Platypus

The Platypus is a unique Australian species. Along with echidnas, Platypuses are grouped in a separate order of mammals known as monotremes, which are distinguished from all other mammals because they lay eggs. When first discovered, the unusual look of a Platypus caused considerable confusion and doubt amongst European naturalists and scientists, many of whom believed that the animal was a fake.

Identification

Platypus is well adapted for semi-aquatic lifestyle. Its streamline body and a broad, flat tail are covered with dense waterproof fur, which provides excellent thermal insulation. The Platypus propels itself through the water by using its front, short, webbed limbs, and the partially-webbed hind feet act as rudders. Behind its distinctive bill are the grooves that house the ear openings and the eyes which close when the animal dives. The Platypus uses its tail for storage of fat reserves and the strong claws on its feet for burrowing and moving on land. In addition, males possess a horny spur on their ankles, which is connected to a venom gland in the upper leg, making the Platypus one of the few venomous mammals.

The skeleton of the Platypus is heavy and has several similarities to that of fossil and modern reptiles. These include pectoral girdles made of five bones, splayed legs and rudimentary ribs on the neck vertebrae.

Distinguishing features of a Platypus are: streamlined body with a bill and broad flat tail; short limbs with webbed feet; dense dark brown to reddish brown fur with light brown/silver underfur.

Habitat

Platypuses occur in freshwater systems from tropical rainforest lowlands and plateaus of far northern Queensland to cold, high altitudes of Tasmania and the Australian Alps. They feed in both slow-moving and rapid (riffle) parts of streams, but show preference to coarser bottom substrates, particularly cobbles and gravel. When not foraging, the Platypus spends most of the time in its burrow in the bank of the river, creek or a pond. At times, the individuals use rocky crevices and stream debris as shelters, or they burrow under the roots of vegetation near the stream. Hence, the ideal habitat for the species includes a river or a stream with earth banks and native vegetation that provides shading of the stream and cover near the bank. The presence of logs, twigs, and roots, as well as cobbled or gravel water substrate result in increased microinvertebrate fauna (a main food source), and the Platypus also tends to be more abundant in areas with pool-riffle sequences.

Distribution

Platypus is endemic to Australia and is dependent on rivers, streams and bodies of freshwater. It is present in eastern Queensland and New South Wales, eastern, central and southwestern Victoria and throughout Tasmania. The western limits of the range are poorly known. The species was once found in the Adelaide Hills and Mount Lofty Ranges of South Australia. Nowadays it is extinct from that state, except for the introduced population on the western end of Kangaroo Island. There is no evidence that the animal occurred naturally in Western Australia, despite several unsuccessful attempts to introduce it there. Within its current distribution, the occurrence of the Platypus is reasonably continuous in some, but discontinuous in other catchments.

Seasonality

Platypuses are active all year round, but mostly during twilight and in the night. During day, individuals shelter in a short burrow in bank. The activity patterns of these animals are determined by a number of factors including: locality, human activity, ambient temperatures, day length and food availability.

Feeding and diet

The Platypus feeds mainly during the night on a wide variety of aquatic invertebrates. The average foraging periods last for 10-12 hours per day, and the distances the animals move during this time vary between individuals and their distribution. The animal closes its eyes, ears and nostrils when foraging underwater and its primary sense organ is the bill, equipped with receptors sensitive to pressure, and with electro-receptors. The precise way in which the Platypus uses the bill to detect prey is still unknown, but the bill serves to find and sift small prey from the substrate, while larger prey is taken individually. The Platypus stays underwater for between 30-140 seconds, collecting the invertebrates from the river bottom and storing them in its cheek-pouches. It then chews the food using its horny, grinding plates, while it floats and rests on the water surface.

Diet of the Platypus consists mainly of the benthic invertebrates, particularly the insect larvae. The species also feeds on free-swimming organisms: shrimps, swimming beetles, water bugs and tadpoles, and at times worms, freshwater pea mussels and snails. Occasionally the animals catch cicadas and moths from the water surface. In captivity, the Platypuses are often fed freshwater crayfish (Yabbies).

Other behaviours and adaptations

When swimming, the Platypus presents a low profile, with three small humps (the head, back and tail) visible above the water surface. The swimming action is smooth, and when the Platypus dives the back is arched as the animal plunges underwater, creating a spreading ring. These characteristics coupled with the absence of visible ears distinguish the Platypus from the dog-paddle style of the Water-rat.

Platypuses can swim through fast waters at the speed of around 1 metre per second, but when foraging the speed is closer to 0.4 metres per second. However, the Platpus is not well adapted for walking on land. The limbs are short, heavy and splayed away from the body, and a Platypus uses almost 30% more energy when moving on land, compared to a terrestrial mammal of similar size.

Communication

The Platypus is largely a solitary animal, but several individuals can share the same body of water. The vocalisation has not been recorded in the wild, but captive animals produce a low-pitched growling sounds when disturbed or handled.

Life history cycle

Young Platypuses do not seem to reproduce in their first year of life, instead, both sexes become reproductive in their second year. Still, many females do not breed until they are at least 4 years old. After mating, a female will lay 1-3 eggs (usually 2) following a 21-days gestation period. She then incubates the eggs for possibly 10 days, after which the lactation period lasts for 3-4 months before the young emerge from the burrow. Platypuses are long-lived animals both in captivity and in the wild, living up to approximately 20 years.

Breeding behaviours

The breeding season of the Platypus varies with distribution and within populations. Studies suggest that breeding occurs earliest in Queensland, followed by New South Wales, Victoria and Tasmania. Mating normally takes place between August to October in New South Wales and Victoria, and lactating females were observed between September and March.

The knowledge of the breeding behaviour generally comes from observations of animals in captivity. In winter (when the water is still cold) males initiate mating interactions. Courtship includes aquatic activities such as: rolling sideways together, diving, touching and passing, and the male is also observed grasping a female’s tail with its bill. The behaviour last from less than a minute to over half an hour and is usually repeated over several days.

After mating, a pregnant female builds a nest in a long complex burrow (possibly re-worked by several females in different seasons) in less than a week. She spends further 4-5 days collecting wet nesting material to prevent her eggs and hatchlings from drying out. During the egg incubation period, a female holds the eggs pressed by her tail to her belly, while curled up. She intermittently leaves the burrow, however, much of this aspect of the animal’s life is still unknown. When the young hatch, the female starts secreting milk and the young Platypuses suckle from the two milk patches covered by fur on the female’s abdomen. The female spends most of this time with her young in the burrow, and as the young grow, she increasingly leaves them to forage. Towards the end of the summer the young emerge from the burrow and their fate as young independent animals is still largely unknown.

Conservation status

The Platypus is protected by legislation in all of the states that it occurs in. Individuals cannot be captured or killed, except for scientific research. The Platypus is a common species with very little apparent change in its historical distribution (except in South Australia). However, there is a general lack of knowledge in the species abundance at local catchment levels to predict population trends. The dependence of Platypuses on the established freshwater systems may lead to their decline in future.

Under IUCN the Platypus has been listed as Near Threatened species (year assessed 2014).

Predators

Platypuses spend most of their time in water or their burrow, so it is difficult to determine their predators. There have been anecdotal reports of the species being predated on by crocodiles, goannas, carpet pythons, eagles and large native fish. In addition, it is likely that foxes, and possibly dogs or dingoes kill Platypuses that move on land or in shallow waters.

Platypuses have a number of ectoparasites in the wild, including their own tick species, Ixodes ornithothynchi. The tick is often found around the hind limbs, and in smaller numbers on the front legs and in the body fur. Severe skin ulcers caused by the amphibian fungal infection have been reported in Tasmanian Platypuses in particular. The fungus can be fatal to the animal if it invades other tissues, particularly the lungs.

Danger to humans

Male Platypuses have a calcaneous, sharp spur about 12 millimetres long on each ankle. The spur is connected via a long duct to a gland that produces venom, particularly in the breeding season. The venom can cause severe pain to humans, and although not lethal, the pain caused has been described as excruciating. Swelling rapidly develops around the wound and gradually spreads throughout the affected limb. Information obtained from case histories and anecdotal evidence indicates that the pain develops into a long-lasting hyperalgesia (temporary increased sensitivity to pain) that persists for days or even months. Therefore, if there is a need to handle a Platypus (helping an injured animal for instance), it should always be picked up by the end half of the tail to avoid the spur in case it is a male.

Fossils description

The fossil record for monotremes is poor in comparison to that of other groups of mammals, and until recently little was known about their evolutionary history. Several fossil discoveries since the early 1970s have shed some light on the origins of monotremes. We now know that monotremes were present in Australia during the Mesozoic Era, when Australia was still part of the supercontinent, Gondwana. The fossil evidence suggests that monotremes originated and diversified in the Australian/Antarctic section of Gondwana, and that there was only a single dispersal to South America before the break up of Gondwana.

Four species related to Platypus have been found in fossil deposits from Australia, including a complete skull of Obdurodon dicksoni and an opalised jaw fragment of Steropodon galmani. The latter is 110 million years old and represents one of Australia's oldest mammals. The only evidence that Platypus ancestors were once present outside Australia came in 1991, when a 61-63 million year old fossil tooth was found in Patagonia, in southern Argentina.

Studies of these fossils indicate that the one remaining living species of Platypus is more specialised than its predecessors. It is smaller, its functional teeth have been replaced by horny pads and other aspects of its anatomy appear simpler. It also appears to have a more restricted distribution, being confined to the river systems of eastern Australia. Although Platypus remains widespread and reasonably common, this trend towards increasing specialisation suggests that it may be moving out onto an evolutionary 'limb' and that its current status should not be taken for granted.

References

Grant, T. (2007) Platypus. Fourth Edition. Australian Natural History Series, University of New South Wales Press, Sydney. Pp.: vii+159.

Menkhorst, P. (2001). A Field Guide to the Mammals of Australia. Oxford University Press, Melbourne. Pp.:44.

van Dyck, S. and Strahan, R. (eds) (2008) The Mammals of Australia. Third edition. Reed New Holland, Sydney. Pp.: 32-35.