ANATOMY AND PHYSIOLOGY
Echidnas have anatomical and physiological characteristics in common with reptiles, birds and mammals (Rens et al. 2007; Alibardi and Rogers 2015; Vogelnest and Allan 2015; Rismiller et al.
2019). Some of these have managed care husbandry and clinical relevance (Table 29.1).2.1 Gastrointestinal system
Echidnas are myrmecophagous and have recently been described as opportunistic, insectivorous herbivores (Shaw 2022; Perry et al. 2022). Multifocal pale smooth nodules identified on the floor of the echidna’s mouth may be lymphoid aggregates but this is yet to be confirmed. The tongue is sticky and, apart from the distal 5-15 mm, which is smooth, is covered in small papillae, giving it a roughened texture. The tongue can extend 18 cm from the mouth, up to 100 times per minute (Nicol
Table 29.1. Selected Uniqueanatomical and physiological traits of short-beaked echidnas (Tachyglossus aculeatus) with management and clinical relevance
| Selected trait | Management or clinical application |
| Musculoskeletal | |
| Complex pectoral girdle provides a robust skeletal basis for digging. Prominent humeral epicondyles provide attachment sites for the extensive forelimb musculature.1 Rotational movement of the humerus is used for digging and locomotion.1,2 Spend around 12% Oftheirtime digging3 | Provide with appropriate substrate to allow digging;3 can quickly bury vertically downwards on natural substrates making retrieval extremely difficult; excellent climbers and strong swimmers4 |
| Skeletal adaptions for digging as a form of defence come at the expense Ofvelocity3 | Do not run, maximal pace of around 0.65 m/s |
| Epipubic bones extend Craniallyfrom mobile articulations with the pubis.2 Epicoracoids overlap in the ventral midline rather than adjoining symmetrically5 | Can appear asymmetrical on radiographs and CT5 |
| Separate centres of endochondral ossification and delayed ossification of physeal cartilage. Ossification occurs in a consecutive manner from distal to proximal in appendicular bones. The femoral capital physis is one of the last to ossify2 | Femoral head and condyles appear later than in other species; patella appears bipartite in juvenile echidnas; prominent physeal space between metaphysis and epiphysis in long bones.2 May underestimate age based on open physes on radiographs, e.g. proximal femoral physeal closure ~3.5 yr6 |
| The os calcaris is a nub of bone upon which the spur sits in adult males; absent in females2 | Visible on the mediolateral view of the talus, caudal to the talus; determine gender |
| Absent tympanic bullae1 | Radiographic interpretation |
| Vertebral formula: 7 cervical, 15-17 thoracic, 3 lumbar, 2-3 sacral and 12 caudal2,5'7 | Radiographic interpretation |
| The large SC muscle panniculus carnosus covers the dorsum and flanks, facilitating quill movement, and individual quill control4 | Allows echidnas to curl and to wedge into corners or similar using quills making safe retrieval difficult |
| Adipose | |
| Fat mass varies between 2.3% and 39.2% bodyweight.8 Adipose underlies the panniculus carnosus and is distributed SC from the base of the skull to the tail base. Intraabdominal fat around the gonads and within mesentery is only present in the most well-conditioned animals | Readily visualised on lateral radiographs and is useful for objectively assessing body condition via serial radiographs (Fig. 29.1); decreased intraabdominal fat decreases radiographic serosal detail |
| Cardiopulmonary | |
| The heart is located at the most cranial point of the thorax and sits left of midline. Three right lung lobes and one left lung Iobe2 | Radiographic interpretation |
| Gastrointestinal | |
| Ingested soil is seen as fine radiopaque particles within the GIT2 | Should not be interpreted as constipation; important feature in juveniles to assess Ifcommenced weaning (see section 3) |
| The beak is a fine skeletal structure covered in glabrous skin. It is used to detect prey by olfaction, noise conduction to the ears and electro- and mechanoreceptors (push-rods).1,4,7 Olfaction and mechanoreceptors are likely more significant than electroreceptors in prey detection.9 Ofthe 400 mucus glands located on the beak tip, 25% contain sensory nerve receptors thought to detect weak electric fields. There are around seven sensory glands per square mm.10 The beak is used to probe, and plough substrate, wedge and split rotten wood, and break apart termite mounds.11 | The beak is vital for prey location, is delicate yet can be used vigorously to forage. This is a major consideration when assessing beak trauma (see section 7.4.1). Will use the beak to explore every aspect of housing, potentially resulting in beak trauma. Inappropriateor poorly placed anaesthetic masks can occlude or damage nares4 |
436 CurrentTherapyin MedicineofAustraIian Mammals
| Selected trait | Management or clinical application |
| Narrow oral gape, oral cavity, oropharynx and nasal passages | Risk of laryngeal obstruction during anaesthesia, especially with blood from a traumatised beak. Care with endoscopic examination as may result in partial obstruction. Difficult to intubate (see Chapter 9) |
| Paired mandibular salivary glands are firm, flattened discs palpable on the ventral cervical region. They are mobile and may also be located over the cranial thorax | Can be mistaken for abnormal masses |
| GIT lactase and sucrase activity is virtually undetectable. Trehalase activity is high to suit high trehalose content Oftermites10 | Echidnas cannot tolerate lactose or sucrose; therefore avoid dairy and fruit |
| Opportunistic foragers12 | Will eat insect larvae and reported to eat egg contents13 |
| Integument | |
| Footpad skin sensitivity; mechanoreceptors in front feet.14 Spines shed intermittently; growth rate unknown15 | Prolonged hospitalisation or inappropriate substrates can cause abrasions or cracks. Suitable enclosure substrates are coarse river sand and soil. Suitable hospital substrate is shredded cardboard, recycled paper cat litter and natural leaf litter (+/- towels). Substrate must be kept dry. Foot pad skin must be healed and healthy prior to release. Cannot be forcibly removed |
| Ocular | |
| Globes small, no nictitating membrane, retina is avascular, 99% decussation of optic fibres at the chiasma4 | Absent consensual pupillary light reflex, anisocoria may be normal |
| Intra-Ocular pressure (IOP) 16.5 (8-21)16 | First report of normal IOPs |
| Hyperopic vision16 | Very poor near vision |
| Reports of good health in partially blind and completely blind echidnas17,18 | Vision is mostly used for predator detection, navigating home ranges and locating refuges9. Due to reports of healthy vision-impaired echidnas, and the importance of other significant sensory modalities, consideration may be given to releasing partiaIIy-blind echidnas19 (if compliant with relevant jurisdiction codes of practice) |
| Central nervous system | |
| The spinal cord is one of the shortest recorded for any mammal, terminating at the mid-thoracic level20 | Interpretation Ofadvanced imaging; assessment of neurological signs and spinal trauma |
| Large, gyrified brain with similar brain size:BMR relationship as primates.21 Cognitive ability at least equal to that of rats or cats22 | Requires mental and sensory enrichment in managed care, e.g. rotten logs, termite mounds for foraging, puzzle feeders |
| Only mammal with a gyrified olfactory bulb23 | Olfaction is primary sensory faculty19 |
| Renal | |
| The kidney can conserve water to produce an extremely concentrated urine allowing survival in arid environments4 | In low humidity echidnas require 40-80 n∩L∕day of water4 |
Table 29.1. (continued)
| Selected trait | Management or clinical application |
| Reproduction | |
| Pouch development is only observed in in the breeding season (regardless Ofconception),4 pregnancy, egg incubation and early lactation.24 The pouch closes in a drawstring manner at oviposition.25 The pouch regresses once the puggle is placed in the burrow but the mammary glands continue to enlarge.26 Lactating echidnas have darkened areolae and mammary gland development that increases the prominence of the lateral margins of the pouch | Ifa rescued female has evidence that it may have a burrow young it should be released as soon as possible, close to the location it was found |
| Urogenital | |
| The ureters enter the urogenital sinus at the base of the bladder and not directly into the bladder. In the female short-beaked echidna this is via a common papilla (L Vogelnest pers. comm.). This has not been described in males. The vasa deferentia enter the urogenital sinus just distal to the ureters. Thus the urogenital sinus conveys both urine to the cloaca and semen to the penis and is connected to the penile urethra by a small orifice at the base of the penis. However, it is unknown how semen is directed at copulation into the penile urethra rather than following the same path as the urine directly into the cloaca27 | Spermaturia common in males; reproductive assessment in males; urogenital imaging |
| Metabolism | |
| Basal metabolic rate is ~1∕3 that of an equivalent-sized dog or cat8 | This may have significant drug pharmacokinetic effects; there are no studies to date |
| No difference in metabolic rate between lactating and non-lactating females21 | Increased caloric intake not required |
| High RBC O2 affinity allows tolerance Ofinspired CO2 concentration of up to 10%3 | O2 requirement is <50% of an equivalent-sized dog or cat.28 Respond to high CO2 by increasing tidal volume rather than respiratory rate; therefore respiratory rate is not as a reliable monitoring parameter as effort4 |
| MISCELLANEOUS | |
| Thyroid glands located in the cranial mediastinum and paired parathyroid glands associated with thymus not thyroids. All three organs embedded in fat28,29 | CTinterpretation |
| Dorsal pole of spleen is ovoid30 | Can be mistaken for a testis or splenic mass |
| Faecal glucocorticoid peaks after enclosure move31 | Environmental moves are stressors. Anorexia in hospitalised echidnas has resolved when returned to familiar environment |
1Augee etal. 2006; 2VogeInest and Allan 2015; 3CIemente etal. 2016; 4MiddIeton 2008; 5WheeIhouse etal. 2022; 6A Naiker (pers.comm.); 7Griffiths 1989; 8LeeHong etal. 2014; 9NicoI 2015; 10HoIz 2015; 11NicoI 2022; 12Sprentand Nicol 2016; 13Robinson etal. 2023; 14Mahnsefa/. 2003; 15Braga etal. 2022; 16McCarthy (pers. comm.); 17Augeeand Gooden 1992; 18Abensperg-Traun 1994; 19S Nicol (pers. comm.); 20AshweII and Zhang 1997; 21NicoI 2017; 22NicoI 2018; 23AshweII 2013; 24Johnston and Keely 2015; 25Dutton-Regester etal. 2021; 26Griffiths 1968; 27FeneIon etal. 2021; 28HoIz 2014; 29Vaasjo etal. 2024; 30Higgins etal. 2018; 31RusseII etal. 2022
438 CurrentTherapyin MedicineofAustraIian Mammals
Fig. 29.1. Lateral radiographs of short-beaked echidnas (Tachyglossus aculeatus) showing the relatively radiolucent SC fat layer that is useful for objectively assessing body condition. (a) Poor body condition, no radiographically apparent SC fat, (b) moderate body condition, thin layer of SC fat and (c) heavy body condition with thick layer of SC fat. Images: Taronga Wildlife Hospital
2015). To prehend a slurry diet the tongue forms a J-shape at full extension. Salivary pH is 8-8.5.
The anatomy and physiology of the echidna stomach has been reviewed (Tong et al. 2017; Shaw et al. 2017). Echidnas have a thin-walled, single-chambered stomach with pale, smooth rugal mucosal folds. The mucosa becomes rougher and more orange in colour within the pylorus. Histologically, healthy gastric mucosa is keratinised squamous epithelium lined with gram-positive cocci. Gastric glands are absent and Brunner’s glands are present in the pylorus.
It is proposed that the echidna stomach functions like a fermentation chamber, which is discussed in Chapter 14. The echidna microbiome has been characterised. Free-ranging echidna samples were highly variable with predominately plant-fermenting and soil bacteria. Microbiomes of echidnas in managed care varied depending on diets fed, and comprised mainly gut commensals, but also featured plant-fermenting bacteria (Perry et al. 2022).
2.2 Scent glands and the crural system
Secretory scent glands are located within the cloacal wall of both sexes. Male echidnas possess a crural system made up of a keratinous spur located on the caudomedial aspect of the tarsus, with a central canal connected via a duct to a crural gland located in the popliteal fossa (Krause 2010). Unlike the platypus spur, the echidna spur is not firmly attached and cannot be used aggressively. The crural gland-spur apparatus is an important secondary sex structure (Nicol et al. 2019a) with crural gland size and plasma testosterone increasing during breeding season (Morrow et al. 2016). Like platypuses, echidna ‘venom’ is expelled from the tip of the spur (Nicol et al. 2019a). The inner aspect of the skin sheath containing the spur, or empty spur pouch in adult females, secretes a waxy discharge year round. In males, it is secreted in greater volumes, is more complex and changes during the breeding system, indicating it functions as a male sex signal (Krause 2010; Harris et al. 2012; Nicol et al. 2019a). The ‘venom’, which contains smaller amounts of the same proteins found in platypuses (Koh et al. 2010), also functions as olfactory communication (Morrow et al. 2016). Male echidnas have been observed wiping the ‘venom’ and exudate onto females, often injuring themselves in the process (Nicol et al. 2019a). Olfactory cues are used for mate attraction and location, intrasexual competition, home range marking, and potentially to induce ovulation (Nicol et al. 2019a).
All juvenile echidnas have sheathed spurs. Rismiller and McKelvey (2003) estimated the age of juvenile echidnas by the pattern of spur sheath loss whereas Nicol et al. (2019a) found too much variability in spur sheath loss and condition for reliable aging of juveniles. Overall, females lose their spurs between 14-31 mo and males lose their spur sheaths between 24-38 mo. The remnant juvenile female spur may persist as a vestigial spur in some individuals. The waxy spur sheath exudate has not been seen in sheathed spurs, and as they are secondary sex structures, it is not unexpected that the earliest age of reproduction in males is the upper age at which juvenile spur sheaths are lost.
2.3 Sex determination
Sex determination and reproduction are discussed in Chapter 5 and Middleton (2008). Sex determination in echidnas is complex because monotremes have an XY chromosome system that is unrelated to that of marsupials or eutherians and shares some homology with the avian Z chromosome (Rens et al. 2007). Echidna males have four Y and five X chromosomes and a Y-linked anti- Mullerian hormone (AMH) gene that determines gender (Cortez et al. 2014).
A vestigial penis with four terminal rosettes was identified in three free-ranging female echidnas at necropsy (ARWH 2024 case nos 12326, 12376, 12398). The organ was discovered within the ventral serosal surface of the urogenital sinus and although there was no direct communication, there was a mucosal depression at the corresponding location within the urogenital sinus. Histologically the organ was structurally similar to that of the male echidna penis (L Tong pers. comm.). The reproductive tracts and mammae of all three females were otherwise normal. This may be an example of intersex variation and may be what was previously reported as a clitoris in the short-beaked echidna (Hughes and Carrick 1978). A clitoris, confirmed histologically, located within the distal urogenital sinus was identified in one long-beaked echidna (Z. bartoni) (L Tong pers. comm.; ARWH 2024 case no. 10667). Some adult female echidnas retain a vestigial spur. Males can reliably be identified by the presence of sharper, narrower, unsheathed adult spurs (Augee et al. 2006; Nicol et al. 2019a). There is one report of up to 25% of Kangaroo Is. echidnas, both males and females, having one adult spur (Rismiller 1993).
2.4 Thermoregulation
Echidnas are heterotherms with a basal body temperature (Tb) range of 29.5-35°C (Brice et al. 2002; Morrow et al. 2009; Barker et al. 2016). They are thermolabile across a range of ambient temperatures (T ) and their thermal preference is 20-30°C (Holz 2015).
Although echidnas do not pant and their sweat glands are restricted to the pouch area (Griffiths 1989; Middleton 2008), they can enhance evaporative water loss (EWL) at high Ta with a similar capacity to other mammals. Evaporative cooling is maximised through a unique evaporative window comprising the moist beak tip and underlying blood sinus. Other thermoregulation strategies include thermal windows (ventrum and medial thighs), spines that provide flexible insulation and increased respiratory and cutaneous EWL (mechanism unconfirmed) (Barker et al. 2016; Cooper and Withers 2023). These thermoregulatory traits, along with observations of echidnas active at 37.4°C (Cooper and Withers 2023) and sheltering in hollow logs at temperatures of 35-40°C for up to 10 hr during bushfires (Nowack et al. 2016), suggest echidnas can tolerate Ta above their supposed critical thermal limit of 35°C (Barker et al. 2016).
Females demonstrate facultative thermoregulation, because they control Tb to within 1°C during incubation (Nicol and Anderson 2006) and when carrying PY within nursery burrows (Wallage et al. 2015).
Echidnas are capable of torpor and hibernation. Pre- emergent juveniles are also capable of entering torpor (Griffiths 1989). Torpor or hibernation typically occurs at Ta <12°C and Tb <27°C (Nowack et al. 2016). During these periods Tb decreases to 4°C, heart rate slows to 4-7 bpm and respiratory rate decreases to 0.3 bpm. Although torpor and hibernation are considered incompatible with reproduction, in Tas. the females that rouse for mating, or are mated early in the breeding season, will re-enter torpor or hibernation up to 5 d later (Morrow and Nicol 2009, 2017). The resulting blastocyst does not develop during torpor/hibernation and gestation is extended relative to time spent in torpor or hibernation (Morrow and Nicol 2017). Echidnas can enter torpor to conserve energy and increase survival in unproductive habitats (e.g. torpor is increased during and after bush fires) (Brice 2009; Nowack et al. 2016).
3.