Duck-Billed Platypus (Ornithorhynchidae)

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Duck-billed platypus

(Ornithorhynchidae)

Class Mammalia

Order Monotremata

Family Ornithorhynchidae

Thumbnail description
Amphibious predator in freshwater habitats, characterized by a broad tail, flat head and body, short limbs adapted to digging and swimming, and conspicuous duck-like bill

Size
16–24 in (0.4–0.6 m); 1.5–6.6 lb (0.7–3 kg)

Number of genera, species
1 genus; 1 species

Habitat
Rivers, lakes, and streams

Conservation status
Not threatened

Distribution
Eastern Australia, including Tasmania

Evolution and systematics

The family Ornithorhynchidae includes just one modern species, the duck-billed platypus. No subspecies or races are known to occur. Several extinct ornithorhynchid species have been described, mainly from fossils found in Australia. At least one type of ancient platypus is also known to have lived in the Patagonian region of South America some 61–63 million years ago (mya), when South America was still physically joined to Australia as part of the giant southern supercontinent, Gondwana. The nearest living relatives are the echidnas (family Tachyglossidae). Based on genetic evidence, it is believed that the platypus and echidna lines have been evolving separately since the late Cretaceous or early Tertiary periods, 63–78 mya.

The taxonomy of this species is Ornithorynchus anatinus (Shaw, 1799), New Holland (Sydney), New South Wales, Australia.

Physical characteristics

The platypus has a flattened, streamlined head and body, well suited to its aquatic lifestyle. The animal's color pattern ensures that the platypus blends in with its watery environment when viewed from either above or below. The fur is dark brown above (apart from a small light-colored spot just in front of each eye) while the chest and belly are silvery cream, sometimes marked with a tawny or reddish streak running along the animal's midline. Interestingly, the platypus relies almost exclusively on its front limbs to propel itself through the water. The end of each front foot is equipped with a broad expanse of webbing, forming a highly effective paddle when the animal swims and dives. In contrast, the back feet are only moderately webbed and mainly used for grooming the fur.

Its most striking feature is undoubtedly its bill. This structure is superficially duck-like—so much so that George Shaw, the first professional zoologist to examine a platypus (a dried skin arrived in England in 1799), felt compelled to probe at the line where the bill joins the rest of the head to see if the specimen had been forged by a clever taxidermist. While the animal's bill may look like a duck's beak, it is actually more like a human thumb in terms of its physical attributes and the way it is used. Like a thumb, the platypus's bill is fleshy and covered by soft, sensitive skin, and is used by the animal to provide essential information about the surrounding environment as well as grab and hold objects.

The platypus is also remarkable in being one of the few mammals known to be poisonous. From a gland (the crural gland) located in the upper thigh, adult males secrete venom, which runs through a duct to a hollow, pointed spur (measuring 0.5–0.8 in [1.2–2 cm] in length) located on the ankle of each hind leg. Platypus venom is produced most abundantly just before and during the annual breeding season, suggesting that it has mainly evolved to help adult males compete for mates. While platypus venom is not considered to be life-threatening to humans, it can cause excruciating pain for a number of days after a person is spurred.

The platypus is a relatively small animal; males are typically 15–20% longer and weigh 60–90% more than females at any given locality. The largest animals measure about 24 in (0.6 m) in total length and weigh 6.6 lb (3 kg). To help reduce its rate of heat loss in the water, a healthy platypus maintains its body temperature at around 89–90°F (32°C), which is about 9°F (5°C) cooler than that of humans. Additionally, there are two layers of fur: a dense, wooly undercoat covered by longer, coarser, waterproof guard hairs. The undercoat traps a layer of air next to the body when a platypus is in the water, helping to keep the animal warm even in freezing winter conditions.

Distribution

The platypus inhabits waterways along the eastern and southeastern coast of mainland Australia (to about as far north as Cooktown, Queensland), and on Tasmania and King Island. An introduced population is also found on Kangaroo Island in South Australia, where 19 individuals were released in Flinders Chase National Park in the years between 1928 and 1946. The absence of platypus populations in central and western Australia reflects the rarity of permanent lakes or rivers in these areas, while predation by crocodiles may plausibly limit its distribution at the northern end of its range.

Habitat

The platypus occupies a wide range of freshwater habitats, including ponds, lakes, rivers, and streams at all elevations. The animals are not adapted to feed on dry land, and so are most commonly found in permanent water bodies. The species will also use humanmade reservoirs as long as the water is not too deep, mainly feeding at a depth of less than 16.5 ft (5 m). The platypus is known to occur in both urban and agricultural areas. The animals are also occasionally seen in river estuaries, though there is no evidence that they occupy saltwater habitats on a permanent basis.

Behavior

Direct observational studies of platypus behavior are exceedingly difficult to undertake: the animals are active mainly at night, and spend most of their time either feeding underwater or resting in underground burrows. Accordingly, much of what is known about the species' movements, habits, and activity patterns has been gained through radio-tracking studies. By fitting animals with special miniature radio-transmitter tags, their location and behavior can be monitored in a consistent manner both during the day and at night.

The platypus is essentially solitary in its habits, though three or four animals may occasionally be found foraging within a few dozen yards (meters) of each other at a spot where food is abundant. Animals residing along a stream or river typically have a home range comprising 0.5–6 mi (1–10 km) of channel. Home-range size varies with an individual's sex

(males have bigger home ranges than females) and habitat productivity. Home-range size shrinks as waterways support more of the small organisms eaten by the platypus, presumably because the animals do not have to travel as far to find enough food.

When a platypus is not feeding, it spends up to 17 hours a day resting in a dry, snug burrow located in a bank at the edge of the water. The animals' front toes are tipped with stout claws, and observations made in captivity have shown that a platypus is capable of digging a new burrow at the rate of around 1.5 ft (0.5 m) per hour. An adult without dependent offspring normally occupies several different burrows (up to about a dozen) within a period of a few weeks. By having numerous burrows scattered along the length of its home range, a platypus is always reasonably close to a safe refuge while feeding.

The platypus rarely vocalizes but, when feeling threatened, the animal can produce a high-pitched growl.

Feeding ecology and diet

The platypus is a predator, mainly feeding on bottom-dwelling aquatic insects such as caddis-fly and mayfly larvae. The platypus is also partial to worms, snails, freshwater shrimps and crayfish, and pea-shell mussels. The size of its prey is limited by the fact that platypus teeth are lost quite early in development and replaced by flat, molar-like grinding pads at the back of the mouth. Unlike most mammalian teeth, these pads grow constantly to compensate for surface wear.

A platypus may find food by digging under banks or snapping up morsels floating on the water surface, as well as searching along the bottom sediments. Small prey is stored temporarily in cheek pouches while an animal is submerged. A foraging platypus typically remains underwater for 10–60

seconds before returning to the surface to breathe and chew its meal with a side-to-side motion of the jaws.

Its eyes and ears are located within shallow, muscular grooves on the sides of the head that automatically pinch shut when an animal dives. The platypus mainly relies on its bill to find food underwater. The surface of this remarkable organ is densely packed with tens of thousands of specialized sensory receptors, sensitive to either touch and vibration (push rods) or electrical currents (mucous sensory glands). It has been shown experimentally that the platypus is capable of registering the tiny amount of electricity generated in the water by the tail flick of a shrimp at a distance of around 2 in (5 cm).

In turn, this information presumably is used to detect as well as track down the location of prey.

Reproductive biology

Together with the echidnas, the platypus is distinguished as a monotreme, or egg-laying mammal. Males and females have a single physical opening (the cloaca) that is used both for reproduction and excretion. To help maintain a streamlined shape, the male's penis and testes are carried inside the body; mating occurs in the water. In the female platypus, the right ovary is small and nonfunctional. At the time of ovulation, the platypus egg is about 0.16 in (4 mm) in diameter. After being fertilized, the first of three shell layers is formed in the fallopian tube before the egg moves into the uterus. There, the egg is supplied with additional nutrients and two more shell layers are secreted, so the egg is about 0.6 in (15 mm) in diameter when it is laid. Though the time required for gestation has never been determined precisely, it is believed that it takes around three weeks.

Platypus eggs are produced in late winter and spring (August–November), with some evidence that breeding occurs later in southern populations as compared to those found in Queensland. The eggs are laid in a burrow typically measuring 10–20 ft (3–6 m) in length, though sometimes much longer. Throughout incubation and juvenile development, the female keeps the burrow's entry tunnel blocked by several plugs of soil. Besides discouraging access by predators such as snakes and Australian water rats (Hydromys chrysogaster), the plugs reduce the likelihood that juveniles drown in the event of a flood. A few days before laying her clutch of one to three eggs, a female drags a large quantity of wet leaves and other vegetation into the rounded burrow chamber to make a nest. It is believed that the female incubates the leathery-shelled eggs for about 10 days, clasping them between her curled-up tail and belly as she lies on her back or side. When they hatch, the young are less than 0.5 in (9 mm) long. Their emergence from the egg is assisted by the presence of a prominent bump (caruncle) at the tip of the snout, an inward-curving egg tooth, and forelimbs armed with tiny claws. When they hatch, the young are less than 0.8 in (20 mm) long.

Juveniles develop in the nursery burrow for about four months before entering the water for the first time. Throughout this period, they are nourished solely on milk. The female does not have nipples. Instead, milk is secreted directly onto the mother's fur from two circular patches of skin located about halfway along her abdomen. An orphaned platypus will drink milk from a human hand by sucking up the liquid while sweeping its short bill rhythmically back and forth against the palm. In the wild, such sweeping movements may help to stimulate the flow of milk.

Both males and females are physically mature at the age of two years, though some females may delay having offspring until they are four years old or more. Courtship involves two individuals swimming alongside or circling each other, sometimes accompanied by nuzzling or rubbing. One animal may use its bill to grasp the tip of the other's tail and be towed or swim behind. Little is known about the platypus breeding system, apart from the fact that the animals do not appear to form long-term pair bonds. Instead, it is believed that males tend to move about widely during the breeding season, trying to mate with as many females as possible. By the same token, adult females appear to rear the young without any help from their mates.

Conservation status

The platypus is a difficult animal to census or survey: burrow entrances are generally well hidden and the animals rarely leave evidence of their activities in the forms of tracks, scats, or food scraps. Live-trapping nets are time consuming to set and must be monitored closely through the night. Accordingly, knowledge of how the species is faring is sketchy in many parts of its range. In broad terms, it is known that the platypus remains fairly common along some waterways, but

has declined or vanished from others. The species is fully protected by law throughout its range.

In most areas, the key factor limiting platypus numbers is likely to be the quality of habitat. Waterways supporting large platypus populations generally have plenty of trees and smaller plants growing on the banks; a varied array of pools, shallow riffles, rocks, and woody materials in the channel; and reliably flowing fresh water throughout the year. All of these attributes favor the small aquatic invertebrates that in turn feed the platypus. Conversely, factors implicated in the decline of platypus populations include erosion, overgrazing by rabbits and livestock, altered water flow regimes, overclearing of native vegetation, and the systematic removal of logs and large branches from the channel.

Besides habitat degradation, the platypus is vulnerable to drowning in nets and traps set illegally for fish and crayfish. Many individuals also die each year after becoming

entangled in garbage such as abandoned loops of nylon fishing line.

Significance to humans

The platypus was hunted in the nineteenth and early twentieth centuries for its soft fur, which (due to the thickness of the skin) was mainly used to make slippers, blankets, and rugs. Today, the animals have no direct economic value apart from their role in attracting tourists to Australia. People living in Australia generally regard the species with great interest and affection. Hence, the platypus also has an important role to play in encouraging landholders and the general community to protect freshwater environments.

Resources

Books

Augee, Michael L., ed. Platypus and Echidnas. Mosman, New South Wales: The Royal Zoological Society of New South Wales, 1992.

Grant, Tom. The Platypus: A Unique Mammal. Sydney: University of New South Wales Press, 1995.

Moyal, Ann. Platypus: The Extraordinary Story of How a Curious Creature Baffled the World. Crows Nest, New South Wales: Allen & Unwin, 2001.

Periodicals

Evans, B. K., D. R. Jones, J. Baldwin, and G. R. J. Gabbott. "Diving Ability of the Platypus." Australian Journal of Zoology 42 (1994): 17–27.

Fenner, P. J., J. A. Williamson, and D. Myers. "Platypus Envenomation—A Painful Learning Experience." The Medical Journal of Australia 157 (1992): 829–832.

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

Proske, U., J. E. Gregory, and A. Iggo. "Sensory Receptors in Monotremes." Royal Siety of London Philosophical Transactions 353 (1998): 1187–1198.

Serena, M. "Use of Time and Space by Platypus (Ornithorhynchus anatinus: Monotremata) along a Victorian Stream." Journal of Zoology, London 232 (1994): 117–131.

—— "Duck-billed Platypus: Australia's Urban Oddity." National Geographic 197, no. 4 (April 2000): 118–129.

Serena, M., and G. Williams. "Rubber and Plastic Rubbish: A Summary of the Hazard Posed to Platypus Ornithorhynchus anatinus in Suburban Habitats." The Victorian Naturalist 115 (1998): 47–49.

Serena, M., M. Worley, M. Swinnerton, and G. A. Williams. "Effect of Food Availability and Habitat on the Distribution of Platypus (Ornithorhynchus anatinus) Foraging Activity." Australian Journal of Zoology 49 (2001): 263–277.

Temple-Smith, P., and T. Grant. "Uncertain Breeding: A Short History of Reproduction in Monotremes." Reproduction, Fertility and Development 13 (2001): 487–497.

Organizations

Australian Platypus Conservancy. P.O. Box 84, Whittlesea, Victoria 3757 Australia. Phone: 613 9716 1626. Fax: 613 9716 1664. E-mail: [email protected] Web site: <http://www.platypus.asn.au>.