Australian Platypus Conservancy

In this section on platypus ecology and behaviour you can read about:

• Foraging behaviour
• Diet and food consumption
• Home range, movements and dispersal
• Burrows
• Reproduction
• Courtship, mating and nest-building
• Social communication
• Mortality factors

Foraging behaviour

Platypus feed only in the water. They find their small invertebrate prey by searching along shallow riffles, gleaning items from submerged logs and branches, digging under banks, and diving repeatedly to the bottom of pools. Animals most commonly feed for one extended session in each 24-hour period, typically remaining active for 8-16 (though occasionally up to 30) hours and completing up to 1600 foraging dives per session.

Platypus foraging behaviour in a pool begins with an animal doing a neat, quiet duck dive (as shown above). The animal swims to the bottom and uses its bill to find and seize prey. The platypus doesn’t swallow food immediately, instead storing its prey in special cheek pouches located at the back of the jaw. It returns to the surface as its oxygen supply becomes depleted (usually within 30-60 seconds of when it dived, though unforced dives of up to 138 seconds have been recorded) and then typically spends 10-20 seconds chewing and swallowing its food before diving again.

Although platypus cheek pouches sometimes hold minor amounts of mud or sand, such material is presumably ingested by accident. In particular, there’s no reason to believe that gritty sediment is retained on purpose to help grind up prey. Instead, inedible material is probably routinely expelled (along with surplus water) through grooves located along the edge of the lower jaw (as shown at left).

An insulating air layer trapped in the platypus’s fur helps to provide positive buoyancy, increasing the amount of energy needed to dive deeply. A study conducted along the Manning River in New South Wales (which has a maximum depth of about 8 metres) found that about 80% of platypus foraging dives reached a depth of 1.6-4.9 metres, with the deepest descending to 6.1 metres. At Lake Lea in Tasmania (which has a maximum depth of more than 10 metres), 98% of platypus dives did not exceed 3 metres, though one dive descended to nearly 9 metres. The use of data loggers has confirmed that platypus feed mainly but by no means exclusively at night, with around 25% of the animals tracked along a small Victorian stream and 40% of those tracked in a Tasmanian lake often recorded to be active during daylight hours.

Photos courtesy of APC (above) and Ann Killeen (below)

Diet and food consumption

The platypus typically has a varied diet dominated by bottom-dwelling (or “benthic”) insects (as shown below). Although several studies have concluded that larval mayflies and caddisflies are particularly important dietary items, this may largely reflect the wide availability of these groups at the sites where the studies were conducted. The platypus also dines on water bugs, water beetles, and larval damselflies, dragonflies, dobsonflies, midges, craneflies and blackflies. Other prey includes freshwater shrimps, snails, “pea shell” mussels, seed-shrimps (or ostracods) and worms. Burrowing crayfish have been found to be an important part of the platypus diet in a Tasmanian lake, and trout eggs were often consumed along the Thredbo River in winter when fish were spawning.

         

The platypus’s ability to prey on fish or other vertebrates is restricted by its lack of true teeth as an adult. Remains of a small frog (which may have been eaten as carrion) have been found in one platypus cheek pouch sample from the Shoalhaven River in New South Wales. A young platypus is equipped with a set of shallow-rooted premolar and molar teeth located at the back of the bill, but these fall out around the time that a juvenile begins to eat solid prey. The teeth are replaced by rough grinding pads which grow continuously to offset natural wear – a very handy feature given that abrasive material such as sand may often accidentally enter the platypus’s mouth when it snaps up its prey from the bottom.

Reflecting the fact that the platypus diet consists of small, soft-bodied prey items that are masticated quite finely even before they are swallowed, the platypus’s stomach is small and lacks the ability to secrete digestive enzymes or hydrochloric acid. However, the platypus’s stomach does contain Brunner’s glands, which produce a mucus-rich secretion to help lubricate the intestinal walls and assist efficient nutrient uptake there.

Because the platypus is a relatively small, warm-blooded animal, it needs a lot of food to serve as fuel. Studies in captivity have shown that adult males need to consume the equivalent of around 15-28% of their body weight in food each day to maintain good physical condition. Similarly, the average daily food intake of animals occupying a Tasmanian lake has been estimated to be 19% of body mass. Not surprisingly, the energy requirements of lactating females increase substantially as their offspring grow. For example, daily food consumption of a mother in captivity was found to increase from around 14% of her body weight in the first month of lactation to slightly more than 36% in the final month of lactation.

Photos courtesy of https://www.mdfrc.org.au/bugguide/resources/howtouse.htm

Home range, movements and dispersal

Based on mark-recapture studies conducted along creeks in Victoria, a male platypus’s home range typically measures 6-11 kilometres in length. By comparison, a female’s home range is generally 2-4 kilometres long. The difference reflects the fact that a male tries to encompass as many female home ranges as possible within his own to improve his prospects for mating. Though adult males try to avoid coming into direct contact with each other, especially just before and during the breeding season, male home ranges often overlap to some extent. Female home ranges also typically overlap, in a well-ordered manner that ensures that each female has enough room to raise her young. The longest platypus home ranges described to date stretched respectively 15.1 kilometres (male) and 6.0 kilometres (female). Most adults occupy stable home ranges for periods of at least several years.

In a Victorian radio-tracking study, males and females typically visited 24-70% of their home range on any given day. Adults have been documented to travel up to 4.0 kilometres (female) or 10.4 kilometres (male, including backtracking) along a creek or river channel in a single activity period. At Lake Lea in Tasmania, daily male activity areas encompassed 3-35 hectares (up to 25% of the total surface area) with females using 2-58 hectares (up to 41% of the surface area). Subsurface swimming has been calculated to be most efficient when a platypus travels at a speed of 0.4 metre/second (1.4 kilometres per hour). However, a feeding platypus typically proceeds at a more leisurely rate of 0.1-0.7 kilometres per hour as it dives and resurfaces.

Juvenile dispersal is believed to be an important mechanism to reduce inbreeding and enable vacant habitat to be repopulated.  It’s well established that young males move farther on average than young females, and that females are more likely than males to settle near where they grew up. Because the number of juveniles captured in live-trapping studies in Victorian streams drops quite sharply in late autumn, it’s also believed that many juveniles initiate dispersal at this time of year. Young dispersing males have been known to travel more than 50 kilometres in the Yarra River system and nearly 45 kilometres in the Wimmera River system, and undoubtedly may sometimes venture much farther.

Photos courtesy of L. Berzins (above), B. Catherine (below)

Burrows

Platypus mainly sleep in burrows located near the water’s edge, though they may also occasionally shelter in a handy hollow log or (in Tasmania) within a dense clump of low-growing vegetation. Platypus burrows are divided into two types: nesting burrows and camping burrows.

A nesting burrow provides shelter for a mother and her offspring for several months. It’s typically 3-6 metres long (measured in a straight line from the entrance to the nesting chamber), though it may be much longer, particularly along rivers prone to major flooding. The entrance is roughly oval in outline and just large enough to allow an adult platypus to enter (as shown at right). Whenever a mother of young juveniles enters or exits her burrow, she blocks the entry tunnel with a series of 2-9 compacted soil plugs (or “pugs”). It’s believed that the pugs both deter predators from entering and help to protect juveniles from drowning if flooding occurs.

Camping burrows are occupied by animals that are not caring for eggs or young. They are typically 1-2 (though can be 4 or more) metres long. Research has shown that camping burrow entrances are sometimes located underwater, with others typically well hidden beneath an undercut bank or overhanging vegetation as illustrated below (entrance locations marked by red arrows).

                    

A platypus will normally occupy two or more camping burrows in a period of a few weeks, including some that may be used by other individuals. For example, a radio-tracking study carried out along a stream in southern Victoria found that between 6 and 12 burrows were occupied by each of five animals (3 males, 2 females) monitored for 28 to 38 days. Three animals occupied at least one burrow known to be used by another animal during the study, though only one burrow was ever occupied by two individuals at the same time.

Photos: APC

Reproduction

Platypus have been recorded mating in late winter and spring (peaking in about September) in Victoria and New South Wales; breeding is believed to occur a few weeks earlier on average in Queensland and a few weeks later in Tasmania. Mates do not form lasting pair bonds: males court as many females as possible, and females rear their young without male assistance.

A clutch of 1-3 whitish, leathery-shelled eggs (15-17 millimetres long) is laid approximately 2-3 weeks after mating. The eggs are then incubated for 10-11 days in an underground nesting burrow, clasped between a female’s curled-up tail and her belly. The young are tiny and very immature when they hatch. 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, the babies develop in the nesting burrow for several months before entering the water for the first time in summer. Throughout this period, they only feed on milk. Because a female platypus doesn’t have nipples, a baby sweeps its stubby bill rhythmically from side to side to slurp up milk secreted directly onto the mother’s belly 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), 22% fat (about six times the average value for cow milk) and 8% protein (more than double the average value for cow milk).

Juveniles are fully furred, well-coordinated and about 80% of their adult length when they first enter the water, (as shown above). They aren’t taught to swim or to find food by their mother, but have to master these skills on their own through trial and error.

Males and females both mature at the age of two years, although some females may not raise young until they are four years old or more. A long-term study conducted along the upper Shoalhaven River in New South Wales concluded that less than half of all females breed on average in any given year (range = 18-80%). Along both the Shoalhaven River and urban streams near Melbourne, more females raise young in years when water flow has been plentiful in the five months before mating begins, suggesting that this is a crucial period for a female to store fat before her body decides to breed. Reproductive success can also be greatly reduced if major flooding occurs in the period when juveniles occupy nesting burrows, presumably because young animals drown when their burrow is inundated.

Photos courtesy of Ann Killeen (above), APC (below)

Courtship, mating and nest-building

Based on studies in captivity, a female platypus decides when courtship occurs. This often results in the male grasping the tip of her tail in his bill and swimming with her in a tight circle (as shown above) or being towed behind her as she twists and turns near the water surface. The actual mating event typically lasts 3-4 minutes, and can occur either while both animals are supported in shallow water by a structure (such as a partly submerged log) or while they’re floating in deeper water.

A number of mating postures have been described. A relatively large male (1.6 kg) mounted his partner from above and behind, wrapping his tail beneath her body and grasping her hind feet and back with his front feet to maintain his position. In contrast, a smaller male (1.1 kg) lay on his side next to his partner while using his bill to grip her neck and his hind feet to grip her body. Pairs that mate while floating in the water may end up facing in opposite directions and upside down relative to one another, forcing them to rotate around their long axis so each can breathe in turn.

A female starts gathering material to build a nest about 1-2 weeks after mating. She continues this activity for 2-5 nights, finishing shortly before she retires to the burrow to lay and incubate her clutch of eggs. In captivity, females were observed using the bill to gather floating grass and leaves from the water surface. This material was then passed under the body to the tail, which was curled forward to hold the bundle firmly against a female’s belly as she swam to the nesting burrow entrance. The finished nest takes the form of a hollow sphere or cup. Because wet materials are used to build a platypus nest, it’s unlikely that the nest serves to keep the eggs and young warm. Instead, its main role is probably to maintain humidity in the burrow so eggs and small hairless juveniles don’t dry out when their mother has to leave to find food or carry out other duties.

Photo courtesy of M. Kirton

Social communication

There is no evidence that platypus use sound to communicate with each other, apart from occasionally producing a querulous growl (similar to the noise of complaint made by a broody hen) when they feel threatened or annoyed.

Although aquatic mammals typically don’t rely much on their sense of smell, the platypus has an exceptionally large number of genes coding for special smell receptors located in the vomeronasal (or Jacobson’s) organ found in the roof of its mouth. In other mammals, the vomeronasal organ is mainly used to detect odours produced by members of the same species.

In the case of the platypus, both sexes have scent glands located at the base of the neck. The glands are much larger in adult males than females and become more active in both sexes during the breeding season. When males are handled at this time of year, the glands often release small drops of a pale yellow fluid with a strong, musky odour. A male held in captivity has also been seen releasing a yellow, mucilaginous liquid from his cloaca after swimming to the bottom of a pool and then pausing above a stone or other object. The person reporting this sequence of events considered it likely to be a form of marking behaviour.

Photo courtesy of APC

Mortality factors

An analysis of 183 platypus mortalities reported to the APC from the 1980s to 2009 in Victoria found that animals died most often after drowning in illegal nets or traps set to capture fish or crayfish/yabbies (56% of all mortalities). Only 18% of victims were killed by more or less natural causes (such as predators, drought and flooding). However, this figure undoubtedly underestimates the actual impact of natural factors, given that many victims of predation are presumably eaten entirely, and those dying from starvation, disease or heat stress are undoubtedly less likely to be found (and the cause of death accurately identified) than those killed directly by human activities. It’s also worth noting that use of enclosed crayfish/yabby traps in Victoria was completely banned beginning in July 2019. Apart from nets and traps, mortality factors identified in the Victorian study (in decreasing order of importance) were as follows:

• Predation by dogs, foxes or birds of prey – 13% of victims
• Irrigation pumps, mini-hydro turbines or other infrastructure – 10% of victims
• Embedded fishing hooks or discarded fishing line – 5% of victims
• Entanglement in other sources of litter – 4% of victims
• Flooding – 3% of victims
• Run over by a car – 3% of victims
• Shot or bludgeoned by humans – 2% of victims
• Drought – 2% of victims
• Other (such as juveniles dug up during earth-moving works) – 2% of victims

By comparison, a study of factors contributing to 23 platypus mortalities in the mid-1990s in Tasmania concluded that the most common cause of death was attack by domesticated dogs (43% of victims). This was followed by being run over by a car (30% of victims), starvation or exposure due to natural causes such as flooding (17% of victims) and infection by the ulcerative fungus Mucor amphibiorum (9% of victims). Differences in the Victorian and Tasmanian findings reflect the fact that:

• Use of crayfish/yabby traps was undoubtedly much less widespread in Tasmania than in Victoria (due to differences in fishing regulations).
• Foxes were not present in Tasmania in the mid-1990s.
• As compared to mainland platypus, Tasmanian animals generally spend more time travelling across land and therefore are more at risk of being attacked by pet dogs or hit by cars.
• There are no known cases of platypus on the mainland becoming sick due to infection by Mucor.

Photo courtesy of B. McNamara