lives from Queensland to Tasmania, including King Island but not the Furneaux Group. On the South Australian mainland it was believed extinct after the last recorded sighting at Renmark in 1975. Then in October 2020 a nesting platypus was filmed in the wild after a previously abandoned sanctuary reopened.

  A population on Kangaroo Island, introduced in the 1920s, is said to number 150 animals in the Rocky River region of Flinders Chase National Park. Large parts of the island burned in the 2019-20 bushfire season. Recovery teams from South Australia's Department for Environment and Water worked to restore habitat, and several sightings were reported by April 2020. In the Murray-Darling Basin the platypus has almost disappeared, possibly because of poor water management.

  Swimming is done with alternating strokes of the front feet, while the webbed hind feet and tail steer. The animal keeps its low body temperature even when feeding in water colder than 5 C. In one study, dives lasted thirty-five seconds on average, with thirteen-second pauses at the surface. Activity is mostly nocturnal, though in summer it extends into dusk and in winter into daytime. A platypus may spend half the day in the water before retreating into a burrow dug into the bank. Burrows range from simple resting spots to complex nesting chambers. One animal may range up to 7 km, and a male's range overlaps those of three or four females. Platypuses are quiet, growling when disturbed and squeaking in pain.

  The diet is meat, gathered by probing the bottom. It includes insect larvae, annelid worms, shrimp, crayfish, bivalves, tadpoles and fish eggs. Captive platypuses have lived up to thirty years, and wild animals have been recaptured at twenty-four. Predators include Murray cod, eels, carpet snakes, goannas, eagles, crocodiles and foxes. Parasites and viruses also take a toll, though platypuses seem to tolerate them well. Fleas, mites and ticks live on the outside, with ticks heavier on the young. The tick Ixodes ornithorhynchi commonly lives on them. Inside, they may host protozoans, trematodes and nematodes. In Tasmania, the fungus Mucor amphibiorum has caused mucormycosis, with skin lesions and ulcers.

  Breeding is seasonal, and southern populations breed later in the year than northern ones. Those in New South Wales mate during fall and winter. Resident and transient animals both occur, and the mating system appears to be polygynous. Females are believed to mature at two years and to keep breeding past nine. In mating, the male swims behind the female, grips her tail in his bill and then holds her neck or shoulder. She then retreats to a grass-lined burrow, which can run 30 m, to lay eggs and rear the young.

  The female's eggs were long doubted by European naturalists. William Hay Caldwell confirmed in 1884 that she does lay them. She normally lays two small, leathery eggs about 17 mm long. Like reptiles and birds, monotremes have an egg that does not divide completely as it develops. The cells at the edge of the yolk stay connected to the egg's cytoplasm, so yolk and embryo can swap nutrients and waste. The eggs develop inside the mother for about twenty-eight days, followed by ten days of external incubation while she curls around them.

  Newly hatched young are blind and hairless. Because teats are absent, milk seeps through pores in the mother's skin and the young lap it from her fur. Platypus milk is thicker than that of placental mammals and supplies everything a growing young one needs. Each hatchling also has a remnant yolk sac that vanishes within four days.

  When she leaves to forage, the mother stops the burrow with thin soil plugs. Pushing back through them on her return squeezes water from her fur and keeps the burrow dry. After five weeks she spends less time with her offspring, who emerge at around four months. By then they are fully furred, at roughly 67% of adult weight and 80% of adult length. Juvenile males disperse farther than females, and full size comes at around two years.

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• 6. Deep Ancestry

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  Between 1934 and 1951, in separate publications, William King Gregory argued that monotremes and marsupials shared a branch that split off after the placentals. Later fossils and research showed this was wrong. Modern monotremes are survivors of an early branch of the mammal tree, and a later branch led to marsupials and placentals. Molecular clock and fossil dating both suggest the platypus split from echidnas some 19-48 million years ago.

  Steropodon, found in New South Wales, is known from an opalised lower jawbone with three molar teeth. Those teeth were first thought to be tribosphenic, which would have supported a version of Gregory's theory. Later work showed they evolved by a separate process. Teinolophos, another Cretaceous monotreme, had an elongated jaw but no beak. Both are considered basal to the platypus and echidnas.

  In 2024, fossils from the same rocks as Steropodon added Opalios and Dharragarra to the family tree. Dharragarra may be the oldest member of Ornithorhynchidae, since it keeps the dental formula of later platypus relatives. Monotrematum and Patagorhynchus come from Argentina, from the latest Cretaceous and the mid-Paleocene. Their presence suggests some monotremes reached South America from Australia while the two continents were joined through Antarctica.

  The closest fossil relative is Obdurodon, from the late Oligocene to the Miocene of Australia. It looked much like the modern platypus apart from having molar teeth. A fossil tooth of the giant Obdurodon tharalkooschild, dated to 5-15 million years ago, implies an animal 1.3 metres long, the largest platypus known. The oldest fossil of the modern platypus is only about 100,000 years old. A limb bone of Ornithorhynchus from Pliocene rock is older.

  The loss of teeth has long puzzled researchers, since the lineage kept a lower molar row for more than 95 million years. Newer studies place the loss around 2.5 million years ago, in the Plio-Pleistocene. That is when the rakali, a large semiaquatic rodent, reached Australia from New Guinea. The platypus had eaten a wide range of hard and soft prey, but the rakali outcompeted it for crayfish and mussels. Horny pads for softer prey may have been the result.

  The platypus is a frequent subject of evolutionary research, and its genome holds surprises. In 2004, researchers at the Australian National University found ten sex chromosomes where most mammals have two. They form five pairs, so males carry XYXYXYXYXY and females carry XX in each pair. One X chromosome closely resembles the Z chromosome of birds. The platypus lacks the mammalian sex-determining gene SRY, and a study found the AMH gene on the oldest Y chromosome decides sex instead.

  A draft genome appeared in Nature in May 2008, with both reptilian and mammalian elements and two genes previously known only from non-mammalian vertebrates. More than 80% of its genes exist in other amniotes that have been compared. An updated genome, the most complete on record, came out in 2021 alongside that of the short-beaked echidna.

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• 7. Under Pressure

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  At Healesville Sanctuary in Victoria, David Fleay achieved the first successful captive breeding in 1943. Success returned in 1998 and 2000 with a stream tank. From 2008 to 2012 the sanctuary bred platypuses regularly, including twins. Warrawong Sanctuary also bred them in 1990-91, and Taronga Zoo in Sydney produced twins in 2003. Taronga has since bred more animals for release into the wild in New South Wales. Outside Australia, only the San Diego Zoo Safari Park in California holds platypuses. In 1947 the Bronx Zoo received three, two females and a male, and one female named Penelope had a false proto-pregnancy.

  Hunters took platypuses for their fur until the early years of the 20th century. Victoria granted legal protection in 1890, and the whole country followed by 1912. Even so, the animals drown in the nets of inland fisheries. Opera house traps, used by recreational fishers to catch yabbies, also drown non-target species such as platypuses. They are banned in the ACT, South Australia, Tasmania and Victoria, and restricted in New South Wales and Queensland.

  In 2016 the International Union for Conservation of Nature rated the species near threatened. Its estimate was that numbers had fallen by about thirty percent on average since European settlement. Other biologists worry the 2016 baseline was wrong and that the loss may be as high as fifty percent. Dams, pollution, urban expansion and runoff continue to disrupt habitat, and droughts and human water demand add to the pressure.

  In January 2020, University of New South Wales researchers presented evidence that the platypus risks extinction. They pointed to water extraction, land clearing, climate change and invasive species. Over fifty years, they predicted abundance would fall by 47-66% and occupancy of habitat networks by 22-32%. Local populations would die out across about 40% of the range. Under climate projections to 2070, drought would cut abundance by 51-73% and occupancy by 36-56%.

  A November 2020 report from the same university found that platypus habitat had shrunk by 22% over thirty years. It was funded by a grant from the Australian Conservation Foundation, with the World Wildlife Fund Australia and Humane Society International Australia. The declines were concentrated in the Murray-Darling basin and in New South Wales generally. The report supported listing the platypus as threatened under the federal EPBC Act, which does not currently cover it. South Australia lists the animal as endangered under its National Parks and Wildlife Act 1972. In January 2021, Victoria adopted the vulnerable designation under its Flora and Fauna Guarantee Act 1988.

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• 8. People and Platypuses

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  For Aboriginal Australians, platypus tails were prized as fatty, nutritious food, and the animal was hunted and eaten. European researchers took a harder line. They captured and killed platypuses or removed their eggs, partly to advance science, but also to win prestige and beat rivals from other countries.

  During the Second World War, Australia gave live platypuses to Allied nations as diplomatic gifts, despite an export ban. The aim was to encourage more military assistance. One animal intended for Winston Churchill died from neglect on the journey.

  In the Dreamtime stories of some Aboriginal Australians, the platypus was a hybrid of a duck and a water rat. One story from the upper Darling River region tells of Biggoon, a large water rat who kidnapped a duck that wandered too far from her tribe. She escaped and returned to lay two eggs, which hatched the first platypuses. They were all exiled to live in the mountains.

  Another upper Darling story has the land animals, water animals and birds all competing to win the platypus to their side. He refused every group, feeling he did not need to belong to one to be special. He wanted to stay friends with them all.

  The platypus also serves as a totem for some Aboriginal peoples, a natural object that a clan or family inherits as its spiritual emblem. For the Wadi Wadi people at the Murray River, it holds special meaning. Because of its significance and its link to country, these Indigenous peoples protect and conserve it. More widely, the platypus has come to represent Australian identity. It has appeared on stamps and currency, and it served as a mascot at the 2000 Summer Olympics in Sydney.

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