Duck-Billed Platypus: Ornithorhynchidae

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DUCK-BILLED PLATYPUS: Ornithorhynchidae

PHYSICAL CHARACTERISTICS

A platypus, at first glance, resembles an otter with a duck's bill on its face and a beaver's tail in back. An adult platypus, about the size of a house cat, weighs from 3 to 5 pounds (1.5 to 2.5 kilograms), its adult head and body length runs 12 to 18 inches (30 to 45 centimeters), and the tail adds another 4 to 6 inches (10 to 15 centimeters). Males are larger than females.

The snout, despite its duckbill shape, is soft, moist, and rubbery in texture, not hard like a bird's beak. The bill has an upper and lower section, like that of a mammal or bird, and the jaw hinging and motions are like those of mammals. The nostrils are set close together on the top of the upper bill.

The word "platypus" means "flat feet," referring to the animal's webbed, somewhat ducklike feet. The scientific name, Ornithorhynchus anatinus, means, in Latin, "bird-snout, resembling a duck." The plural is "platypuses" or just "platypus."

Most of the body is covered with fine, soft fur. The pelt color varies from dark amber to very dark brown on the platypus's back and sides, and from grayish white to yellowish brown on the underbelly. Platypus fur is fine, soft, and dense, with up to 900 hairs per square inch of skin. The fur has two layers, an undercoat with a woolly texture and an overcoat of coarser hair. As the platypus dives, the two fur layers trap a layer of air next to the skin, thus keeping the body dry and helping to insulate it against cold while the platypus swims, often throughout the night, and sometimes in temperatures close to freezing.

The body is somewhat flattened and streamlined. The limbs are short and muscular. As in other monotremes, the limbs of the platypus are set in a permanent push-up position, the upper limb bones extending out from the sides of the body, horizontal to the ground, the lower limb bones going straight down. Although an excellent swimmer, the platypus is clumsy when trying to walk on land, and seldom does so anyway, except within its tunnels, since it burns up twice as much body energy moving about on land as it does swimming underwater.

All four feet have five claws apiece and are webbed, but the webbing of the front feet extends in a flat flange beyond the toes when the platypus swims. Back on land or in its burrow, the animal folds the extra webbing under its forefeet and walks on its knuckles. The platypus uses the forelimbs and forefeet for swimming and digging, while using the hind feet and claws as combs to keep the fur clean and waterproof.

The eyes are small and the external ears are mere holes in the skull, although the internal structure of the ears is like that of other mammals. There are two long grooves for protecting the eyes and ears, a single groove surrounding both the eye and ear on each side. These grooves are closed underwater, shutting both eyes and ears, when the platypus dives to hunt for food. Out of water, the senses of sight and hearing are sharp.

Both hind limbs of the male bear hollow, pointed, poison spurs mounted on the insides of the ankles, just above the heels. There are venom glands, one in each thigh, called the "crural glands" because they are controlled by the crural nerves, which are major motor nerves of the hindlimbs. The glands secrete venom that is passed through ducts to the sharp spurs, which the platypus can erect like jacknife blades and stab into other animals.

Both sexes have the spurs when they are young. At four months of age, male spurs are protected by a covering of whitish, chalky material that sloughs off completely by the end of the first year of age. Females bear smaller, useless spurs, without venom, that they shed by ten months of age.

The platypus's flat, beaverlike tail is used as a swimming rudder, a shovel, for fat storage, and by the mother for keeping eggs and young warm. The tail can store up to fifty percent of a platypus's total load of body fat. Female platypus use the tail to carry leaves to the nesting chamber, and both sexes use it to sweep loosened soil out of the way when digging. The tail has no fine fur, only coarse, bristly hair on its upper surface to aid in carrying or sweeping.

GEOGRAPHIC RANGE

Platypus are found only in mainland Australia and the southern island of Tasmania. Platypus are distributed along Australia's east coast, to about 500 miles (800 kilometers) inland, from Cook-town, Queensland to Melbourne, Victoria, and into Tasmania.

HABITAT

All platypus live on the edges of freshwater bodies like lakes, ponds, rivers, and streams, in tropical and temperate regions.

DIET

The platypus eats small freshwater animals, which it hunts at night, underwater, with its eyes and ears closed. It finds and catches underwater creatures that are swimming or sunken in the bottom mud by tracking them down with its sensitive bill, which can detect electricity and motion.

Diet is varied, including adult and larval water insects, crayfish (called "yabbies" in Australia), fish, frogs, tadpoles, snails, spiders, freshwater mussels, worms, fish eggs, and unlucky insects that fall into the water from overhanging trees. Occasionally, platypus probe for food at the edge of the water, grubbing under rocks or among roots of plants. A platypus must eat one third to one half of its body weight in food every day.

BEHAVIOR AND REPRODUCTION

Platypus are either solitary, or a male and female may live together, sharing a burrow. Platypus build two types of burrows along the banks of creeks and ponds. One is a "camping burrows," an all-purpose shelter for male and female; the other is the "nesting burrow," built only by the female, and containing a breeding chamber, or room, for birthing and raising the young. Both sorts of burrows keep their entrances at, slightly above, or below water level, the entrance tunnel climbing at an angle a few feet above water level to prevent flooding of the burrow. The openings can be difficult to spot, since platypus prefer to build them as hidden as possible in sturdy, concave banks with reeds and other aquatic plants at the water's edge, and overhanging sod and tree roots.

A burrow's entrance tunnel is barely wide enough to allow the platypus to pass, so that when the animal emerges from water and forces itself through the entrance tunnel, water on the pelt is squeezed and sponged off, and the platypus's fur is dry when it enters the main tunnel. A platypus may have up to a dozen camping burrows strung along the banks of its territory, providing numerous nearby, safe havens. The animal rotates the burrows for shelter, staying at each a few days, probably to keep down its population of parasites.

A nesting burrow can be as long as 90 feet (30 meters), with two or more branching tunnels that circle about and eventually lead to the central nesting chamber.

Platypus normally hunt and feed at night, but have been seen doing so in the daytime. In the water, a platypus propels itself with powerful strokes of its forelimbs, the extended webbing adding extra push to the motions. It uses the hindlimbs and tail only for steering. As it swims, the platypus swings its head from side to side, allowing a full scan of its surroundings with its sensitive bill. The platypus feeds by snagging swimming creatures with its bill and by rousting them out of stream bottom mud and gravel, shoveling it up with its bill to put buried creatures to flight, then catching them as they try to escape.

Since the platypus must breathe air, it combines underwater hunting with trips to the surface to exhale and inhale. It will usually stay submerged for about a minute at a time, although it can stay submerged for up to five minutes. Platypus blood is especially rich in red cells and hemoglobin, the substance in blood that carries oxygen. The platypus can also ration its blood oxygen supply by reducing its heartbeat from two hundred beats per minute to ten beats per minute.

When not out hunting, a platypus rests in its burrow for up to seventeen hours a day. Platypus are active throughout the year, even in cooler southern Australia and Tasmania, where water temperature drops nearly to the freezing point. Individuals have been known to go into periods of torpor, or sluggishness and reduced activity with a lowering of body temperature, during the coldest months. Such a period, which can last up to six days, is not true hibernation but allows the animal to conserve energy in cold times.

Platypus are for the most part silent. Some naturalists have heard threatened platypus make soft, growling sounds that are only audible at close range. Lifespans for platypus in captivity and in the wild can reach sixteen years.

Platypus mate from August to October. Following an elaborate courtship ritual that includes the male holding on to the female's tail, and the pair swimming in slow circles, the two copulate in the water. Then the female tends to the nesting burrow and chamber, carrying wet leaves and moss with her folded tail for lining the chamber, to prevent the eggs from drying out. The female lays one to three eggs in the chamber two to four weeks after mating. A typical egg is slightly oval, about half an inch in diameter (13 millimeters), with a soft, leathery shell like a reptile's.

The mother incubates the eggs by holding them against her belly fur with her tail, maintaining a constant temperature of 90°F (31.5°C). The young hatch in about ten days, each tearing through the eggshell with a temporary egg tooth. The newly hatched, inch-long young are fragile and translucent, blind and furless, and at about the same stage of development as a newly born marsupial young.

SEEING ELECTRICITY AND PRESSURE?

The monotremes, the echidnas and platypus, are the only living mammals that are known to have an ability to sense electricity. The platypus bill is something unique in nature, so sophisticated and advanced that no one can call the platypus "primitive." The skin surface of the bill contains 40,000 tiny electroreceptors, or specialized sensory nerve endings, arranged in rows along the length of the bill. These detect tiny, underwater bursts of electricity from the muscles of swimming creatures that the platypus hunts. The electroreceptors are intermingled with 60,000 mechanoreceptors, nerves ending at the skin in tiny "push rods" that respond to small pressure changes and detect the movements of prey animals underwater and on the bottoms of streams and ponds. Together, the two senses allow the platypus to home in on prey.

In addition to detecting bursts of electricity from prey animals, the electroreceptors in a platypus's bill can probably detect the tiny electric currents made by water flowing over and around rocks and sunken logs, thus producing a three-dimensional map of a river or lake bottom within the platypus brain. In the platypus, the combined abilities of electroreception and mechanoreception are so sensitive and detailed that they have become something like vision, providing a three-dimensional "view" of the platypus's underwater world, and enabling the platypus to pinpoint, in all three dimensions, the exact locations of its prey.

The mother, having no nipples, nurses the young with milk that comes directly from her belly skin. In about four months, the young emerge for the first time from the burrow, each about a foot long and with a full coat of fur.

Predators of platypus, other than humans, include birds of prey such as hawks, eagles, and owls; Murray cod, a freshwater fish; and crocodiles. Carpet pythons, goanna lizards and rakali, or Australian water-rats, prey on young platypus in burrows. Carnivorous mammal species introduced by European settlers, including foxes, dogs, and cats, prey on platypus, although some of these predators are dealt painful ends by the poison spurs of male platypus.

PLATYPUS AND PEOPLE

The platypus, almost as much as the kangaroo, has become a national symbol of Australia and of the odd, weird, and outright bizarre creatures native to that continent and country. The platypus is a symbol, as well, for the unique, the quirky, and the unexpected in nature, which makes the animal and its behavior a subject of curiosity and science education.

Platypus were nearly wiped out by hunting, into the early twentieth century, for their fine, soft, waterproof fur. Nevertheless, humans, out of carelessness and ignorance, continue to make life miserable for the platypus. The animals become entangled in fishing hooks and lines, and in fishing nets; such encounters end in drowning or in the scarring of the bill. Tasmania's platypuses are being impacted by infection from an introduced fungus and by chemical pollutants.

Well-meaning people may try to rescue a platypus that is wandering and seems to be lost, a move that often proves harmful to people and platypus. A wild platypus captured by humans will probably die of shock. The rescuers may end up with days of pain and misery from a platypus sting. Wildlife education in Australia stresses leaving lost animals alone and calling a local office of the Australian Government Department of the Environment and Heritage so that someone can professionally capture and care for a lost platypus.

Recently, platypus have started invading human-made urban waterways in Melbourne, Victoria, while disappearing from some wild areas, for reasons still not understood. The urban platypus most likely have been forced into artificial waterways due to destruction of their habitat by development, and there is enough live platypus food in the waterways to feed a platypus population. The Australian Platypus Conservancy and the Melbourne Water Department together have surveyed and taken counts of the urban platypus populations. They found that platypus in the waterways were as healthy and well-fed as those in the wild, while some individual platypus from the waterways have migrated and re-colonized river banks with improved habitat.

CONSERVATION STATUS

Platypus are considered "common but vulnerable" by the government of Australia. It is plentiful in some areas, but is considered vulnerable due to habitat destruction from dams, irrigation projects, being caught in fish nets and lines, and water pollution.

Platypus are strictly protected by law and harsh penalties in Australia, which is agreeable with most, if not all, Australians, since the animals are not pests and are now national emblems. The Australian government and private groups like the Australian Platypus Conservancy keep close eyes on platypus populations and have proposed relocating some of the urban platypus to suitable natural areas where they have been driven from by development in the past.

FOR MORE INFORMATION

Books:

Augie, M. L. Platypus and Echidnas. Mosman, Australia: The Royal Zoological Society of New South Wales, 1992.

Moyal, Ann Mozley. Platypus: The Extraordinary Story of How a Curious Creature Baffled the World. Washington, D.C.: Smithsonian Institution Press, 2001.

Short, Joan. Platypus. New York: Mondo Publishing, 1997.

Periodicals:

Hughes, R.L. and L. S. Hall. "Early Development and Embryology of the Platypus." Philosophical Transactions of the Royal Society of London 347 (1998): 1101–1114.

Musser, A. M. "Evolution, Biogeography and Paleontology of the Ornithorhynchidae." Australian Mammalogy 20 (1998): 147–162.

Pettigrew, J.D., P. R. Manger, and S. L. B. Fine. "The Sensory World of the Platypus." Philosophical Transactions of the Royal Society of London 347 (1998): 1199–1210.

Siegel, J. M., P. R. Manger, R. Nienhuis, H. M. Fahringer, and J. D. Pettigrew. "Monotremes and the Evolution of Rapid Eye Movement Sleep." Philosophical Transactions of the Royal Society of London 347 (1998): 1147–1157.

Strahan, R. and D. E. Thomas. "Courtship of the Platypus, Ornithorhynchus anatinus." Australian Zoologist 18, 165–178. 1975.