Rosemary Booth and Joanne Connolly

Platypuses are sexually dimorphic. Males are larger than females (45–63 cm long and weigh 1000–3000 g, females are 39 – 55 cm long and weigh 700–1750 g) and can be distinguished from adult females by the presence of a spur on the medial side of the tarsus (Grant 1989; Connolly & Obendorf 1998). Bergmann’s rule applies to the platypus, with larger animals found in the cooler parts of their range (Dunn 1949).

Monotremes are long-lived for small mammals and have been maintained for up to 21 years in captivity. Grant et al. (2004a) recorded a female platypus surviving 21 years in the wild.

Platypuses occur in freshwater streams and lakes along the eastern seaboard of Australia from Cooktown to Tasmania, mainly east of the Great Dividing Range (west only in permanent rivers) and were introduced to Kangaroo Island in 1940. They occupy a wide range of habitats, from tropical to cool temperate and pristine to degraded, and are classified as common but are potentially vulnerable to environmental perturbation (Grant & Temple-Smith 1998). Prior to their protection, thousands of platypuses were killed for the fur trade. Legal protection of platypuses was first given in Victoria in 1892, with all other states following suit by 1912 (Grant 1995).

Platypuses have a solitary nature (except when breeding or suckling) and are mainly nocturnal, but may be crepuscular during the winter. In some locations they are regularly diurnal. They depend on relatively undisturbed stream banks to support their resting and nesting burrows and show site fidelity with riverine home ranges of up to 7 km (Gardner & Serena 1995). Juvenile platypuses disperse, moving away from their natal stream.

2 ANATOMY AND PHYSIOLOGY

2.1 External features

The platypus has a streamlined body that is compressed dorsoventrally and is covered by fur except for the bill and feet. Platypuses moult seasonally with patchy loss of guard hairs. Females may also lose significant hair on the dorsal surface of the tail during the breeding season due to pugging of burrows (the process of building a mud partition within the tunnel). The platypus snout is covered by soft leathery pigmented skin (Fig. 6.1). The dorsal nostril position allows the platypus to breathe while most of its body is underwater.

Figure 6.1 The head of the platypus. The eyes (1) and ears (2) are closed. The pores in the skin of the bill and frontal shield (3) are the sites of the electro-receptors and mechano-receptors. Photo: Taronga Zoo.

Male platypuses possess a crural system consisting of a venom gland on the dorsal aspect of the thigh, connected by a duct to a hollow keratinous spur on the tarsus. The spurs are used offensively by bringing them together in the ventral midline with powerful jabbing movements. Venom is pumped through a fine barely perceptible duct in the centre of the spur. The size and activity of the gland is seasonal, consistent with androgenic control. Venom appears to be produced only by sexually mature males from about 4 yrs of age. Aggressive use of the spurs in fights between males is confined to the breeding season. Reports of dogs being spurred shows that male platypuses can use their spurs to defend themselves from predators (Burrell 1927).

The toxin, of which a major component is a natriuretic peptide, can cause death in mice, dogs and captive platypuses. It causes severe and prolonged pain, oedema and localised tissue necrosis in humans, but no fatalities have been reported (de Plater et al. 1995; Fenner et al. 1992). Up to 1994 there were 15 recorded cases of human envenomation and the Commonwealth Serum Laboratory received at least one inquiry regarding treatment annually. There is currently no available antivenene (Tonkin & Negrine 1994). Supportive therapy in people has included ice, elevation, splinting, opioid IV infusion, regional nerve blocks, prednisolone and/or antibiotics. See section 8.1 for prevention by correct handling.

Juvenile females have a rudimentary spur sheath on the tarsus which is lost at around 8 mo age. There is no crural gland in females. The size and shape of the spur can be used to estimate the age of male platypuses (Fig. 6.2).

Venom can be collected from adult male platypuses (>4 yr) during the breeding season for research purposes. In some cases the venom drips from the opening in the spur. It is also possible to gently milk it down from the crural gland at the back of the hind leg. Venom can be collected into microhaematocrit tubes or via a 2 mL syringe connected to a section of drip tubing connected to the spur, to draw out up to 0.05 mL (L Vogelnest pers. comm.).

2.3 Skeleton

The platypus skull has a large braincase to house the well-developed forebrain (which lacks sulci and gyri, and a corpus callosum between hemispheres). A reptilian feature of the platypus skull is the ectopterygoid bones. There are no tympanic bullae. The vertebral column of the platypus comprises 49 bones (7C 17T 2L 2S 21Co). The cervical vertebrae bear cervical ribs and each thoracic rib has ossified sternal and vertebral portions, with cartilage between the two, like reptiles. The pectoral girdle of the platypus provides a sturdy base for the digging action of the fore limbs and is more like that of lizards. It consists of paired scapulae, clavicles, coracoids, epicoracoids and a T-shaped interclavicle attached to the sternum. The pelvic girdle is typically mammalian with paired ilia, pubes and ischia, but there are also epipubic bones (as in marsupials) projecting forward from pubic bones (Griffiths 1978).

Figure 6.2 The presence of spurs in platypuses confirms age and sex. a) Subadult female, 3–8 mo. b) Juvenile male, 0–6 mo. c) Juvenile male, 6–9 mo. d) Subadult male, 9–12 mo. e) Adult male, >2 yr. f) Old adult male. Photo: P Temple-Smith.

The skin of the platypus’ bill contains over 100 000 electro-receptors and mechano-receptors, first reported by Scheich et al. 1986, which allows it to locate prey underwater with its eyes, ears and nostrils closed. The bill’s sensory representation in the brain is nearly three-fold that of the visual and auditory representation (Pettigrew et al. 1998). The platypus bill receives its massive innervation from the well-developed trigeminal nerve (cranial nerve V). Electro-receptors are large sensory mucous glands that are distributed over the entire surface of the bill, including the frontal shields (Gregory et al. 1989) (Fig. 6.1). These receptors are extremely sensitive and allow detection of the electric field generated by the single tail-flick of a shrimp at a distance of 10 cm (Scheich et al. 1986). Mechano-receptors are pushrods which respond to tactile stimulation.

Adult platypuses do not have teeth. In early life they possess 1 premolar and 2 molars in each maxilla and 2 – 3 molars in each mandible. These small teeth are resorbed and replaced by keratinous grinding pads soon after the young platypuses emerge from the nesting burrow (Fig. 6.3). Despite the lack of true teeth, mastication is a significant component of monotreme digestion. Insect and crustacean prey are collected into the cheek pouches and thoroughly ground and sifted to remove much of the exoskeletons.

The tongue of the platypus has specialisations to direct food to and from the cheek pouches, from where it is repeatedly ground and filtered so that only the more digestible components of the diet are swallowed (Fig. 6.3).

The remainder of the digestive system is simple. The stomach of platypuses is lined with cornified stratified epithelium to further grind insect prey. The stomach is very small and receives ground and filtered material from the cheek pouches. This is significant during hand-rearing or tube-feeding, where volumes fed should be conservative (see 4.4). A small (2.5 cm) caecum is located at the ileocolic junction (Whittington 1988; Connollly et al. 1999a). Because of the readily assimilated ingesta, the gut transit time is short. A barium study of a mature healthy captive platypus revealed a transit time of about 5 hr (Fig. 6.4).

2.5 Cardiopulmonary system

The platypus has a four-chambered heart with a closed double circulation. The systemic aorta is derived from the left fourth arterial arch. Great cardiac veins open directly into the right atrium. A coronary vein, which is not present in other mammals, also empties directly into the right atrium. A pelvic rete mirabile is present (see 2.9).

Figure 6.3 The bottom jaw and some of the oral specialisations of the platypus. 1) Filtering serrations on bottom jaw. 2) Bilateral secateuring ridges in top and bottom jaw. 3) Cheek pouches. 4) Keratinous grinding pads. 5) Torus linguae. 6) Body of tongue.

Figure 6.4 Barium study of a conscious captive platypus (DV views). a) Time 0 showing barium in stomach already entering small intestine. b) 1 hr, showing the barium in the small intestine. c) 5 hr later, showing barium in transverse and descending colon. Photos: R. Booth.

Platypus lungs are alveolar and have the typical mammalian tree-like branching system of intrapulmonary bronchi, with one lobe on the left and two lobes on the right. A diaphragm is present and is functional in respiration.

The female platypus has paired ovaries, but only the left ovary is functional (the right is rudimentary, as in birds). The ovary bears many large follicles projecting from its surface, as in reptiles. Each ovary is enclosed by the infundibulum of the lightly convoluted oviduct. Each oviduct leads to a separate uterus, which opens independently into the urogenital sinus, thence the cloaca. The platypus is oviparous, laying 1–3 leathery eggs, 14 3 17 mm in size and weighing 1.5–2 g (Griffiths 1978; Grant 1995).

The male platypus is testicond, the abdominal testes lie caudo-dorsal to the kidneys. Seminiferous tubules in the testis drain via efferent ducts to the large epididymis and via a short vas deferens to the rostral end of the urogenital sinus. The penis lies in the caudal urogenital sinus, within a preputial sac ventral to the cloaca. The erect penis is about 7 cm long, and is extruded through the cloaca and the cloacal sphincter. The shaft of the penis bears spines and each half of the bifid glans has four evertible foliate papillae. The penile urethra, which communicates with the urogenital sinus, carries only semen (not urine). There are no ampullae, coagulating glands or seminal vesicles. Also, the prostate is disseminate rather than discrete, and bulbourethral glands are present at the base of the penile urethra (Carrick & Hughes 1978; Griffiths 1978).

2.7 Immune system

The platypus has the full repertoire of lymphoid tissues present in mammals, including a grossly visible spleen, thymus and lymphoid nodules (0.222 mm in cervical, thoracic, pelvic and mesentery sites), as well as histologically detectible gut-associated lymphoid tissue (tonsils, Peyer’s patches and caecal lymphoid tissue) and bronchus-associated lymphoid tissue (Connolly et al. 1999a). These lymphoid tissues contain all the key elements (T and B lymphocytes and plasma cells) to mount an effective immune response against foreign antigens.

IgM, IgG (two subclasses), IgA (two subclasses) and IgE heavy chains have been isolated from the platypus (Belov & Hellman 2003). At this stage it is not known whether the platypus has IgD. Immunoglobulins have been purified from platypus serum and used to detect the immune response to Mucor amphibiorum (Whittington et al. 2002).

2.8 Endocrine and exocrine systems

The pituitary of the platypus is a large pear-shaped body lying adjacent to the tuber cinereum of the hypothalamus. It contains two separate endocrine glands, the pars nervosa and the pars anterior or distalis (Griffiths 1978).

The normal thyroid gland in the platypus has been described as a reddish flattened body, 2 cm 3 1.75 cm, lying just proximal to the arch of the aorta in the thoracic cavity (McKenzie 1921). The parathyroids are 3 mm 3 2 mm and sit either side of the proximal trachea just distal to the thyroid cartilage. The thymus lies around the base of the heart and extends onto the ventral surface of the pericardium, and is paler than the thyroid.

The paired adrenal glands sit anterior to the kidneys and measure 18 mm 3 8 mm. Chromaffin tissue is located at the broader caudal end of the pear-shaped adrenal gland, with cortical tissue at the anterior end (Griffiths 1978). Platypuses have large adrenal glands in proportion to body size. The adrenal gland body weight ratio for the platypus is 260 mg/kg compared to 40 mg/kg in echidnas (McDonald 1978). The reason is unknown, but large adrenal glands would support adaptation to change.

Platypus skin has abundant eccrine and apocrine sweat glands, but these are thought to be ineffective in evaporative cooling due to the presence of the dense fur (Griffiths 1978). Sweating from the poorly furred ventral side of the tail has been observed in the platypus in the laboratory (Dawson 1983).

There is a subcutaneous elliptical, translucent yellow, prescapular scent gland which is used for territorial marking. Captive males have been observed rubbing their neck and shoulders on tussocks in their exhibits during the breeding season. The size and activity of the gland is seasonal, consistent with androgenic control, and the gland is larger in males than females.

Platypus mammary glands lack teats but have two areas called areolae (milk patches), onto which the ducts of about 150 mammary lobules open independently and where the young suckles the milk. There are marked seasonal changes in the anatomy and histology of the mammary glands. At peak lactation, the platypus mammary glands are palpable extending from the axilla to the groin (Griffiths 1978).

Other exocrine glands noted in the platypus include paired bulbourethral and disseminate prostatic glands in the male reproductive tract; uterine glands in the female reproductive tract; Brunner’s glands, intestinal glands and an exocrine pancreas in the digestive system; and crural glands in the male.

2.9 Physiological adaptations for diving and the aquatic environment

Platypuses have a number of anatomical and physiological adaptations to diving. When feeding, repeated short dives of 30 sec–3 min duration are made (Griffiths 1978; Grant 1995). Early studies of involuntary submersion in platypuses showed rapid changes and restoration of arterial oxygen and carbon dioxide. Blood pH fell along with the rise in carbon dioxide during submersion. Recovery in arterial oxygen levels occurred in 2 min, although restoration of pH to pre-submersion levels took 9–10 min.

Bradycardia develops gradually during submersion, and there is sudden release of cardiac inhibition on surfacing (Griffiths 1978). The normal heart rate when swimming on the surface is 140–230 beats/min, compared with 10–120 beats/min during forced dives (Grant 1995).

The platypus is able to thermoregulate in an aquatic environment even in alpine regions. Its thermoregulatory abilities at high temperatures are poor. It has adaptive mechanisms to reduce heat loss and thus minimise energy expenditure (Dawson 1983). The platypus has an extensive pelvic network (rete mirabile) of small parallel arterial and venous vessels that supply the muscles of the hind limbs and tail (Grant 1989). This vascular specialisation is believed to be a mechanism of counter-current heat exchange, serving to restrict heat loss and enable efficient thermoregulation. This rete is also thought to reduce the circulation and therefore oxygen supply to the hind limbs during a dive (Grant & Dawson 1978) to conserve oxygen for the brain.

The platypus has highly insulative fur and can forage for up to 12 hr in water at 0°C (Grant 1995). The guard hairs and dense underfur of the platypus trap a layer of air close to the skin, reducing heat loss.

The tail is the principal fat storage site in platypuses (about 40% of body fat). Body condition can be assessed by the tail fat index (Table 6.2).

Haemoglobin levels are also higher in platypuses (highest in nestlings) than in terrestrial mammals, believed to be an adaptation to low oxygen in the burrow and diving (see 7.1.2).

The platypus has a low average resting body temperature and a low basal metabolic rate. Physiological data for adult platypuses is presented in Table 6.1.

3 REPRODUCTION AND REPRODUCTIVE MANAGEMENT

There are still gaps in our knowledge of reproduction in platypuses. Although platypuses have been kept in captivity since the 1800s, they have been successfully bred in captivity only six times in three separate and distinctly different circumstances. The temperament of individual platypuses is the most critical factor in whether captive platypuses will breed. It would be misleading to write a ‘how to breed platypuses in captivity’ section based on these infrequent successes.

Table 6.1 Useful physiological values for clinically normal adult platypuses

a Grigg et al. (1992) using implanted radiotransmitters on 5 wild platypuses in Thredbo between April and October. Cloacal temperature measured under anaesthesia is consistent with this range, although temperatures above 33°C would cause concern.
b Grant (1995).

In captivity, success has been achieved with males and females housed with separate tunnels and nest boxes and feeding tanks, and with a communal feeding tank which allows the animals to choose to be together. They have also been bred in semi-natural situations such as a walk-through aviary and in farm dams. All successful breeding has occurred when females have had access to earth to build their own burrows. Cave-ins have occurred, so earth should have high clay content and have plant roots and logs to provide support.

Platypuses in the wild do not live in pairs, rather, a dominant male appears to patrol a length of stream inhabited by a number of females. Mating occurs in late winter or early spring (Grant et al. 2004a). Breeding season varies with latitude, with platypuses in north Queensland commencing breeding earlier than in colder regions such as Victoria (Connolly & Obendorf 1998). Breeding usually commences in August in Queensland, September in New South Wales, and October in Tasmania (Grant et al. 2004a). In New South Wales the highest proportion of lactating females are captured in December and January About 40% of female platypuses do not breed in consecutive seasons (Grant et al. 2004a). Female platypuses can breed in the wild as early as 2 yr of age and up to 16 yr of age (Grant 1995). In captivity, the average age of females involved in any kind of reproductive behaviour was 6.2 yr (Holland & Jackson 2002).

Reproductive seasonality in the platypus has been assessed by measuring progesterone and androgen concentrations in blood, faecal and urine samples using radioimmunoassay (Jakubowski et al. 1998; New et al. 1998). Progesterone concentrations in females from plasma and excreta were higher in July–September, indicating significant ovarian activity during this period. Androgen concentrations in adult males from plasma and excreta were higher in July–August. Cortisol concentrations were higher and males lost body condition during this period, indicating stress due to rivalry during the breeding season.

The courtship ritual of the platypus involves processional swimming with the male holding the extremity of the female’s tail in his bill. The two animals swim in circles and intermittently preen and nuzzle each other. From the tail-biting position, the male doubles his body under to insert the penis into the female’s cloaca. Mating lasts for 10 min (Fleay 1980). The male spurs are not used in the mating ritual. The male plays no part in rearing the young and in captivity should be separated from the female after mating has been confirmed. Courtship behaviour can last several days. Mating occurred during three consecutive days in the 1998/99 Healesville Sanctuary breeding (Holland & Jackson 2002).

3.2 Gestation and incubation

The female may excavate a nursery burrow 4 mo in advance of mating. The female increases her feed intake for 9–11 d following mating then commences nest-building (i.e. lining the nursery burrow), requiring a supply of leaves, reeds and grass which must be provided in the water. Nesting material is carried in the prehensile tail. Nest-building stops abruptly after 3–5 d (Fleay 1980; Holland & Jackson 2002). In 1945, the female platypus Jill fed all day for 2 d from completion of her nest and disappeared into the nursery burrow 14 d after mating to lay and incubate her eggs. Five days later two eggs were dug out of her nest, the breeding season after the famous first breeding of Corrie (Fleay 1980). Gestation therefore is greater than 14 d and less than 19 d. Platypuses lay 1–3 eggs (Burrell 1927). The female spends 11–12 d almost exclusively on the nest, which probably represents the incubation period. In 1944, Jill did not leave the nursery burrow at all for 5 d, then left for 30 min toilet and grooming breaks every few days. Not until 12 d after presumed laying did she spend increasing time away from the nursery burrow and her hatchling (Fleay 1944). In 1998, Koorina also spent 5 d confined to the burrow before emerging for progressively longer intervals (Holland & Jackson 2002). The female instinctively blocks (pugs) the nursery burrow when she comes and goes during lactation, which emphasises the need for access to soil to facilitate captive breeding. Pugging involves using the tail to fashion multiple packed-earth blockades (pugs) in the tunnel, which causes progressive hair loss on the dorsal tail tip. On all occasions where platypuses have bred in captivity, females have had access to deep soil to construct their own nursery burrows. During the first 6 wk of lactation, the female’s appetite gradually increases to almost her own body weight in daily intake.

From hatching, after an 11–12 d incubation period the young platypus feeds by nuzzling the mammary area of the female’s abdomen. The curled posture of the mother platypus probably holds the baby firmly in place and keeps it warm. The eyes remain closed for 11 wk, and the baby platypus is fat and unco-ordinated until 15–17 wk age when it emerges for its first clumsy swim and introduction to solid food (123–136 days, n = 3) (Fleay 1980; Holland & Jackson 2002). From 17–20 wk the appetite for solids increases to maintenance levels. The young platypus continues to den with its mother until it is 5 mo old (Fleay 1944).

3.4 Lactation

Lactation occurs in October–February in New South Wales (Grant & Griffiths 1992) and November–May in Tasmania (Connolly & Obendorf 1998). Lactation can be detected by administering oxytocin at 1–2 IU/kg IM. Five minutes after injection, milk can be collected from the areolae using haematocrit tubes or pipettes. During lactation the fur covering the mammary region of a lactating female is discoloured (brownish yellow) and worn compared to fur elsewhere on the abdomen.

Platypus milk is made up of 38% total solids, mainly comprised of 22% crude lipid, 8% crude protein and 3% total hexose (n = 20) (Griffiths et al. 1984). Analysis of the fatty acid components of the triglyceride fractions of platypus milk indicated no major change in composition during lactation and the predominant fatty acids were oleic acid (25%) and palmitic acid (18%) (Grant et al. 1983). Platypus milk, like echidna milk and marsupial milk, exhibits very high concentrations of iron, up to 21 mg/kg of milk (n = 7). Given the minute size of the monotreme and marsupial liver at birth, the storage of iron would be limited and the need for iron to synthesize haemoglobin essential (Griffiths 1988).

4 PLATYPUS RESCUE

There are relatively few platypuses in captivity in Australian zoos. As platypuses are widespread and common, it is not unusual for veterinarians and wild-life carers to be presented with rescued animals. The following section aims to provide some guidance on how to assess and manage a rescued platypus.

4.1 Decision-making

1 Is the platypus an adult male and do I need to be careful handling it to avoid being spurred?

Grab the animal firmly by the tail and suspend it away from yourself and look for a spur on the heel of either hind leg (Fig. 6.2). If present, handle the animal with great care. Adult males are more likely to try to spur you during the breeding season from

July–December.

If no, go to 2.

If yes, go to 5.

2 Is the platypus a lactating female?

During the breeding season, attention should be paid to whether an adult female platypus is lactating because, if so, priority should be given to returning her to the close vicinity of her young if she is healthy enough. Signs of lactation include loss of guard hairs in the mammary region of the abdomen and oozing of milk in response to gentle pressure. The lactation status of female platypuses can also be assessed using oxytocin at 1 IU IM to induce milk letdown. Lactation occurs in October-February in New South Wales (Grant & Griffiths 1992) and November-May in Tasmania (Connolly & Obendorf 1998).
If no, go to 3.
If yes, go to 5.

3 Is the platypus a milk-dependent juvenile?

Unweaned juveniles have a very short bill only as long as it is wide. If it has a square bill <30 mm × 30 mm, it is still milk-dependent (Fig. 6.5). Arrows indicate where to take measurements. A) Length of bill from tip to crease (do not include bill shield). B) Maximum bill width. Another telltale feature of a milk-dependent juvenile is that it will try to stimulate milk letdown from your hand if you put it in the position shown in Figure 6.5, and if offered milk it will slurp it up. They are generally tame and unafraid of people. Young that have not yet left the burrow will often be very fat. Body weight is not a useful indicator of age because platypuses vary enormously in weight with geographic location, age, sex and condition. A platypus weighing 600 g could be a subadult, a fat juvenile or an emaciated adult. A milk-dependent juvenile platypus will require intensive care for some weeks.

Figure 6.5 A milk-dependent juvenile Victorian platypus weighing 680 g and with a bill measuring 29 mm at A and B. This is the typical suckling posture of monotremes. Photo: R Booth.

Most platypuses that come into care are dispersing juveniles (Fig. 6.6). If they are not sick or injured, rapid return (within 24 hr) to suitable habitat close to point of origin should be considered a priority.

If yes, check thoroughly for evidence of disease or injury (see 5) before release.

5 Is the platypus sick or injured?

A strap-like tail, tameness and poor waterproofing are classic signs of ill health in platypuses. The animal’s condition can be assessed using the tail fat index (Table 6.2). If the animal is in poor condition it may be diseased or it may be a dispersing juvenile which has had trouble finding enough food. An experienced wildlife veterinarian should be enlisted to help determine whether a rescued platypus is sick or injured. A healthy platypus often growls on handling and wriggles constantly, and its fur dries within 10–15 min of leaving the water.

Figure 6.6 A dispersing juvenile with the bill now more rectangular than square. A) 37 mm. B) 32 mm. Photo: R Booth.

Table 6.2 The tail fat index of the platypus as a measure of condition

• 50 g yabby tails (sick or young platypuses cannot usually kill and break up yabbies without help);
  
• fly pupae, crickets and mudeyes (dragonfly larvae, which are available from good fishing bait stores).