Matters of Life and Death

Size and appearance

Like many other animals, platypus living at the warm end of their range in Queensland is generally somewhat smaller than those found at the cold end of their range in Victoria and Tasmania. The largest platypus recorded to date (in Tasmania) was a male weighing 3.0 kilograms and measuring 0.6 metres in length. On the mainland, adult males typically weigh 1.2-2.4 kilograms and are on average 0.5 metres long, whereas adult females typically weigh 0.7-1.6 kilograms and are on average 0.4 metres long.

The platypus’s general appearance is quite distinctive, combining a streamlined, furry body with a broad, paddle-shaped tail, four short legs, and a superficially ducklike bill. To help reduce drag in the water, the male’s testes and penis are normally held within the body. A platypus also lacks ear flaps (or pinnae): the ear and eye are both located in a muscular groove placed at the side of the head, which automatically pinches shut when an animal dives to protect the eye and ear underwater.

The platypus’s body is covered by dense fur apart from its bill, feet and tail. The bill is covered by smooth skin with a soft, suede-like texture and (unlike a duck’s bill) is quite pliable and fleshy around the edges. The upper surface of the tail is covered by coarse hairs which can stand up to the wear and tear involved in using the tail to help push aside and tamp down soil when a platypus digs or remodels a burrow (see 1.3.5, Burrows ). In contrast, the tail’s lower surface is covered by short, fine hairs which are replaced each year when an animal moults but then wear away as the tail is dragged over stones and gravel, leaving the tail mostly bald below.

Across their entire range, platypus is dark brown above (except for a small patch of light-coloured fur located next to each eye) and creamy white below (sometimes tinged rusty-red). When the platypus’s eyes are closed underwater, the light-coloured patches give the appearance of eyes remaining open, presumably fooling predators into being less likely to attack.

The upper surface of the bill is uniformly dark grey, with two nostrils located near the tip. The bill’s lower surface can either be uniformly pigmented or quite mottled.

The platypus’s front foot is furnished with a broad expanse of skin which extends past the front claws to form a large and efficient paddle (below left). The webbing folds under the foot when out of the water, making it easier for a platypus to walk and use the sturdy front claws to dig burrows. The hind feet are used to help change direction and maintain balance in the water. In addition, they are equipped with sharp, curved claws used to groom the fur (below right).
size and appearance     size and appearance

Further reading:
Connolly, J.H. and Obendorf, D.L. (1998). Distribution, captures and physical characteristics of the platypus (Ornithorhynchus anatinus) in Tasmania. Australian Mammalogy 20: 231-237.
Grant, T.R. and Temple-Smith, P.D. (1983). Size, seasonal weight change and growth in platypuses, Ornithorhynchus anatinus (Monotremata: Ornithorhynchidae), from rivers and lakes of New South Wales. Australian Mammalogy 6: 51-60.

Sensory systems

Vision. The platypus’s eye is small (6 millimetres in diameter) and equipped with a round pupil. The internal structure of the eye is typically mammalian in most respects but includes some reptilian features, such as the presence of double (as opposed to single) retinal cone cells used to perceive colour. The lens resembles those found in otters and sea lions, in being fairly flat at the front and much more curved at the back. This shape reduces the eye’s ability to see fine detail, but improves underwater vision.

Interestingly, the platypus rarely uses sight when submerged - its eyes normally close automatically as soon as it dives. One possible explanation is that the platypus’s ancestors relied on underwater vision more extensively than the modern species does.

Hearing. Platypus ears resemble those of other mammals in most respects but do have a few primitive features, such as the fact that the ear region is encased in cartilage rather than bone. The platypus ear is most sensitive to sound frequencies around 4 kilohertz (exactly the same as in humans) but can hear frequencies as high as 15 kilohertz.

Smell and taste. Aquatic mammals typically don’t rely much on smell to find food or detect predators, as chemical cues tend to be washed away by water. In the case of the platypus, only about half as many genes are linked to standard odour detection as compared to most land-based mammals. However, scientists have been intrigued to learn that the platypus has an exceptionally large number of genes coding for specialised smell receptors in the vomeronasal (or Jacobson’s) organ – paired pouch-like structures located in the roof of the mouth. Vomeronasal organs are found in both reptiles and mammals and are mainly important in social communication – detecting odours produced by other individuals of the same species. Accordingly, these receptors are likely to be used by a platypus to track chemical signs left to mark territorial boundaries or advertise reproductive status, though it’s possible they may also help the platypus find its prey underwater.

The platypus has two grooves at the back of the tongue which are lined with sensory papillae (tiny projections) which are believed to be used to taste food.

Bill sensory receptors. The skin of a platypus bill holds tens of thousands of specialised sensory structures providing information needed to navigate underwater and capture prey. Receptors known as “push rods” are sensitive to touch or pressure, either as an outcome of solid objects contacting the skin or water movement. Nerves are activated when the tip of a push rod receptor is displaced by as little as 20 microns (0.00002 metres), which means a platypus can detect the movements of edible invertebrates such as freshwater shrimp or crayfish at a distance of 15-20 centimetres, simply by sensing the associated movement of water.

The bill surface is also thickly dotted with acutely sensitive electroreceptors (“sensory mucous glands”), which respond to the tiny amount of electricity generated when the muscles of aquatic invertebrates contract. Because electricity moves so rapidly through water, the tail flick of a shrimp will be recorded a fraction of an instant earlier by bill electroreceptors as compared to push rods, providing a way for a platypus to judge the distance to a prey item.

Further reading:
Pettigrew, J.D., Manger, P.R. and Fine, S.L.B. (1998). The sensory world of the platypus. Philosophical Transactions of the Royal Society of London, Biological Sciences 353: 199-1210.
Proske, U., Gregory, J.E. and Iggo, A. (1998). Sensory receptors in monotremes. Philosophical Transactions of the Royal Society of London, Biological Sciences 353: 1187-1198.
Scheich, H., Langner, G., Tidemann, C., Coles, R.B. and Guppy, A. (1986). Electroreception and electrolocation in platypus. Nature 319: 401-402.

Body temperature and torpor

The platypus normally maintains a body temperature close to 32°C. This is a bit lower than the body temperature of most other mammals – for example, the temperature of a healthy human is usually about 37°C. The platypus’s relatively low body temperature is believed to be an adaptation to conserve energy, particularly when an animal is swimming in cold water.

To further reduce heat loss, platypus fur is made up of two layers: an extremely dense undercoat (including up to 900 individual hairs per square millimetre of skin surface) and coarser overlying guard hairs. These layers work together to trap air next to the platypus’s skin when an animal enters the water, so most of the body surface actually remains dry. The combined insulation value of the fur and air layer has been estimated to be similar to a three millimetre layer of neoprene wetsuit material.

Secondly, the platypus has a special network of small intertwined veins and arteries in the pelvic region (known to scientists as a rete mirabile or literally “miraculous network”). This network serves as a counter current heat exchange system: cooled blood returning to the heart from the animal’s legs and tail absorbs some warmth from blood being pumped from the chest, reducing the overall loss of body heat to the environment.

One disadvantage of being so well adapted to surviving cold conditions is that the platypus has a propensity to overheat: in captivity, animals become “noticeably lethargic” when the water in display tanks exceeds 29°C, and a platypus has reportedly lost consciousness after being exposed to an air temperature of 35°C for 17 minutes. Overheating is not normally a problem for platypus in the wild, as they prefer to spend their time either immersed in substantial bodies of water or resting in burrows, where average air temperatures typically do not exceed 18-20°C even in summer. However, it does mean that platypus is likely to overheat badly if they try to travel long distances across land in summer, for example to find new feeding sites during a drought.

Observations in both captivity and along a small stream in Victoria suggest that platypus may periodically enter a state of torpor in which the animals allow their body temperature to drop, remaining inactive for up to about six days. This behaviour has only been recorded in the colder months of the year (late May to early September). Interestingly, no records of inactivity have been recorded in the course of platypus radio-tracking studies undertaken in winter along two rivers in New South Wales or a sub-alpine lake in Tasmania, suggesting that low ambient temperatures are necessary but not sufficient to trigger torpid behaviour in this species.

Further reading:
Grant, T.R. and Dawson, T.J. (1978). Temperature regulation in the platypus, Ornithorhynchus anatinus: maintenance of body temperature in air and water. Physiological Zoology 51: 1-6.
Grant, T.R. and Dawson, T.J. (1978). Temperature regulation in the platypus, Ornithorhynchus anatinus: production and loss of metabolic heat in air and water. Physiological Zoology 51: 315-332.
Grigg, G., Beard, L., Grant, T. and Augee, M. (1992). Body temperature and diurnal activity patterns in the platypus (Ornithorhynchus anatinus) during winter. Australian Journal of Zoology 40: 135-142.

Reproduction and life history

The platypus is a monotreme, or egg-laying mammal. Males and females have a single physical opening (known as the cloaca) which is used both for reproduction and excretion.

Platypus have been observed mating in the wild in Victoria and New South Wales from early August to early November, with animals believed to breed a few weeks earlier in Queensland and a few weeks later in Tasmania. The animals do not appear to form lasting pair bonds: males probably court as many females as possible, and females rear their young without any assistance from their mates. Based on observations made in captivity, a female becomes receptive to males for a period of 4-6 days. Afterwards, she digs or renovates a nesting burrow and then spends 2-5 days collecting vegetation from the water (leaves, grass, bark strips, etc.) to line the nest. It is believed that wet nesting material is required to help keep platypus eggs and newly hatched young from drying out.

A clutch of 1-3 whitish, leathery-shelled eggs (like those of lizards and snakes) is laid approximately 2-3 weeks after mating. The eggs are incubated underground for around 10 days, clasped between a female’s curled-up tail and belly as she lies on her back or side. The eggs are about 15 millimetres in diameter, and the young are correspondingly small when they hatch (about 9 millimetres in length). Their exit from the egg is assisted by a prominent bump (or caruncle) at the end of the snout, an inwardly curving egg tooth and tiny claws on the front feet.

After hatching, juveniles (there is no well-established special term for a baby platypus) develop in the nesting burrow for about 3-4 months before entering the water for the first time. Throughout this period, they are nourished only on milk. A female platypus does not have nipples. Instead, milk is secreted directly onto her belly fur from two round patches of skin. Platypus milk is thick and rich, containing on average about 39% solids (as compared to 12% solids in cow milk). The average fat content of platypus milk (22%) is about six times greater than that of cow milk, while its protein content (8%) is more than double the average value for cow milk.

The newly emerged juveniles are fully furred, well co-ordinated and about 80% of their adult length. They apparently are not taught to swim or how to feed by their mother, but have to learn by them through trial and error.

Males and females both become mature at the age of two years. However, some females may not produce young until they are four years old or more, with a long-term study carried out by Dr Tom Grant along the Shoalhaven River in New South Wales indicating that less than half of females breed on average in a given year (range = 18-80% over 27 years).

The juvenile mortality rate generally appears to be high, with only a small proportion of young platypus surviving to adulthood. However, it is not uncommon for adults to live for a decade or more. The oldest known platypus (a female) survived to the age of at least 21 years in the wild.

Further reading:
Grant, T.R., Griffiths, M. and Temple-Smith, P.D. (2004). Breeding in a free-ranging population of platypuses, Ornithorhynchus anatinus, in the upper Shoalhaven River, New South Wales – a 27 year study. Proceedings of the Linnean Society of New South Wales 125: 227-234.
Griffiths, M., Green, B., Leckie, R.M.C., Messer, M. and Newgrain, K.W. (1984). Constituents of platypus and echidna milk, with particular reference to the fatty acid complement of the triglycerides. Australian Journal of Biological Sciences 37: 323-329.
Hawkins, M. and Battaglia, A. (2009). Breeding behaviour of the platypus (Ornithorhynchus anatinus) in captivity. Australian Journal of Zoology 57: 283-293.

Venom and spurs

Spurs and venom. The male platypus has a conspicuous spur (similar in size and shape to a dog’s canine tooth) located on the inner hind ankles (right). Adult spurs are typically 12-18 millimetres long and made of keratin, the structural protein found in feathers and human fingernails. The spur is connected to a venom-secreting gland, known as the crural gland. Platypus venom is first produced when a male becomes mature, and more venom is secreted during the spring breeding season than at other times of year. Accordingly, it is believed that platypus spurs and venom have mainly evolved to help adult males compete for mates.

Platypus venom is a clear, slightly sticky fluid. It contains at least 19 different compounds which appear to have evolved quite independently from those found in snake venoms. Platypus venom is not life-threatening to humans, but can cause severe localised swelling and excruciating pain which gradually abates over a period of a few weeks. At its worst, the pain is not very effectively relieved by standard analgesics such as morphine and is only made worse by application of ice packs. However, it can be treated successfully with drugs such as bupivacaine, which act by blocking nerve transmission.
Venom and Suprs
Venom leaking from the tip of an adult platypus spur.

Platypus spurs are normally held in a relaxed position, folded back against the inner ankle. Particularly during the breeding season, a spurring response will be initiated if the male is touched or stroked on its abdomen in the area between the hind legs. The hind feet are rapidly rotated outwards and upwards, pulling each spur erect and locking it into position against the lower limb bones. Both spurs are then jabbed inwards with great force, impaling any object in their path from two directions.

Although platypus are not particularly aggressive animals, great care should be taken whenever picking up either an adult male or an individual of unknown age and sex. In particular, such an animal should NEVER be supported from below. Instead, grasp the animal firmly by the END half of the tail (which cannot be reached by the spurs) before lifting it up and transferring it to a cloth bag, lidded box or other secure container.
Venom and Suprs

When holding a platypus by the tail, it should be easy to determine if the animal is a male (based on the presence of conspicuous spurs on the ankles).

The appearance of male spurs changes with age. In the case of young juveniles, spurs are relatively short and stubby and covered in a sheath of whitish keratin. This covering gradually wears away, exposing the true spur which continues to grow. The spurs of subadult (second year) males can normally be distinguished from older individuals by the presence of a pink collar of skin which initially extends about one-third up the length of the spur. The collar skin gradually regresses and is very much reduced by the time that males mature at the age of two years.

Venom and Suprs     Venom and Suprs
Examples of a juvenile male spur (left) and subadult male spur (right)

Adult females of any age are easily told apart from males because they do not possess true spurs. However, juvenile females do have a tiny pointed brown or whitish “spur” – typically 1-2 millimetres in length – on their hind ankles. This structure generally disappears within about 8-10 months of a young female’s emergence from a nursery burrow, leaving behind a small pit in the skin.

Venom and Suprs     Venom and Suprs
Juvenile females have a tiny false spur (left, circled) which is lost by the time a female is one year old (right).

Further reading:
Fenner, P.J., Williamson, J.A. and Myers, D. (1992). Platypus envenomation – a painful learning experience. The Medical Journal of Australia 157: 829-832.
Koh, J.M.S., Bansal, P.S., Torres, A.M. and Kuchel, P.W. (2009). Platypus venom: source of novel compounds. Australian Journal of Zoology 57: 203-210.

Social behaviour and communication

Although the home ranges of several platypuses may overlap at any given spot, individuals (including mothers and their offspring) normally forage independently of each other. Males have been observed grappling vigorously in the water during the spring breeding season, presumably in order to work out who is dominant. In places where several platypus regularly feed within sight of each other (such as some lakes), it is not unusual for one animal to swim directly towards another during the breeding season, generally starting from a distance of 30-100 metres. The second animal sometimes responds by leaving the area (with or without the first animal in hot pursuit). On other occasions, two animals will swim side by side for a short distance or feed near each other for a few minutes before again moving apart.

In captivity, platypus courtship behaviour may be initiated by females as well as males. A pair will gently nuzzle each other’s bill or face one another on the surface with bills nearly touching for up to ten minutes. One animal will rub against the length of the other while gliding past, and a male will use his bill to grasp the tip of the female’s tail and be towed behind her as she swims on or near the water surface, with the pair often travelling in a tight circle. Mating has only been recorded to occur in the water, for periods lasting from a few minutes up to nearly half an hour.

When feeling threatened or annoyed, a platypus will voice its displeasure by emitting a querulous growl, similar to the sound made by a broody bantam hen disturbed on her nest. Olfactory cues may also assist communication: in the case of males, scent glands located at the base of the neck become particularly active during the breeding season, emitting a strong, musky odour. Captive males have also been observed producing a yellow, mucilaginous liquid from the cloaca after swimming to a stone or similar object. The liquid settles in a cloud over the object, presumably helping to mark the male’s territory.

Further reading:
De-La-Warr, M. and Serena, M. (1999). Observations of platypus Ornithorhynchus anatinus mating behaviour. The Victorian Naturalist 116: 172-174.
Easton, L., Williams, G. and Serena, M. (2008). Monthly variation in observed activity of the platypus Ornithorhynchus anatinus. The Victorian Naturalist 125: 104-109.
Strahan, R. and Thomas, D.E. (1975). Courtship of the platypus, Ornithorhynchus anatinus. Australian Zoologist 18: 165-178.


Platypus resting sites most often consist of burrows located in the consolidated earthen banks of a river, creek or lake. However, the animals have also occasionally been recorded sleeping in a hollow log or within a large pile of twigs and branches emerging from the water, in a natural cave, or (in Tasmania) in a burrow constructed within dense vegetation such as sedge tussocks.

"Nesting" burrows provide shelter for a mother and her offspring for several months, from the time that eggs are laid to the time that young become independent. These burrows are typically 3-6 metres in length (measured in a straight line from the entrance to the nesting chamber), though they can be much longer. The entrance to a nesting burrow is roughly oval in cross-section and just large enough to allow an adult platypus to enter. It also tends to be elevated well above the water along a reasonably steep bank, with its height probably helping to reduce the risk of inundation after storms. Whenever she enters or exits the burrow, a mother of young juveniles blocks the entry tunnel at 2-9 points with compacted soil plugs (or “pugs”), each measuring about 30 centimetres in length. The tunnel often changes direction immediately after a pug, suggesting that its main role is to fool predators into thinking they have come to the end of the burrow.

"Camping" burrows mainly provide a safe place for an adult or subadult to sleep. They are shorter than nesting burrows, typically measuring 1-4 metres in length. Based on radio-tracking studies, some camping burrow entrances are located underwater, with the rest typically well hidden by thick vegetation or beneath a stably undercut bank or overhanging tree roots. Besides helping to camouflage burrow entrances, such sites provide a relatively secure, hidden route for a platypus to approach or leave a burrow without being seen.

An adult platypus will normally occupy several different camping burrows within a period of a few weeks, with a given burrow sometimes used by different animals at the same or different times. For example, a study carried out in Victoria found that eight radio-tagged platypus each occupied between two and eight burrows over periods of 8-58 days. One burrow was occupied by a subadult male and an adult male for four days in early January (i.e. well outside of the breeding season), and a second burrow was occupied by two grown females for five days in early February. The burrow occupied by the two males was also subsequently occupied by an adult female, more than a year after it was used by the males.

Further reading:
Grant, T.R., Grigg, G.C., Beard, L.A. and Augee, M.L. (1992). Movements and burrow use by platypuses, Ornithorhynchus anatinus, in the Thredbo River, New South Wales. Pp. 263-267 in Platypus and Echidnas (edited by M.L. Augee). The Royal Zoological Society of NSW, Sydney.
Otley, H.M., Munks, S.A. and Hindell, M.A. (2000). Activity patterns, movements and burrows of platypuses (Ornithorhynchus anatinus) in a sub-alpine Tasmanian lake. Australian Journal of Zoology 48: 701-713.
Serena, M., Thomas, J.L., Williams, G.A. and Officer, R.C.E. (1998). Use of stream and river habitats by the platypus, Ornithorhynchus anatinus, in an urban fringe environment. Australian Journal of Zoology 46: 267-282.


Platypus genes are packaged in a set of 52 chromosomes, twelve of which are relatively large and the rest quite small. Like other mammals, the sex of a platypus is determined by inheriting X and Y chromosomes, with females having five pairs of X chromosomes and males having five X chromosomes and five Y chromosomes. However, much of the genetic information contained in platypus sex chromosomes appears to be different from that contained in the sex chromosomes of marsupials and placental mammals, with some evidence suggesting that a gene involved in determining the sex of birds may also be involved in determining the sex of a platypus.

The platypus genome has been estimated to include approximately 18,500 protein-coding genes, which is at the lower end of the range of estimates for the number of human genes. A map of the platypus genome was published in 2008, based on research carried out by more than 100 scientists based at 32 universities and research institutes located in nine different countries (Australia, New Zealand, United States, United Kingdom, Germany, France, Spain, Japan and Israel). It showed that most platypus genes (82%) also occur in other vertebrate animals such as mice, dogs, chickens, humans and opossums (a North American marsupial). These genes presumably are involved in basic biological functions that haven’t altered for hundred of millions of years. The remaining 18% include genes that have developed since the platypus lineage began evolving independently of other modern vertebrates, along with genes that have been retained by the platypus but lost by other species over evolutionary time.

An analysis of microsatellite DNA sampled in two neighbouring river basins in New South Wales (Shoalhaven and Hawkesbury-Nepean) concluded that the two systems were not very divergent, suggesting that platypus move reasonably frequently between them. In another study, Tasmanian platypus were found to be genetically less variable than animals found on the Australian mainland, with even less variability recorded in the small and very isolated platypus population occupying King Island in Bass Strait.

Further reading:
Akiyama, S. (2000). Molecular ecology of the platypus in Tasmania. Australian Mammalogy 21: 263.
Kolomyjec, S.H., Chong, J.Y.T., Blair, D., Gongora, J., Grant, T.R., Johnson, C.N. and Moran, C. (2009). Population genetics of the platypus (Ornithorhynchus anatinus); a fine-scale look at adjacent river systems. Australian Journal of Zoology 57: 225-234.
Warren, W.C. et al. (2008). Genome analysis of the platypus reveals unique signatures of evolution. Nature 453: 175-184.

Predators and Disease

The earliest evidence that platypus were hunted and eaten by aboriginal Australians consists of bones found in caves occupied between 13,000 and 30,000 years ago.

Spotted-tailed quolls (Dasyurus maculatus), Tasmanian devils (Sarcophilus harrisii), white-breasted sea eagles (Haliaeetus leucogaster), wedge-tailed eagles (Aquila audax), grey goshawks ( Accipiter novaehollandiae) and carpet pythons (Morelia spilota) have all occasionally been reported to capture and/or consume a platypus. Circumstantial evidence (the nature of injuries sustained by dead animals) suggests that domesticated or feral house cats may be responsible for some platypus mortalities. It has been suggested that Australian water-rats ( Hydromys chrysogaster) may possibly prey on platypus (particularly young juveniles), given that water-rats are known to kill reasonably large waterbirds, but there is no documented evidence to support this hypothesis.

The most significant platypus predators in recent decades are almost certainly wild and domesticated dogs and foxes (Vulpes vulpes). Dog attacks were deemed to be the commonest reason for platypus being killed in the years before foxes were introduced to Tasmania, accounting for 40% of the carcasses examined in a veterinary study in the 1990s. The Australian Platypus Conservancy has also recorded many cases of platypus being killed by dogs or foxes: an analysis of platypus mortalities in Victoria from the 1980s to 2009 indicated that nearly one-fifth of all reported deaths were due to predation, with most of these incidents (21/24) attributable to dogs or foxes, and the rest (3/24) to birds of prey.

While many micro-organisms and parasites have been detected in platypus tissue, few are known to be responsible for causing disease. In 1982, people began noticing that platypus in parts of Tasmania were starting to develop skin ulcers which resulted in some animals dying. The causative agent was eventually identified as a fungus, Mucor amphibiorum, which is known to infect frogs and also has been found to occur naturally in Queensland soil samples. It has therefore been suggested that the fungus may have been introduced to Tasmania via infected frogs carried in shipments of tropical produce, such as bananas. Fortunately, mucormycosis appears to becoming less of a problem over time: the incidence of infection in Tasmanian platypus declined by a factor of four from the 1990s to 2008-2009, suggesting that either animals are becoming more resistant and/or that the fungus is becoming less virulent. Interestingly, this disease has never been reported to affect platypus anywhere on the Australian mainland.

The platypus has its own species of tick (known to scientists as Ixodes ornithorhynchi) which does not occur on any other animal. The ticks are mainly found around the platypus’s lower hind legs (i.e. the part of the platypus’s body which is most difficult to groom with the claws of the back feet) and do not cause their host any obvious physical harm.

Further reading:
Connolly, J.H., Obendorf, D.L., Whittington, R.J. and Muir, D.B. (1998). Causes of morbidity and mortality in platypus (Ornithorhynchus anatinus) from Tasmania, with particular reference to Mucor amphibiorum infection. Australian Mammalogy 20: 177-187.
Gust, N., Griffiths, J., Driessen, M., Philips, A., Stewart, N. and Geraghty, D. (2009). Distribution, prevalence and persistence of mucormycosis in Tasmanian platypuses. Australian Journal of Zoology 57: 245-254.
Marshall, B. (1992). Late Pleistocene human exploitation of the platypus in southern Tasmania. Pp. 268-276 in Platypus and Echidnas. (edited by M.L. Augee). The Royal Zoological Society of NSW, Sydney.
Serena, M. and Williams, G. (in press). Factors contributing to platypus mortality in Victoria. The Victorian Naturalist.

Effects of flooding

In theory, depending on their magnitude and duration, floods could have either a positive or negative impact on platypus populations. The effect of minor flooding is likely to be relatively benign and could even improve the quality of platypus habitat, for example by flushing accumulated silt from pools.

By comparison, severe flooding is much more likely to affect platypus populations adversely. The animals may drown, contract pneumonia after inhaling water, or be swept downstream and have to find their way back through unfamiliar terrain. Their burrows may also be inundated for a substantial period of time and food supplies badly depleted due to invertebrates being washed away.

Flooding can also degrade the quality of platypus habitat if it causes banks to erode, pools to become filled with sediment, or in-stream woody habitat (logs and branches) to be deposited on land as flood waters recede.

A study conducted by the Australian Platypus Conservancy in mid-2008 examined how platypus populations in four Gippsland rivers were faring approximately 9-11 months after substantial floods occurred. In each case, flooding peaked at an estimated flow rate of more than 10,000 megalitres/day. In brief, the severity of flood-related habitat damage was inversely related to platypus population density and reproductive success: the river suffering the greatest damage had the lowest numbers of platypus and the smallest proportion of juveniles (none), whereas the least damaged area had the highest density of platypus and the largest proportion of juveniles. It was concluded that flood-related impacts can have a measurable adverse effect on platypus populations, particularly when (as was true in this study) the vegetation on adjoining slopes has recently been damaged by wildfire.

The fact that juvenile platypus are weaker and less accomplished swimmers than older animals suggests that they may be more likely to be killed by floods, particularly if these occur around the time that juveniles first emerge from the nesting burrow in summer. This is supported by the results of live-trapping surveys carried out in the Melbourne area after more than 120 millimetres of rain fell on the city in less than 24 hours in early February 2005 (the highest one-day total since weather records were first kept in 1855). The mean juvenile capture rate from February to June 2005 was less than 10% of the corresponding mean capture rate from 2001-2004. In contrast, the capture rate for adults and subadults occupying the same five water bodies from February to June 2005 was actually slightly higher than the corresponding mean capture rate from 2001-2004.

Further reading:
Serena, M. and Williams, G.A. (2008). The status of platypus in flood- and fire-affected catchments in Gippsland, 2008. Report to Department of Sustainability and Environment and Parks Victoria. (Australian Platypus Conservancy, Wiseleigh).

Effects of fire and drought

Platypus mainly shelter underground in burrows which should provide good protection from the flames, smoke and radiant heat generated by bushfires. In accordance with these facts, a study carried out by the Australian Platypus Conservancy along four Gippsland rivers in 2008 to examine the effect of wildfires and floods in the previous year on platypus populations concluded that no relationship was apparent between the amount of fire damage sustained by trees growing on the banks and platypus density and reproductive success. However, the water bodies included in this study were all reasonably large (typically 6-8 metres wide) and reliably flowing systems which would have helped buffer their aquatic ecosystems against direct fire impacts.

In contrast, anecdotal information provided by landholders living along Cardinia Creek – a small (typically 2 metres wide) stream which flows directly into Western Port in Victoria – indicates that platypus became locally extinct following the intense “Ash Wednesday” bushfire of 1983. In this case, the heat of the fire (in combination with drought) apparently resulted in parts of the channel drying out for some time, which would have made it highly problematic for platypus to find food and also increased their vulnerability to terrestrial predators. Given that the small size of Cardinia Creek would also have limited the number of platypus residing there, these circumstances could plausibly have resulted in the post-fire population declining to an unsustainable level.

Although a platypus requires adequate surface water in which to feed, there is no reason why these animals cannot survive periods of drought in isolated pools, as long as the pools are large enough to provide a reliable food supply in the form of aquatic invertebrates. As a pool and its food resources shrink, platypus presumably must weigh up the risk of starving versus that of being killed by a predator if they choose to go looking for better feeding opportunities elsewhere. Female platypus are likely to suffer disproportionately when competing with adult males for a dwindling food supply, given that males are generally both larger and equipped with venomous spurs.

Unfortunately, the amount of run-off reaching rivers after rainfall has dramatically declined in many systems due to construction of livestock dams on gullies as well as major headwater impoundments. At the same time, the availability of substantial natural pools has often been vastly reduced due to loss of riparian trees (and the large woody debris they generate), de-snagging, channelisation and/or increased rates of erosion in both the channel and its catchment area. The capacity of platypus populations to cope with natural and anthropogenic drought has diminished accordingly.

Given the trend of increasing competition for freshwater resources in temperate Australia, the occurrence of platypus in many areas will depend on sympathetic and informed management which ensures that enough reliable surface water is available to enable animals to survive and breed.

Further reading:
Grant, T.R. and Bishop, K.A. (1998). Instream flow requirements for the platypus (Ornithorhynchus anatinus): a review. Australian Mammalogy 20: 267-280.

Effects of on-stream dams and weirs

Dams and weirs may affect the size and survival of platypus populations in a number of different ways.

Firstly, platypus prefer to feed in relatively shallow water (ideally 1-3 metres deep), so most of the area within the deep water storages impounded by large dams will typically not be suitable for their use.

Secondly, large dams may support very different invertebrate communities as compared to the river habitats they replace, resulting in changes to the platypus diet. For example, animals captured in Bendora Dam (in the Australian Capital Territory) and Lake Jindabyne (in New South Wales) have been found to consume a much higher proportion of molluscs (snail, clams and/or mussels) than platypus captured in New South Wales rivers. However, it remains unknown how these dietary differences may be reflected in population density or reproductive success.

In contrast to deep impoundments, the shallower pools formed behind smaller on-stream weirs often are excellent foraging sites for platypus, and particularly in the case of relatively degraded streams may provide critical habitat for breeding and surviving drought.

Thirdly, although platypus is surprisingly good at scrambling up steep banks, they are generally unable to negotiate vertical (or nearly vertical) concrete structures. Accordingly, many dams and weir walls (or steeply pitched outlet structures) are unlikely to be passable to animals engaged either in routine foraging or longer range movements, such as those undertaken by breeding males or dispersing juveniles. Platypus may elect to leave the water to bypass such obstacles, but this will increase their exposure to predators and other dangers. The actual amount of risk presumably will reflect how far they have to travel across land, how much protective cover exists en route, and whether or not they need to cross a road to reach water again.

Fourthly, it has been suggested that platypus foraging efficiency in Victoria could be affected by the unnatural seasonal pattern of irrigation flows from dams, whereby water is typically retained in the dams through winter (when rainfall is usually highest) and released downstream in summer. For example, high flows might require platypus to forage over larger areas or for longer periods of time to meet the increased energy cost of feeding in deeper, colder, faster water. In practice, a study carried out along the Goulburn River in Victoria failed to find any measurable difference in how far adult males moved or how long they foraged in summer/early autumn (when flows were relatively high) as compared to late autumn/winter (when flows were relatively low). However, all radio-tagged animals spent time feeding in a backwater during the high flow period (with two of three animals concentrating their activity there), whereas only two of six animals visited the backwater during the low flow period. The researchers concluded that platypus may prefer to avoid foraging in strong currents if habitats with slower moving or still water are available.

Lastly, development of dams and weirs may adversely affect the platypus populations living downstream if less water is available to maintain pools (and connectivity between pools) than was historically the case. Unfortunately, the species is most likely to rely on sympathetic management of shared water resources at precisely those times when water is most critically limited by prolonged droughts.

One of the most characteristic outcomes of maintaining highly regulated flow regimes downstream of large dams is that – in the absence of occasional flushing flows – natural pools tend to fill with sediment. This in turn reduces the resilience of associated aquatic ecosystems when challenged by drought. Accordingly, in the absence of management which ensures either that a reliable environmental flow or adequate platypus refuge areas are maintained in highly regulated systems, platypus populations are predicted at best to contract and at worst to become extinct.

Further reading:
Grant, T.R. (1982). Food of the platypus, Ornithorhynchus anatinus (Monotremata: Ornithorhynchidae), from various water bodies in New South Wales. Australian Mammalogy 5: 235-236.
Gust, N. and Handasyde, K. (1995). Seasonal variation in the ranging behaviour of the platypus (Ornithorhynchus anatinus) on the Goulburn River, Victoria. Australian Journal of Zoology 43: 193-208.

Human activities contributing directly to platypus mortality

Platypus is completely protected by law across Australia. Nevertheless, more than 80% of all platypus mortalities reported to the Australian Platypus Conservancy from 1989 to 2009 where the cause of death could be reliably assigned were directly related to human activities.

The single biggest problem, accounting for more than half of platypus deaths, was animals being drowned in nets or traps set illegally to capture fish or crustaceans (yabbies or crayfish). Many of these incidents involved several animals being killed at one time. For example, at least eight platypus are known to have died in a pair of rectangular gill nets set overnight in a large reservoir located in north-central Victoria in the mid-1990s. More recently, APC staff confirmed that the remains of 17 platypus were contained in a single unlicensed fyke net that had been set and then a bandoned along a small stream, and up to three animals have drowned overnight in one opera house trap. Although platypus are in theory protected across much of their range by laws restricting the use of nets and enclosed yabby and cray traps, these laws continue to be widely flouted, sometimes by persons who aren’t aware that regulations exist and apply to them.

Human Activity
A platypus found dead in an "opera house" yabby trap.
If a breeding female drowns in late spring or summer,
any dependent juveniles will also die.
(Photo: Joanne Connolly).

A substantial number of platypus also died as a by-product of recreational angling, in some cases due to exhaustion after being caught on baited lines left overnight (illegally) to catch fish. Other animals were found dead with fishing line wrapped around the body or fishing hooks embedded in the bill or front foot. In some cases it appeared that the platypus died of stress or infection – for example, a loop of fishing line had gradually cut so deeply into one animal’s chest that the lung cavity was exposed. Still other platypus drowned after fishing line attached to a hook became tangled in submerged branches or other objects.

Human Activity
Many platypus die slow and painful deaths after discarded
fishing line becomes tightly looped around their neck or chest.

In addition, many deaths were caused by litter. Live-trapping surveys carried out by the Australian Platypus Conservancy from 1998 to 2007 found that nearly 5% of the platypus captured in suburban habitats around Melbourne carry loops of litter around their neck or chest: plastic cable-ties, six-pack holders, elastic bands, canning jar seals, knotted loops of twine, flexible engine gaskets, miscellaneous circular fittings (in one case, for a bicycle headlamp), tamper-proof rings from food containers, plastic bangle-type bracelets, elastic hair-ties, and a wide assortment of loops or rings of unknown origin. In country areas, the main problem tends to involve nylon fishing line, as described above.

Human Activity
Plastic rings and loops are responsible for many platypus deaths.

Because the platypus mainly feeds on bottom-dwelling insects, much of an animal’s time is spent investigating the channel bed where litter tends to accumulate. A second problem is that the platypus mainly swims using two broad flaps of skin that unfurl beyond its front toes. Although the front feet are very efficient at paddling, they have virtually no ability to grasp or manipulate objects. The back feet are more dexterous, but their location means that they can’t remove an object by pulling it forward if it gets tangled around the front half of the body. An item of litter that accidentally finds its way around a platypus’s head therefore tends to work its way back along the body and then remain there until it breaks or the platypus dies - often due to horrific injuries that develop as constricting loops gradually cut through skin and underlying tissues.

Other examples of platypus mortalities linked to human activities include animals drowning after entering irrigation pumps or becoming stuck in a narrow space between irrigation gates, being run over by motor vehicles, and being shot.

Further reading:
Grant, T.R. (1993). The past and present freshwater fishery in New South Wales and the distribution and status of the platypus Ornithorhynchus anatinus. Australian Zoologist 29: 105-113.
Serena, M. and Williams, G.A. (1998). Rubber and plastic rubbish: a summary of the hazard posed to platypus Ornithorhynchus anatinus in suburban habitats. The Victorian Naturalist 115: 47-49.

Platypus habitat relationships: vegetation and catchment imperviousness

As long as adequate surface water is available, the number of platypus found in any given area is most likely to be limited by food. Factors which encourage the development of productive and abundant populations of aquatic insects and other invertebrates (the platypus’s main food supply) should therefore also favour platypus populations.

Studies conducted by the Australian Platypus Conservancy along urban, rural and forested water bodies have consistently found that platypus numbers and foraging activity show a strong (statistically significant) positive relationship with the number of indigenous trees (eucalypts and wattles) growing on the banks. The same relationship holds for the amount of cover provided by shrubs and lower-growing plants overhanging the water. Similarly, a study carried out in New South Wales along the Macquarie River concluded that the amount of overhanging bank vegetation was positively linked to platypus usage.

Numerous studies have confirmed that indigenous vegetation contributes to the health of Australian streams and rivers. Trees, shrubs and ground covers work together to stabilise banks, keep sediment out of the channel, contribute organic matter to the aquatic food chain, and shade the channel in summer (thereby helping to keep water well-oxygenated). These processes benefit aquatic invertebrates which in turn feed the platypus. Overhanging vegetation also helps to disguise platypus burrow entrances and provides protective cover for the animals themselves when they’re active.

Platypus demonstrably can and do occur in water bodies that are lined with substantial numbers of willows or other exotic trees. Nonetheless, two studies by APC researchers have detected a negative relationship between the occurrence of willows (Salix spp.) and platypus foraging activity in summer and early autumn. The streams where this research took place were quite small and the willows were old and well established, so the channel under the trees was typically dominated by a nearly impenetrable mat of tough, fibrous roots – undoubtedly making it difficult for a platypus to detect and capture prey.

The occurrence of trees and other plants in a catchment assists infiltration of rain into soil, storing water which is only gradually released to streams and rivers. When vegetation is replaced by hard surfaces (such as roofs and roads), more rain ends up as surface run-off, promoting channel erosion and reducing inflow between storms. In practice, a study has found that platypus do not regularly inhabit water bodies where more than 11% of the catchment area has been converted to impervious surfaces. Similarly, researchers at the University of Melbourne have concluded that platypus populations disappear from streams or rivers characterised by more than 2.2% direct connected imperviousness (defined as the proportion of a catchment covered by impervious surfaces that are in turn directly connected to a natural water body by pipes or sealed drains).

Further reading:
Danger, A. and Walsh, C.J. (2008). Management options for conserving and restoring fauna and other ecological values of urban streams in the Melbourne Water Region. (Report to Melbourne Water). Department of Resource Management and Geography, University of Melbourne, Parkville.
Ellem, B.A., Bryant, A. and O’Connor, A. (1998). Statistical modelling of platypus (Ornithorhynchus anatinus) habitat preferences using generalised linear models. Australian Mammalogy 20: 281-285.
Serena, M., Swinnerton, M., Worley, M. and Williams, G.A. (2001). Attributes of preferred foraging habitats of platypus (Ornithorhynchus anatinus). Pp. 565-570 in Proceedings of the Third Australian Stream Management Conference (The Value of Healthy Streams). (edited by I. Rutherfurd, F. Sheldon, G. Brierley and C. Kenyon). CRC for Catchment Hydrology.
Serena, M., Worley, M., Swinnerton, M. and Williams, G.A. (2001). Effect of food availability and habitat on the distribution of platypus (Ornithorhynchus anatinus) foraging activity. Australian Journal of Zoology 49: 263-277.

Platypus habitat relationships: in-stream habitats and water quality

The capacity of a water body to support platypus is influenced by water quality and the quality of in-stream habitats.

Stably undercut banks. A positive relationship has been found to exist between platypus population density and the occurrence of consolidated soil banks undercut to a depth of 8 centimetres or more. Platypus foraging activity has also been positively linked to the distribution of stably undercut banks along creeks near Melbourne. These findings presumably reflect the fact that undercut banks support a wide range of aquatic organisms, many of which are consumed by platypus.

Channel depth. Studies carried out in Victoria, NSW and Tasmania agrees that platypus prefer to concentrate their feeding activity in water that is approximately 1-3 metres deep.

Logs, branches and leaves. Research carried out by the APC has shown that platypus numbers and activity are both positively associated with the occurrence of logs, branches and finer organic materials such as bark, twigs and leaves in the channel. This presumably reflects the fact that many aquatic invertebrates rely on organic substrates to provide food, attachment sites, materials for cases or egg sacs, or shelter from strong currents. By marking sites in the water where edible invertebrates are likely to be concentrated, the presence of woody or leafy material may also help platypus to locate food more efficiently.

Inorganic substrates. Along the Hastings River of New South Wales, a study of platypus foraging behaviour concluded that the animals prefer sites where the channel bed is covered mainly by cobble-sized stones (as opposed to finer gravel, sand or silt). By comparison, platypus occupying a small stream in Victoria appeared to prefer feeding at sites dominated by gravel, pebbles, cobbles and larger rocks.

Water pollution. The amount of phosphate and suspended sediment in water have been found to be inversely related to the distribution of platypus around Melbourne: the more phosphate and suspended sediment that is present, the fewer platypus are captured. This plausibly reflects the effects of pollution on the platypus food supply, with substantial nutrient enrichment typically reducing populations of preferred prey species. High levels of suspended sediment can deplete populations of aquatic invertebrates by triggering them to drift downstream, and also contributes to silt being deposited in the channel (see above).

Salinity. The electroreceptors used by platypus to navigate underwater and locate prey presumably function over a limited salinity range and are predicted to perform best in fresh water. Although platypus are not known to regularly inhabit highly saline coastal environments, the animals do occur in rivers and stream where salinity sometimes reaches 10,000 to 14,000 Electrical Conductivity units (1 EC unit = 1 microsiemens/centimetre). By comparison, horses and sheep can respectively tolerate up to around 9,000 and 16,000 EC units in their drinking water without suffering a decline in condition. The salinity of ocean water is typically 51,500 EC units.

Further reading:
Grant, T. (2004). Depth and substrate selection by platypuses, Ornithorhynchus anatinus, in the lower Hastings River, New South Wales. Proceedings of the Linnean Society of New South Wales 125: 235-241.
Serena, M. and Pettigrove, V. (2005). Relationship of sediment toxicants and water quality to the distribution of platypus populations in urban streams. Journal of the North American Benthological Society 24: 679-689.

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