David Middleton

1 TAXONOMY, DISTRIBUTION AND BIOLOGY

The short-beaked echidna (Tachyglossus aculeatus) is native to Australia and New Guinea. The larger, rare long-beaked echidna (Zaglossus bruijni) is confined to the New Guinea highlands. Together, these two species are the only extant representatives of their genera. Echidnas are well-represented in the Australian fossil record and, along with the platypus, are the only living remnants of the egg-laying monotremes, thought to represent the most ancient mammalian lineage surviving to the present day. Echidnas probably evolved from the platypus during the Cretaceous period (Augee & Gooden 1993) and it is possible that monotremes have been around for more than 50 million years. There are five recognised subspecies of Tachyglossus aculeatus: T.a. acanthion from the Northern Territory, northern Queensland, inland Australia and Western Australia, T.a. aculeatus from eastern New South Wales, Victoria and southern Queensland, T.a. lawesii from New Guinea, T.a. multiaculeatus from South Australia and T.a. setosus from Tasmania (Augee et al. 2006).

The name echidna derives from Ekhidna, a Greek goddess who was half woman and half reptile. Monotremes possess unique bones in their pectoral girdles: the epicoracoids and the interclavicle, both structural adaptations for digging. The monotreme pectoral girdle thus has similarities to the extinct cyanodonts and to living crocodilians. The echidna was initially identified as a relative of South America’s anteater (a placental mammal) simply through lifestyle similarity, and it was not until 1802 that its close relationship to the platypus was appreciated.

The earliest record of a short-beaked echidna in captivity was from Philadelphia Zoo in 1903, an individual that was still alive in 1953, 50 yr later. This represents the longest recorded lifetime of a short-beaked echidna (Augee et al. 2006).

The short-beaked echidna has been able to exploit all terrestrial habitats including those of Tasmania and many off-shore islands and is today the most widely distributed Australian mammal. In spite of its terrestrial nature the echidna is a strong swimmer and climber. The author once encountered a healthy specimen in fresh water 1 km from the nearest land, and a captive echidna at Healesville Sanctuary (Vic.) was observed to climb a tree in its enclosure to a height of 2 m or more on a regular basis. The species is currently secure throughout most of its present range.

2 ANATOMY AND PHYSIOLOGY

2.1 External features, sensory organs, skin and associated structures

The short-beaked echidna is rotund and furred with a small head terminating in a cylindrical beak or snout. Adults can grow to a body length of 40 cm and weigh up to 7 kg. The dorsum and flanks are covered by long quills. These quills are under neuromuscular control and can be erected in defence as well as being used to grip surroundings. The quills are fixed, do not readily fall out and cannot be ejected from the skin. The body, apart from the beak, is covered in coarse hair. The density of the hair varies from heavy to sparse in different individuals. Tasmanian echidnas are smaller and have a denser hair coat. When threatened, the echidna can burrow vertically downwards quite rapidly, leaving only the spine-covered dorsum exposed. When on impenetrable substrates, the head and neck can be flexed ventro-caudally and the body curled to produce a spine-covered ball, making it very difficult for a predator to gain access to unprotected areas of skin. This is accomplished by the enormous SC muscle panniculus carnosus, which covers the sides and dorsum. This adaptation is most helpful for a creature that spends its foraging hours with its entire head buried in soil, searching for ants and termites.

The skin is thick, with a fat storage layer, the panniculus adiposus, underlying the panniculus carnosus. Sweat glands are restricted to the pouch area. Nipples are absent and milk is expressed directly onto the skin of the anterior pouch via specialised hair follicles concentrated into two regions (patches or areolae) of the anterior pouch skin. During feeding, the juvenile echidna sucks the milk from the skin and hair rather than licking it (Augee et al. 2006). The echidna’s ‘beak’ is a highly refined structure of bone, cartilage and soft tissue covered in dark shiny hairless skin. Its tip is equipped with a number of sensitive electro-receptors and mechano-receptors which may be used in the location of invertebrate prey items. The beak electro-receptors can detect very small electrical impulses emitted by muscular contractions in prey (Proske 1990). The paired nostrils face upwards at the tip of the beak and the mouth opening is small. The echidna produces copious amounts of tenacious saliva from the sublingual glands. This aids in food procurement, but when combined with the narrowness of the external nares results in almost constant nasal discharge accompanied by bubbling and sniffling (Fig. 5.10). These signs are present to a degree in most normal echidnas and do not usually indicate respiratory disease.

The external ear canal is well-disguised by hair and opens as a vertical slit protected by a ridge of muscle and cartilage (Fig. 5.1). Echidnas have acute hearing aided by the arrangement of the auditory ossicles. The incus and malleus are fused, and attached to the petrosal bone. The body of the echidna conducts sound quite efficiently to the ear in particular via the snout which, when in contact with the substrate, can detect low noise and vibration (Griffiths 1989; Augee et al. 2006).

Figure 5.1 Echidnas have well-developed ears which are disguised by SC musculature as narrow vertical slits. The arrowhead indicates the site for needle insertion for blood collection from the beak sinus.

The echidna’s eyes are small but well-developed. The retina is equipped with a greater proportion of cones than the human retina, and may provide good colour vision. There is almost complete decussation of the nerve fibres from each eye to the opposite side of the brain, which may limit binocular vision. In addition to electro- and mechano-receptors, echidnas probably rely heavily on olfaction for the location of prey and for seeking out oestrous females during the mating season. Blind echidnas have been reported to survive in the wild (Augee & Gooden 1993). The cerebrum of the echidna appears well-developed, deeply folded and relatively large. The frontal cortex of the echidna’s brain is proportionately larger that of other mammals, though the significance of this is unclear (Griffiths 1989). This may be explained to some degree by the amount and nature of the various sensory inputs from the snout. Echidnas have been described by zoo keepers as sensitive, complex and intelligent and this may correlate with the size of the frontal cortex (Augee & Gooden 1993).

The legs and feet are stout and powerfully equipped for digging with well-developed blunt flattened claws. There are five clawed digits on the manus and four on the pes, with the most medial hind digit (digit one) greatly reduced. The second digit on the pes is elongated and, although often recognised as a useful tool for digging and excavation of foraging grounds, is also used for grooming as the shorter digits are insufficient to reach areas of the skin between the spines (Wood Jones 1923).

All male and some female echidnas possess spurs on the medial aspect of the tarsus. These are considered a remnant of an ancestral defence mechanism. A recent study concluded that the presence of spurs can be used to confirm the sex of adult echidnas, since no externally detectable spurs were present in any females in that study (Johnston et al. 2006b). However, this method may not be reliable in juvenile echidnas.

2.2 Digestive system

Echidnas have no teeth—food items are secured through the use of an elongated tongue constantly moistened with sticky saliva. The tenacious saliva, rich in glycoprotein, is produced in large amounts by the paired sublingual salivary glands which open to the floor of the mouth via multiple ducts. This ‘glue’ assists in the normal prehension and mastication of food items. The paired submandibular salivary glands are palpable as two distinct firm flattened mobile SC masses on the ventral neck. The tongue is long and slender, can be extended at least 18 cm beyond the tip of the snout and has a sophisticated arrangement of longitudinal and circular muscle bundles. It has a cavernous vascular network that gives it an erectile capability to forcefully probe for food in such substrates as rotten logs and termite mounds. It can protrude and retract up to 100 times per min and probe arthropod-rich crevices with great efficiency. With its strong fore limbs the echidna breaks open termite mounds, rotten logs and ant nests to probe for food. During foraging, the eyes remain closed. Prey items are crushed between backward-directed keratinous spines on the base of the tongue and above on the hard palate. This mastication can be detected audibly as a distinct grinding sound. As a result of this method of feeding, much soil and sand are ingested.

The digestive tract has a simple monogastric layout. The stomach is largely non-glandular and not involved in peptic digestion, exhibiting a relatively high pH, measured in one study at 6.2 (Griffiths 1989). It is lined by a cornified stratified epithelium which can grind invertebrate bodies with the aid of the ever-present dirt particles. The intestine of the echidna is 3.5 m long and exhibits a slow rate of throughput, with a clearance of 2 d for a termite and ant-based meal (Griffiths 1989). Faeces are solid, smooth and cylindrical and contain the undigested exoskeletons of prey items as well as large quantities of dirt. The small intestinal mucosa has high trehalase activity corresponding to the high trehalose content of termites and ants. These prey items lack lactose and sucrose, hence lactase and sucrase activity is virtually indetectable (Griffiths 1989).

Figure 5.2 Whole-body radiograph of a sub-adult short-beaked echidna demonstrating some of its unique skeletal features.

The echidna’s skeleton exhibits a number of interesting radiographic features (Fig. 5.2). The pectoral girdle has two additional bones, the epicoracoids and the interclavicle (structural adaptations for digging). There are cervical ribs, cranially projecting scapulae, an unfused acetabulum with a central foramen, outward-rotated hind limbs (that result in a backward-facing pes), transposition of the tibia and fibula, epipubic bones forward of the pubis and a short flattened femur. The tibia and fibula are of equivalent size and can be confused radio-graphically The fore limbs are massive, with the radius and ulna easily matching the femur for size and strength. The scapulae are large. The humerus is horizontal and the glenoid broad and shallow. The paired mandibles are delicate and assist in the opening of the mouth by rotating medially. The vertebral formula of the echidna is 7C, 16T, 3L, 3S and 12Co (Griffith 1989).

2.4 Genitourinary system

The kidney of the echidna can produce concentrated urine that may be as high as 2300 mOsm/kg. The kidney’s ability to conserve water in this way plays an important part in the echidna’s capacity to live and thrive in arid environments.

2.4.1 Male

The reproductive tract of the male echidna is unique among mammals. The two ovoid testicles are suspended within the abdominal cavity caudal to the kidneys. There are no seminal vesicles and no discrete prostate gland. Sperm are produced by the testes and pass directly into the epididymis and on into the ductus deferens before entering the urogenital sinus and subsequently the penile urethra. The penile urethra passes only sperm. Urine is passed via the ureters to the urogenital sinus and has the opportunity to mix with sperm for a short time as the ureters and ductus deferentia join immediately prior to entering the urogenital sinus. The process of sperm being shed into the urine is known as spermatorrhoea and has been used recently as an indicator of sperm production. Urine is not conveyed by the penile urethra but enters the cloaca directly via the urinary papilla, situated on the cloacal wall at the ventral base of the penis (Johnston et al. 2007).

Figure 5.3 The echidna penis. The glans is divided into four bulbous portions by three dorso-ventral grooves. Each portion is adorned with fine epidermal projections and each receives a branch of the penile urethra, which divides laterally to produce two branches each of which divides again.

Figure 5.3 provides a dorsal view of the extruded echidna penis. The penis is securely retracted into the preputial sac inside the cloaca when not in use. The penis in adult males is around 7 cm long. Mackenzie (1919) describes the glans as being divided into four bulbous portions by three dorso-ventral grooves. Each portion is adorned with fine epidermal projections and each receives a branch of the penile urethra, which divides laterally to produce two branches each of which divides again.

2.4.2 Female

The female echidna is oviparous. The female reproductive tract consists of two functioning ovaries (unlike the platypus, in which only the left is functional), each associated with an oviduct which terminates in an infundibulum. The oviducts join directly into the urogenital sinus. The bladder also joins directly to the urogenital sinus opposite and dorsal to the entry of the ureters.

2.5 Thermoregulation and hibernation

The echidna has a relatively low body temperature among the terrestrial vertebrates (32°C) and a metabolic rate one-third that of an equivalent sized dog or cat (Griffith 1989). The body temperature is variable and echidnas are able to enter torpor and to hibernate during cold periods. Torpor is a term more applicable to short-term cold snaps after which the animal can return to activity quickly. Hibernation more correctly relates to seasonal endurance with minimal periods of awakening, a period of metabolic preparation (e.g. deposition of fat reserves) and a more protracted return to activity at the end of winter. In the colder alpine parts of their range echidnas lose 3% of body weight per month during hibernation and regularly regain activity every 12 d before re-entering their hypothermic state. Individuals engaged in hibernation can be aroused by warming gradually over a period of days (Grigg et al. 1990).

Interestingly, the echidna demonstrates a variable dependence upon hibernation in accordance with their lifecycle needs such as reproduction. Immature individuals and adults not involved in mating have been shown to hibernate for longer periods (Grigg et al. 1990). Body temperatures as low as 4°C are possible in adult echidnas and even nestlings can enter torpor for some days to endure maternal foraging absences (Griffiths 1989). All this has great relevance for the clinician as the echidna is classified as a ‘heterotherm’ and as such is much affected by environmental temperature. Their thermoneutral range is 20–30°C. Below 20°C muscular contraction must generate heat if the echidna is to remain active; above around 28°C heat loss is minimal and hyperthermia begins to develop (Griffith 1989). Although the echidna is unable to use sweating or panting as a means of heat loss, it is particularly adept at finding shelter in hollow logs, rock crevices or underground and restricts its activity to the cooler parts of the day and night to avoid heat stress. It will also make use of free water for cooling but this is not essential, as evidenced by the presence of echidnas in dry habitats. Daily water intake of adult echidnas held at 28°C environmental temperature has been measured at 40–80 mL/d (Griffiths 1989).

2.6 Cardiopulmonary system

The heart rate of the echidna is about 115 beats per minute, blood pressure has been measured at 123/96 mmHg and cardiac output at 59 mL/kg/min (Griffiths 1989). The heart has two unique features: coronary veins that carry blood from the heart to the right atrium (a feature not found in other mammals or in birds) and papillary muscles that insert directly onto the heart valves in the absence of chordae-tendinae. The diaphragm is broad and muscular with a tendinous centre over the apex of the heart.

Echidnas are tolerant of high CO₂ concentrations in their inspired air (to 10%), and have a metabolic requirement for oxygen that is less than half that of eutherian mammals (Schmidt-Nielsen et al. 1966). At thermoneutrality, the adult echidna consumes 0.22 mL of oxygen per gram body weight per hour. This is further evidence that the echidna is very well adapted to environmental variation and capable of exploiting low oxygen/high carbon dioxide situations.

2.7 Immune system

The general anatomical layout of the lymphatic system is the same as in other mammals (Diener et al. 1967a). Monotremes have a diffuse lymphoid system composed of nodules which are suspended by a pedicle, dispersed throughout the lymphatic vessels. The lymphoid nodules within the intestinal mesentery are grossly visible as small white raised foci near the attachment of the mesentery to the intestinal serosa (Rose 1999).

The immunological response of the echidna is also similar to the general mammalian response, however, the anamnestic response is less intense and described as similar to that demonstrated by reptiles (Diener et al. 1967b). Wild echidnas seem to enjoy relatively good health and a low incidence of infectious disease. In spite of this, echidnas in captivity are susceptible to infection, in particular bacterial septicaemia. This may be partly due to a low average body temperature (32°C), low metabolic rate and additional aspects of sluggish immunity such as a reduced level of phagocytosis as reported by Wronski et al. (2003). They demonstrated an inferior immune response in the echidna (relative to the rabbit) based upon antibody production and secondary response to antigen administration.

3 REPRODUCTION

3.1 Sex determination

Determining the sex of short-beaked echidnas can be difficult as the sex organs and associated structures are internal and the female pouch is only present when in breeding condition. Johnston et al. (2006b) describe five techniques for sex determination in adults. Only one technique—hind limb spur morphology—was suitable for use in conscious adult echidnas. Three techniques—penile palpation/extrusion, testicular palpation and ultrasound of the testes—require general anaesthesia. The latter two techniques are also subject to false negative results in young males and in males with inactive or immature testes. A fifth technique—faecal steroid analysis—requires laboratory facilities and reference ranges and is not practical for clinical use. Adult male echidnas have well-developed hind limb spurs that are evident externally while females do not (although some females have small spurs). Palpation of the penis in the cloaca with external extrusion is probably the most reliable method to use in the clinical setting, but general anaesthesia will be required to achieve an adequate level of relaxation.

3.2 Breeding and development of young

Little data is available on the breeding biology of the short-beaked echidna. This is primarily due to their cryptic nature in the wild and lack of breeding success in captivity. Echidnas show evidence of a seasonal breeding pattern. The lifecycle shows a defined period of courtship and mating during July, August and early September regardless of geographic locality (Augee et al. 2006). Although authors seem reluctant to classify the female reproductive pattern, it is likely to be seasonally polyoestrous. Females have been known to conceive twice in a single season. Sperm production is seasonal and correlates well with testicular size; production peaks in August and is lowest in late spring (Johnston et al. 2007).

The echidna is mostly solitary with adults occupying large, vaguely defined and overlapping home ranges of up to 70 ha. Mating occurs in late winter and during this time several males may be seen lined up snout-to-tail, following an oestrous female. Three weeks later, the female lays a grape sized egg directly into her pouch, which has developed into a depression in the ventral midline bounded by muscular ridges. Twins are rare. All females develop pouches during the breeding season regardless of conception. Prior to egg-laying, the female excavates a burrow which consists of a short tunnel terminating in a nesting chamber. These burrows are extremely difficult to detect as the female plugs the entrance while in residence. Hatching occurs around 10 d after laying and the young echidna, commonly referred to as a puggle, remains within the pouch sucking milk from the ‘milk patch in the anterior pouch area. At some time, prior to the development of the quills and at around 45–55 d of age (about 200 g body weight), the young exits the pouch and is deposited in a nursery burrow Fig. 5.7. It is suckled relatively infrequently, perhaps only every 5 d, when the mother returns from her foraging expeditions. The young echidna grows and develops rapidly, aided by the large quantity of milk ingested at a single suckling—up to 20% of body weight in 1–2 h. The young gain 0.4 g for each millilitre of milk ingested—an impressive conversion rate. At 100 d the puggle can weigh as much as 550 g (Griffiths 1989). Lactation lasts around 200 d and permanent nest exit is thought to occur by 240 d (Augee & Gooden 1993). In studies on Kangaroo Island, South Australia, it has been estimated that young echidnas leave the nest at between 180 and 205 d at which time they weigh 800–1300 g. Weaning is instigated by the mother failing to return to the nest. Once weaned, the mother has no further contact with her young (Augee et al. 2006).

4 HUSBANDRY

4.1 Housing

Echidnas are inquisitive and responsive to environmental stimuli and express individual personality traits through learned behaviours. In exhibit design and construction, every effort should be made to respect this most fascinating and beguiling of creatures and provide a captive environment with a rich array of possible activities based on natural behaviours.

Echidnas are enormously strong and can pull cage furniture to pieces with ease. In the hospital, echidnas are best housed in smooth plastic tubs. The vertical walls should be at least 50 cm tall to prevent escape. For short-term hospitalisation, shelter and security can be provided by large amounts of straw or shredded paper which can be easily changed. For long-term patients, consider round plastic tubs about 2 m in diameter which can be partially buried if outside. Although the beak is a delicate and sensitive organ, the echidna will use it to explore the hospital environment. Care must be taken to eliminate any sharp-edged surfaces or cavities in which the beak may become jammed and injured. Echidna enclosures should be kept dry and the base substrate should not be abrasive as it may cause ulceration of the feet. Rubber floor covering may be worked loose and the echidna may get under it. A substrate that allows burrowing, such as soil or leaf litter, should be added to provide enrichment, including the opportunity to forage for invertebrates.

Outside enclosures should have a mesh or solid impenetrable layer beneath the digging substrate to prevent escape. Alternatively, the walls can be continued vertically down to a depth of 50 cm (Jackson 2003). The perimeter walls of outdoor enclosures should be solid, smooth and vertical to at least 50 cm to prevent escape. Overhanging structures and trees should be avoided as echidnas can climb.

The thermal preference for adult echidnas is between 20–30°C. Echidnas are far more prone to heat stress than to cold stress and in most climates can be housed outside provided shelter from heat is available. This must also be considered in the hospital situation, and careful monitoring of ambient temperature is essential.

Several echidnas can be housed together in the same enclosure (including in hospital) if a minimum of 5 m²floor space is provided for each individual and food, shelter and foraging opportunities are sufficient to minimise direct competition. Additional food dishes and shelter locations should be provided if dominance behaviour is observed.

4.2 Feeding

In hospital and when housed without other species, food can be placed in bowls. Tubes are an alternative to bowls. PVC pipe (50 mm diameter, 150 mm long) with a cap on one end and a removable cap with a hole the diameter of an echidna’s beak on the other are secured or partially buried and filled with echidna mix (L Vogelnest pers. comm.). When housed in mixed exhibits (e.g. with small macropods or birds), access to echidna food by other species must be prevented. PVC feeding tubes or artificial termite mounds with smooth-edged holes in the base can be used to cover and protect food bowls. Natural food items such as pieces of termite-infested wood or termite mounds will add greatly to the quality of the captive environment. Water should be available ad lib.

Hygiene is essential in maintaining the echidna’s hospital environment. Daily cleaning and regular disposal of substrates is essential. The increased incidence of faecaloral infections such as coccidiosis and salmonellosis in captive echidnas may be associated with environmental stress and hygiene (McOrist & Smales 1986).

4.4 Individual marking and identification techniques

Passive integrated transponders (PIT tags) or microchips are most commonly used to permanently mark individual echidnas. These are inserted SC on the dorsal midline between the scapulae. Coloured plastic tubing fitted over individual spines is also commonly used and has the advantage of allowing identification from a distance. Historically, tattoos on the ventral abdominal skin have been used. Paint markers can be used for temporary identification. Radio-tracking devices have also been fitted to the dorsum, anchored by various techniques including glues and ligatures.

4.5 Transport

For short-term (up to 2 hr) transport, 20 L plastic buckets or tubs with high sides (minimum 40 cm) are suitable as long as they are secured and prevented from falling over. For longer trips, strongly constructed transport boxes with smooth continuous internal surfaces free from deep corners, protrusions, cavities, holes and free edges are ideal. Some substrate material such as shredded paper should be provided to absorb fluids and enable the echidna to seek security. Provision of food and water is only necessary for extended trips (>4 hr) and then only via purpose-designed systems that prevent spillage in transit. Cardboard boxes, hessian bags and wire crates are not suitable for transporting echidnas (Jackson 2003).

5 NUTRITION

In the wild, adult echidnas are myrmecophagous—they feed almost exclusively on ants and termites. Echidnas seem to prefer termites (Augee & Gooden 1993), however, in some parts of their range (e.g. Tasmania) ants are far more common and form the greater part of the diet (Griffiths 1989). They will also eat other invertebrates such as earthworms and the larvae of beetles and moths (Booth 1999).

Invertebrate diets are characteristically high in protein and fat and most captive diets follow this basic principle. Oyarzun et al. (1996) analysed the nutritional composition of termites of Nasutitermes spp., which are high in protein (67%), phosphorus and iron and low in fat (2%) and calcium. Interestingly, they discovered that the dominant lipid fraction in all termite stages of development was oleic acid, which mirrors the lipid composition of echidna milk, reflecting the influence of dietary intake on milk composition (see section 8).

Termites contain considerably more water than ants (80% vs 64%). The water requirement of echidnas has been studied. It was found that at moderate humidity (9–15.6 g H₂O/m³) in the temperature range 21–28°C echidnas feeding on termites did not require free water. However, as humidity fell the ability to maintain fluid balance without free-water declined. At 3.3 g H₂O/m³, echidnas needed to drink 40–80 mL of water per day to remain adequately hydrated (Bentley & Schmidt-Nielsen 1967).

Due to the difficulty in maintaining a regular and adequate supply of termites and ants to captive echidnas, a wide range of alternative diets has been formulated. There has been a lot of experimentation with the formulation of captive echidna diets, with no firm indications of the ideal. Captive diets are often based on minced meat mixed with other ingredients to form a slurry. Most wild echidnas will accept a captive diet within a few days unless seriously ill. Echidnas that have suffered damage to the delicate structures of the beak may have difficulty feeding and in severe cases will be unable to survive. Formic acid (as produced by some ant species) or paw-paw (L Vogelnest pers. comm.) have been added to captive diets to encourage feeding, however, the real benefits of these additives are unknown.

Breeding success in captivity has usually been low and obesity is common. Although diet is certainly not the only factor in breeding success, it most probably plays an important role, suggesting that further work is required on echidna nutrition. Different zoos and fauna parks have used different diets with variable success regarding breeding and obesity (Jackson 2003). Adelaide Zoo has bred echidnas on a regular basis, an unusual achievement, and its diet is presented in Table 5.1 (D Schultz pers. comm.). Healesville Sanctuary has maintained echidnas in captivity for many years with good health but low breeding success. The diets offered are presented in Table 5.2.

Table 5.1 Echidna diet at Adelaide Zoo

Table 5.2 Echidna diets at Healesville Sanctuary, Zoos Victoria. Quantities are per animal per day