In this section on platypus biology you can find information about:
- Vital statistics
- The bill and its senses
- Vision and hearing
- Body temperature
- Spurs and venom
- Sleep and dreaming
Adult length: Typical total length = 0.5 metres (males), 0.4 metres (females). Maximum recorded length = 0.6 metres.
Adult weight: Typical weight = 1.0-2.4 kilograms (males), 0.6-1.4 kilograms (females). Maximum recorded weight = 3.0 kilograms. Animals living in the colder/southern parts of the platypus’s range are generally larger than those found in the warmer/northern parts of its range.
Age when first mature: 2 years (males and females)
Mating period: Late winter to spring
Size of eggs: 15-17 millimetres long
Number of babies in a litter: 1-3 (most typically 2)
Length of egg incubation period: 10-11 days
Length of lactational period: 16-20 weeks (in captivity)
Life span: Up to 21 years in the wild (males and females). More than 25 years in captivity. A long-term study near Melbourne found that 40% of adults were last captured when 3-5 years old, 36% when 6-8 years old and 24% when 9 or more years old.
Photo courtesy of John Bundock
The platypus has a streamlined body and a superficially duck-like bill. The eyes and ears are both located in a muscular groove that normally pinches shut when a platypus dives. The fur is dark brown above (except for a small light-coloured patch of fur next to each eye) and creamy white below (sometimes tinged rusty red or brassy yellow). The light-coloured patches make it look as if the eyes remain open, reducing the risk of attack by a predator (as shown above).
The platypus’s hind foot is equipped with sharp, curved claws that are used like a comb to groom the fur (as shown at left). The front foot ends in a broad expanse of skin that unfurls in the water to form a large and efficient paddle (below). The extra skin folds under the foot when an animal leaves the water, making it easier to walk or to dig using the sturdy front claws.
The bill and its senses
The platypus’s 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 bill’s upper surface is uniformly dark grey; its lower surface can either be uniformly grey or quite mottled.
The skin of the bill contains tens of thousands of specialised sensory receptors that provide a platypus with the information it needs to navigate underwater and capture its prey. The receptors known as “push rods” are sensitive to touch or pressure. Nerves are activated when the tip of a push rod receptor is displaced by as little as 20 microns (0.00002 metre), which means that a platypus can detect movement by prey such as a freshwater shrimp from a distance of 15-20 centimetres, simply by sensing associated water movement.
The bill surface is also thickly dotted with electroreceptors (technically known as “sensory mucous glands”) that are able to register the tiny amounts of electricity generated when the muscles of edible aquatic invertebrates contract. Because electricity travels so rapidly through water, the tail flick of a shrimp is recorded a fraction of an instant earlier by bill electroreceptors as compared to push rods, making it possible for a platypus to judge the distance to its prey.
Photo courtesy of Ann Killeen (right), other photo: APC
Vision and hearing
The platypus’s eye is small (6 millimetres in diameter) and has a round pupil. Its structure is mostly like that of other mammals but has some reptilian features, such as double retinal cone cells used to see colour. The lens is fairly flat at the front and much more curved at the back, like those found in otters and sea lions. This shape improves underwater vision.
Interestingly, the platypus rarely uses sight when underwater – its eyes normally close automatically as soon as it dives. It’s believed that this occurs to help protect the eyes from being injured accidentally by underwater branches and the like, and makes sense given that the platypus feeds mainly at night and often in murky water. However, a platypus very much relies on its eyesight as a way to detect predators when it’s swimming or resting on the water surface – the animals instantly dive if they see a bird (even a small, harmless one) swooping over them.
The platypus ear is located in a muscular groove just behind the eye. Platypus ears are generally like those of other mammals but do have a few primitive structural features -for example, 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.
Photo courtesy of Barry Baker
The platypus normally maintains a body temperature close to 32oC. This is lower than the body temperature of most other mammals – for example, the temperature of a healthy human is usually about 37oC. The platypus’s relatively low body temperature helps it to conserve energy, especially when swimming in cold water.
To further reduce heat loss, platypus fur consists of an extremely dense undercoat (with up to 900 individual hairs occupying a square millimetre of skin) and coarser overlying guard hairs. These layers work together to trap air next to the platypus’s skin, so most of the platypus’s body actually remains dry in the water. The insulation provided by the fur and trapped air layer is comparable to that of a 3-millimetre layer of neoprene wetsuit material.
The platypus also has a special network of small intertwined veins and arteries in the pelvic region (known to scientists as a rete mirabile or “miraculous network”). This network provides an internal heat exchange system: cooled blood returning to the heart from the legs and tail absorbs warmth from blood being pumped to the legs and tail through arteries, thereby helping to retain heat inside the body.
Because the platypus is so well adapted to surviving cold conditions, it can also easily overheat. This isn’t normally a problem when animals spend time in the water or rest in an underground burrow. However, heat stress may be a real concern if a platypus is forced to walk a long distance in warm to hot conditions, for example to find a suitable place to feed during a drought.
Observations made in both captivity and along a small Victorian stream indicate that a platypus may sometimes enter a state of torpor in which body temperature drops and an animal remains inactive in a burrow for up to around six days. This behaviour has only been recorded in the colder months of the year (late May to early September). However, animals were not found to become inactive in radio-tracking studies undertaken in winter along rivers in New South Wales and a subalpine lake in Tasmania, implying that cold weather is necessary but not sufficient to trigger torpor in this species.
Spurs and venom
The male platypus has a conspicuous spur located on each inner hind ankle (as shown below at left). Adult spurs are typically 12-18 millimetres long, similar in shape to a dog’s canine tooth and are connected to a venom-producing crural gland. The developing spurs of juvenile males are typically shorter and thicker than those of older animals (as shown below at right). Females (both juveniles and adults) lack conspicuous spurs.
Platypus venom is a clear, slightly sticky fluid containing different active compounds than those found in snake venoms. Although not life-threatening, it causes severe pain and swelling which can take several weeks to subside. The pain is not relieved by standard analgesics such as morphine. However, it can be treated successfully using drugs which act to block nerve transmission.
Platypus venom is only produced by mature males, and venom production peaks during the breeding season. It is therefore believed that platypus venom mainly serves to help adult males compete for mates. A spurring response is triggered when a male is touched or stroked on its belly between the hind legs. The hind feet rotate 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.
To avoid being spurred by a platypus, never try to hold or support an adult male (or a platypus of unknown age and sex) from below. Instead, lift the animal by grasping it firmly around the end half of the tail (not the base of the tail, which can be reached by the spurs – see illustration at right). While holding a platypus in this manner, it should be easy to see if spurs are present.
Photos: APC, drawing by Peter Marsack
The platypus has 52 chromosomes, including 10 sex chromosomes (5 X and 5 Y chromosomes in males; 10 X chromosomes in females). The DNA coding sequence for the platypus’s entire genome (apart from the Y chromosomes) was described in 2008, based on tissue obtained from a female captured along the Barnard River in New South Wales. This showed that the platypus has approximately 18,500 protein-coding genes, most of which (82%) also occur in other vertebrate animals such as mice, dogs, chickens and humans. These genes presumably contribute to basic biological functions that haven’t changed for hundreds of millions of years.
Subsequent genetic studies suggest that there are four major platypus population units in Australia, respectively inhabiting Tasmania and King Island, New South Wales and Victoria, central Queensland, and north Queensland. Genetic differences among these units appear to have been caused by substantial geographic barriers that have blocked successful dispersal between them. The animals occupying Tasmania, New South Wales-Victoria and north Queensland seem to have been genetically isolated from each other for up to 1 million years; those from central Queensland and New South Wales-Victoria may have been separated for around 300,000 years.
Genetic differences commonly occur among populations occupying neighbouring river basins on the Australian mainland, suggesting that walking overland from one river system to another is a fairly difficult and dangerous task for a platypus. Following on from this, a pattern of “isolation by distance” – populations becoming less genetically similar to each other as the distance between river basins increases – is evident both in Tasmania and on the mainland. Interestingly, there is no indication that the Great Dividing Range has ever acted as much of a genetic barrier to populations located on either side of these mountains, suggesting that it’s fairly easy for a dispersing platypus to travel between headwater streams located near the top of the Divide, at least in wet years.
Genetic studies of island populations indicate that platypus can thrive despite high levels of inbreeding and small effective population size (Ne, a measure of the number of individuals making an effective genetic contribution to the next generation). The level of platypus genetic diversity on King Island (which has been isolated from both the Australian mainland and Tasmania for at least 10,000 years) is among the lowest ever recorded in a wild vertebrate population. However, animals are widespread and abundant in suitable habitat and no signs of physical abnormalities or poor physical condition are evident. Similarly, the platypus population on Kangaroo Island (descended from 16 founders released in the 1940s) has been estimated to have an effective population size of just 10-11 individuals today. Nonetheless, animals are again entirely normal in terms of their physical appearance and condition, and population densities are similar to (or slightly higher than) those reported for Victorian streams of comparable size.
Photos courtesy of Mike Sverns (above), B. Catherine (below)
Sleep and dreaming
A platypus sleeping in a cool burrow looks like a furry ball: it tucks its legs up against the body, drops its bill against the chest, and then wraps its tail over its head and bill. This posture reduces the amount of heat lost from the platypus’s body surface while it sleeps and, in particular, means that the bare skin of the bill is kept warm under its tail. However, if the temperature inside the burrow rises substantially (for example, during a heat wave), a sleeping platypus may partly or entirely stretch out while reclining on its side, stomach or back.
Mammalian sleep is typically divided into two distinct and alternating stages: REM (“Rapid Eye Movement”) sleep and non-REM sleep. In REM sleep, nerve cells in the brain fire in rapid bursts, the face, eyes and limbs twitch and (at least in humans) dreaming occurs. Interestingly, researchers have found that a platypus engages in more REM sleep than any other adult mammal in which this has been studied (more than 8 hours per day on average). There are some differences between REM sleep in the platypus as compared to most other mammals (for example, the platypus brain cortex has a higher voltage when monitored using an electroencephalogram or EEG). However, the platypus resembles other mammals in vigorously twitching its bill and head and rapidly moving its closed eyes during REM sleep. It remains unknown if a platypus actually dreams.
Photo courtesy of Erin Whitford