The platypus is roughly half the size of a household cat. The adult male's average length is about 50cm and its weight is approximately 1.7kg. The female, however, is smaller and will reach an average length of 44cm and a weigh of about 0.9kg. This difference in size and weight is called sexual dimorphism. See the table below for more information about the size of platypuses.
|Total length||45.0 - 60.0cm, average 50.0cm||39.0 - 55.0cm, average 43.0cm|
|Tail length||10.5 - 15.2cm, average 12.5cm||8.5 - 13.0cm, average 11.2cm|
|Bill length||4.9 - 7.0cm, average 5.8cm||4.5 - 5.9cm, average 5.2cm|
|Weight||1.0 - 2.4kg, average 1.7kg||0.7 - 1.6kg, average 0.9kg|
(Table adapted from Grant, 1989)
The tail of the platypus is mainly made up of a fatty tissue that is used to store energy supplies, which the animal can use when there is a shortage of food, such as in the winter months. It also acts as a form of insulation. The top is covered by coarse hairs, whereas underneath there is only a sparse growth of hair. The platypus's tail differs from that of a beaver's both in it's shape and the purpose it is used for. The beaver's tail is flatter, broader and covered in special scales, and it is used to help the animal propel itself through the water. The platypus, on the other hand, uses its tail only for steering while swimming. This form of movement is further assisted because its body is streamlined and compressed dorsoventrally (Nowak & Paradiso, 1983).
Research undertaken on Tasmanian platypuses has highlighted two interesting facts about the animal's size. The first is that platypuses found in Tasmania are larger than those living in Australia (Connolly & Obendorf, 1998). However, the size is not great enough to be able to say that there are two sub-species of platypus. The second point of interest is that Tasmanian male platypuses are significantly larger(p<0.001) than the females (Connolly & Obendorf, 1998).
The brain of the platypus is small and is made up of two smooth cerebral hemispheres (Nowak & Paradiso, 1983). Within which there are specific areas responsible for the platypus' behaviour, as can be seen in the diagram below:
(Diagram adapted from Grant, 1989)
The different parts of the body do not require the same amount of brain area in order to function. This is because some body parts are very complicated in function, e.g. the bill which needs to masticate and use its electroreceptors to find food, whereas others, such as the tail have a much simpler purpose. These differences can be seen in the diagram above. There is some limited overlap between these areas of the brain, because the platypus' body parts do not operate totally independently of each other.
The platypus body, apart from the bill and the feet, has a thick covering of waterproof fur, which is made up of a woolly underfur overlain by blade-like guard hairs. On the platypus' back the long, coarse, flat-bladed guard hairs, range from deep amber to blackish brown, whereas on its underside they are a yellowish colour. These hairs also ensure that the fur structure remains intact. The fine, kinked, dense underfur, has a similar feel to wool, and ranges in colour from grey to dark brown, with it usually being lighter on the platypus' underside.
The platypus' fur provides it with insulation through a layer of air held by the underfur and the guard hairs. This air is trapped in the kinks the underfur and between the underfur & the blades of the guard hairs. It is a major contribution to the body's insulation. The fur on the tail is much sparser and so it offers little insulation. However, the fat it stores in this area does provide it with insulation.
Some fishermen used to believe that the platypus could breathe through its back because when it was submerged small bubbles of air would escape from where it was trapped and appear on the surface.
The legs of the platypus are short and each foot has five clawed digits. There is webbing on the front feet making them ideally suitable for swimming. The hind feet, which are only partially webbed, act as steering rudders. When out of the water and moving around the webbing is folded under the animal's feet, in order to prevent damage occurring and to uncover broad nails, which are ideally suited for digging.
The webbed feet are most likely to be the reason why the platypus got its name. This is because the webbed feet may give the impression that the platypus is flat-footed, which is what its name means.
The platypus' bill is covered with a soft, moist, naked, leathery skin, which, when touched, feels soft and rubbery. It is reported that the bill has a certain amount of flexibility, but the bones of the upper and lower jaw limit this. These are further apart at the tip than at the head. This structure can be found in all mammals, whereas other vertebrates, e.g. reptiles, have a lower jaw that is made up of several pairs of bones. The top of the bill is a blue-grey colour and slightly back from its tip are two nostril holes. The positioning of these allow it to breathe while the rest of its body is submerged. The lower bill, which is a pale pink or mottled colour on its underside, is smaller than the upper bill. At the back of the bill is the frontal shield that stretches slightly up and over the forehead. It is unknown as to what its purpose its.
The bill's entire surface is covered with openings to sensitive nerve endings (Nowak & Paradiso, 1983), which makes it very sensitive to touch. The platypus uses its bill in order to search for food and to find its way around when it is submerged. The platypus does not have any teeth instead, it has horn-like grinding pads, which cover most of each jaw. At the front of the bill these pads are sharp ridges, but as they progress towards the head they become more flattened, which makes them suitable for grinding food. These pads get worn down fairly rapidly, but this is compensated by the fact that they are continuously growing.
The teeth of young platypuses are small and calcified with little enamel and numerous stubby roots. At least two pairs of teeth have replacement buds beneath them (Nowak & Paradiso, 1983). On their progression to adulthood they develop into grinding pads. The tongue is flattened, and works against the grinding pads to aid mastication.
There are two grooves situated on either side of the platypus's head, just behind the bill. The eyes are situated at the front of each groove, and at the back are the ear openings. The platypus has no external ear lobes.
When diving, the platypus closes both its ears and eyes. This means it has to rely on other organs for finding its way about underwater, hence the sensitivity of its bill. However, when on land it has the use of its eyes, which are very acute over long distances. However, because of their location, it is unable to see what is literally 'under its nose'.
The platypus' ability to smell is not as developed as the echidnas (Nowak & Paradiso, 1983).
The adult male has an inwardly directed, hollow spur on the ankles of his hind legs, which is about 1.5cm long. It is hollow and connected to venom glands.
The venom is secreted by a gland, which is attached to the spur via a duct. It is then injected by erecting the spur. It is a colourless fluid containing a mixture of proteins and peptides. When injected the person feels an agonising pain that seems to suggest that it contains one or more extremely potent neuroirritants. As to what these neuroirritants are and the way in which they act is not yet known. However, one protein has been identified from the red blood cells of a human victim. It is the enzyme phospholipase D, which is found in some snakes, including the Australian Tiger Snake, but no other mammals. This strongly points to an evolutionary link between the platypus and reptiles.
The venom is potent enough to kill dogs, but in a human it causes severe pain, which is not alleviated by analgesics. The pain can last for at least 36 hours and the limbs may swell up like balloons. Following this, a rash can last for several months. A potential gamma-globulin antibody has been found for the venom, which could lead to an antivenene to treat a person who has been spurred.
The platypus uses its spur mainly to fight for females during the breeding season. It can also be used as a potentially effective mechanism for males to ensure that they have spatial separation from each other.
Looking at the way the spur changes during the first few years of the life can be used to age male platypuses. Up to the age of 6 months, the spur is enclosed in a sheath. Between 6 and 9 months about a third of the spur can be seen poking out of the sheath, which has begun to break down. Between 9 and 12 months (sub-adult) the sheath has broken down to such an extent that about two-thirds of the spur is showing. When the male has reached about 2 years the sheath can still be seen covering the base of the spur (Grant, 1989).
Juvenile females have a rudimentary version of the spur, but, unlike juvenile males, they lose it within their first year. The spur of the female does not change in size during the 8 to 10 months it can be seen, after which it is shed.
The spleen, thymus and gut-associated lymphoid tissue in the platypus are well developed. When contrasted with those in therian mammals, the histological structures are comparable (Connolly et al, 1999).
The lymphoid tissue of the platypus has all the necessary cell types, which are T and B lymphocytes and plasma cells. So when a foreign antigen is present the lymphoid tissue can provide an immune response.
The platypus has a skeleton that is heavy and strong enough to support large muscles, which it uses for both digging and swimming. The front limbs, especially, use large muscles in order to assist in digging and swimming. In order to assist the platypusí movement the leg bones and the shoulder girdles, which support these bones, are all fairly large. These girdles are similar to those of the goanna and other modern reptiles, and within them there is a bone called the interclavicle, which is shaped like a T. The pectoral girdles, on the other hand, have a similarity to a group of reptiles which are now extinct, but are thought to have been ancestors of the mammals. The platypus has a similarity to another group of animals, the marsupials. This is in its epipubic bones, which stick down at the base of the tail and hind legs, and are attached to its pelvic girdle. The purpose of these epipubic bones is unclear. It has been suggested that they are used to support a pouch, but this has been rejected because not all marsupials have a pouch, yet they still have these bones. The legs of the platypus are similar to both reptiles and mammals. Their splayed nature provides a link to reptiles, whereas the fact that they rotate in a socket suggests a mammalian connection. There are some rudimentary ribs in the platypusí neck, which are similar to those of some reptiles.
(Diagram adapted from Grant, 1989)
Research by Connolly et al (1999) have found a variety of information about the make-up of the platypusí blood. This is described in this section.
It has been found that the blood of the platypus contains levels of Vitamins D and A, which are well below the normal range for other animals. This might be because they spend the majority of their time underwater or in a burrow. This means they have very limited exposure to the sunís radiation, which is an important factor in the production of these vitamins. However, the blood levels of Vitamin E are within the usual normal range.
Compared to other animals, platypuses have higher packed red cell volumes, erythrocyte, and haemoglobulin concentrations and total leukocyte count. Whereas reticulocytes are not seen. Other haematological and serum biochemical values are relatively similar.
The greater concentration erythrocytes are probably due to the way the platypus has adapted to the hypoxia (reduction in the amount of oxygen delivered to the body), which occurs while they are in their burrow or diving.
The normal lymphocyte count in a wild platypus falls significantly within a few hours of capture.
It has been found that there is a difference in the haematological and serum ranges between Tasmanian and mainland Australian platypus populations. This is thought to be due to differences in specimen handling, types of laboratory used and geographical variations.
The platypusí method of excreting waste products is the same in both sexes. The bladder and the ureters enter a urinogenital sinus, which is connected to the cloaca, and through which urine is passed. Faeces, on the other hand, enter the cloaca at an opening behind the urinogenital sinus and are then excreted.
In order to determine an individual's sexuality, i.e. males or females, sex chromosomes are required. The platypus has 26 pairs of chromosomes, which is 3 more pairs than humans have. Of these, five of the pairs are sex chromosomes - humans only have one pair. One of the platypus's pairs of sex chromosomes is similar to humans X and Y sex chromosomes, but another pair is similar to that found in birds, which is the ZZ/ZW sex chromosome system. As a result of this evidence it questions the widely held belief that mammals' and birds' sex chromosomes evolved independently (GrŁtzner et al, 2004).
The penis of the male platypus is located inside the cloaca and is everted through this opening for the purpose of reproduction. Semen is carried from the testes to the penis by the urethra. The testes are located within the platypusí body and this condition is called testicond. Only a few other mammals, e.g. the elephant, some shrews, have a similar set-up. Other mammals have external testes contained in a scrotum, which hangs from the body.
(Diagram adapted from Grant, 1989)
As with other mammals the female platypus has two ovaries, but the right one is not very well developed and is non-functional. In the other monotremes, both their ovaries are developed. Female marsupials have separate uteri, which curve around and over the ureters, entering the urinogenital sinus at separate openings behind the opening of the bladder. During the birthing process these two openings become connected (Grant, 1989).
The female suckles its young using mammary glands. During the long period when lactation, the production of milk, is not required these glands are quite small, i.e. only spread over a small part of the femaleís abdomen. However, when milk is required by the young during the lactating season they become extremely large, and can cover up to a third of this area.
(Diagram adapted from Grant, 1989)
Diving & Swimming
The platypus' swimming technique involves it kicking its front legs alternatively, propelling it through the water, and the hind legs, which are only partially webbed, and the tail act as steering rudders.
On average, a platypus tends to remain submerged for about 1 minute. In 1983, Nowak & Paradiso observed a platypus remaining submerged for 5 minutes while holding onto an object. Evans et al (1994) observed another example of a platypus using an object to remain submerged. It submerged and then rested under an object for 11 minutes, surfaced for 10 seconds, dived for a further 7 minutes, resurfaced for 20 seconds and then submerged itself for a further 5 minutes. All these dives were inactive and the platypus was one living in captivity.
Before, during and after dives the heart rate of the platypus changes. Its normal resting heart rate is between 140 and 150 beats per minute. During active dives of one minute or less the heart rate is usually fairly active. After the platypus has completed a dive, its heartbeat is usually greater than its resting rate.
According to Evans et al (1994), at the beginning of a dive the heart rate decreases rapidly, its lowest level of 30 beats per minute after it has been submerged for about 15 seconds. However, on anticipation of surfacing, the heart rate increases (tachycardia). In dives of between 1 and 3.5 minutes, the heart rate can fall to 12 beats per minute while it is inactive. There is no increase in heart rate towards the end of the dive, and there are no signs of tachycardia after the dive.
Before the platypus dives it will take a deep breath, probably filling about half of its lung capacity, and then when it resurfaces it will exhale. It is roughly estimated that at the beginning of a dive a platypus has about 5mL of oxygen in its lungs, about 15mL in its blood, and about 5mL in its muscles. This amount of oxygen would appear to be suitable to sustain an active dive of about 60 seconds or an inactive one of about 3-4 minutes. When longer dives are undertaken, it is very likely that many parts of the platypusí body will have a limited supply of blood. Evans et al (1994) suggest that from this information it would seem to be that the platypus is capable of longer dives, but they do not state how long this could be.
The research by Evans et al (1994), some of which is described above, raises two questions. The first is that their study was done on captive platypuses. So, would wild ones demonstrate the same diving behaviour, heart rate and oxygen usage? The second question concerns the platypusí desire to remain submerged. Could it be doing this in order to hide from a predator or could it be behaving in this way in order to sift food that would be carried in the flow of the river?
Apart from the seal, the platypus is the most aquatically adapted animal in Australia.
It has been reported that the platypus is capable of making noises, but these have rarely been heard. They include a growl that is similar to the one a puppy would make and a noise that is comparable with that of a brooding hen. It is thought that they would use this when they are in danger.
The platypus will usually sleep in a curled position on its side. While in this state it has been found that the platypus has rapid-eye movement (REM). In research on humans it seems to be that REM is important in allowing the brain to either develop nerve endings or restore their sensitivities (Bernstein et al, 1991). The fact that the platypus displays REM is important for two reasons. The first is that it seems to indicate that it uses sleep to allow its brain to develop/recharge itself. The second, which is possibly the more significant, is that REM has been around since a very early stage in animalsí evolution.
Bernstein, D.A., Roy, E.J., Srull, T.K., & Wickens, C.D., 1991. Psychology. Boston: Houghton Mifflin Company.
Connolly, J.H., Canfield, P.J., McClure, S.J., & Whittington, R.J., 1999. Histological and immunohistological investigation of lymphoid tissue in the platypus (Ornithorhynchus Anatinus). Journal of Anatomy. 195. Pp.161-171.
Connolly, J.H., & Obendorf, D.L., 1998. Distribution, Captures and Physical Characteristics of the Platypus (Ornithorhynchus Anatinus) in Tasmania. Australian Mammology. 20, pp.231-237.
GrŁtzner, F., Rens, W., Tsend-Ayush, E., El-Mogharbel, N., O'Brien, P.C.M., Jones, R.C., Ferguson-Smith, M.A., & Marshall Graves, J.A., 2004. In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes. Nature. 432, pp.913-917
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