Short-beaked Echidna (Tachyglossus aculeatus) Fact Sheet: Physical Characteristics

Body measurements

Data sources: Rismiller (1999); Augee et al. (2006); Nicol and Anderson (2007b); Nicol (2015)

*Some sources (e.g., Courtney 1999, Scott 2013) state short-beaked echidnas can weigh more — up to 8-10 kg (18-22 lb)

Body mass

• Changes throughout the year (Nicol 2015a)
  • May fluctuate by up to 35% in a year (Rismiller and McKelvey 2000)
• Lose weight during the breeding season (Rismiller and McKelvey 2000; Nicol 2015a)

Body

• A medium-sized mammal (Augee et al. 2006)
  • Remarkably strong for its body size (also see Husbandry)
• Body robust and dome-shaped (Nicol 2015a)
• Body covered with hair and barbless spines (= modified hairs) (Nicol 2015a)
  • Amount and length of hair varies among short-beaked echidna subspecies and long-beaked echidna species
    • Echidnas in colder climates typically have thicker fur and fewer spines
  • Also see Coloration and Spines
• Middle part of the belly has little hair and no spines
  • Belly is flat or even concave, depending on an individual’s body weight/fat stores (Nicol 2015a)
• Unique skeleton shows traits of both reptiles and placental mammals/marsupials (Clemente et al. 2016)
  • See Locomotion
• No discernable neck (Nicol 2015a)

Coloration

• Typically dark in color; ranges from black to light brown to reddish-brown to honey (Augee et al. 2006; Nicol 2015a)
• Blonde short-beaked echidnas seen on Flinders and Kangaroo Islands (Nicol 2015b)

Spines

• Spines on back and sides only; not on belly (Nicol 2015a)
• Each spine has a long root that is embedded in a special muscle layer; allows spines to be moved individually or as small groups (Nicol 2015a)
  • In warding off predators, echidnas use their spines to anchor themselves in a log, in the ground (Augee et al. 2006)
• Echidnas can bend their body to produce a variety of shapes to protect more vulnerable parts of the body, like the head or hindquarters (Nicol 2015a)
• Spines do not detach when used for defense, unlike porcupine spines (Nicol 2015a)
  • Not molted seasonally; can remain on the body for years (Nicol 2015a)
• Also see Agonistic Behavior and Defense

Head and snout

• Small head, relative to its body size (Clemente et al. 2016)
• Eyes near the base of the snout (Nicol 2015a, except as noted)
  • Eyes small, black, and round; about 9 mm (0.35 in) in diameter (Augee et al. 2006)
  • Angled forward; likely have stereoscopic vision
  • Bare skin around eyes in the short-beaked echidna
• Ear openings are small, vertical slits (Nicol 2015a)
  • Not easily visible; spines and hair usually cover them (Augee et al. 2006)
  • Echidnas have an external ear structure different from other mammals; cartilaginous funnel mostly buried in a superficial muscle (Augee et al. 2006)
• Snout
  • Usually referred to as a “beak” (Nicol 2015a)
    • Structure not the same as beak of birds (SC Nicol, personal communication, 2017)
      • Bird beak
        • Keratinized layer over bone
      • Echidna “beak”
        • Formed by bones of the skull and lower mandible
        • Covered with skin that is well-supplied with blood vessels and nerves
  • Adaptive functions of the snout (Augee et al. 2006)
    • Used to prod and move soil (capture prey, create air spaces for breathing underground)
    • Holds sensory organs–smell receptors, mechanoreceptors, and electroreceptors
    • Helps echidnas conserve water in their bodies
  • Shape
    • Slender, tubular (Nicol 2015a)
    • Wedge-shaped; penetrates soil efficiently (Augee et al. 2006), like a golf tee or spike
  • No hair on snout (Nicol 2015a)
  • Skin
    • Coloration: black (Augee et al. 2006)
    • Texture: slightly moist and leathery; outer skin is keratinized for protection against abrasion; smoother towards tip (Augee et al. 2006)
    • Also see Senses
  • Nostrils are above (Nicol 2015a)
  • Very small, V-shaped mouth is underneath (Nicol 2015a)
• Mouth and tongue (Nicol 2015a)
  • Jaw bones light but very strong
    • Use to pry apart decomposing logs, move rocks, and move soil while foraging
  • No teeth
  • Tongue is sticky and worm-like; can extend far out of the mouth—helpful for sticking into termite mounds and ant dens
    • Also see Feeding

Limbs and claws

• Short, strong legs for digging and tearing apart logs, termite mounds, and underground insect nests (Nicol 2015a)
  • Forelegs are shorter and more muscular than the hindlegs
  • Hindleg bones are rotated; an echidna’s feet point backward
• Claws on forefeet
  • Broad, flattened, spade-like for digging and tearing open termite mounds and logs (Nicol 2015a)
• Claws on hindfeet
  • Long and thin compared to claws on forefeet (Augee et al. 2006)
  • Enlarged, curved “grooming claws” on digits vary between subspecies (SC Nicol, personal communication, 2017)
    • Used to comb through spines to remove dirt, insects, and debris (Nicol 2015a)
    • Echidnas have the remarkable ability to rotate their hindlimbs and groom in hard-to-reach places, such as behind their head (Augee et al. 2006)
• Gland-spur (Augee et al. 2006; Harris et al. 2012; Harris et al. 2014; Nicol 2015a)
  • Adult males possess a gland-spur (“crural spur”) on their hind ankle
  • Characteristics
    • Length: 0.5-1.0 cm (0.2-0.4 in)
    • Located under a fold of skin; hard to see until revealed
    • Made of keratin
  • Function
    • Connected via a duct to a gland in the thigh, which produces a non-toxic, milky fluid during the breeding season
    • Fluid may be used in scent marking; more behavioral research needed
      • See Breeding
    • Platypus produce venom from similar spurs
  • Development
    • In young, both sexes develop spurs; these are lost in adult females (Augee et al. 2006)
    • In young males, spur is covered by a sheath, which is lost before four years of age

Tail

• Thick and stubby (Augee et al. 2006)
• Above, covered with hair and spines (Nicol 2015a)
• Below, no spines and hair density/length varies (Augee et al. 2006)

Little sexual dimorphism

• Male on average slightly heavier than females, but not useful as a distinguishing feature (SC Nicol, personal communication, 2017)

Males vs. females

• Sexes look alike (Augee et al. 2006)
• No outward sign of reproductive organs; must closely inspect underside (Augee et al. 2006)
  • A male’s penis can be located by feel under the skin, near the cloaca
• Female distinguished by presence of pouch (SC Nicol, personal communication, 2017)
• Spurs (SC Nicol, personal communication, 2017)
  • Absent in females, in most populations
  • Present in males, in most populations

Differences among subspecies

• Distinguished by density of hair (pelage) and length of grooming claws on second and third digits on hindlegs (Nicol 2015b)
  • Different densities of hair affect each subspecies’ metabolic rate and thermoregulation

Subspecies of the short-beaked echidna, Tachyglossus aculeatus

(from Augee et al. 2006, p. 6)

Vision

• Important sense for detecting predators, navigating, and locating shelter (Nicol 2015a)
• Unusual visual system: mix of reptilian and mammalian characteristics (Augee et al. 2006)
• Surface layers of the cornea have keratin (Augee et al. 2006)
  • Likely protects the cornea from irritation/injury from ant bites, termites, soil/vegetation debris, and echidna spines
  • Similar to aardvarks
• Can forage in the dark and even survive blindness (Augee et al. 2006)
  • Remain healthy by using other senses for extended periods
• Visual fields of the two eyes overlap, indicating a degree of binocular vision (Augee et al. 2006)
• Very flat eye lens; adaptive for seeing distant objects (Augee et al. 2006)
• Dichromatic color vision, as found in many mammals (Nicol 2015a)

Hearing

• Echidnas use hearing to detect predators and prey, rather than for communication (Nicol 2015a)
  • May perceive sound better through their jawbones than with their ears (Nicol 2015a)
• Ear openings covered by hairs (Augee et al. 2006)
  • Reduces entry of debris and biting insects into the ear canal
• Outer ear structures have a low-profile (Augee et al. 2006)
  • Reduces risk of injury while burrowing

Senses of the snout

• Smell
  • Thought to be important in defense and locating prey (Augee et al. 2006)
  • Nasal cavity contains complex, vertically-folded structures called ethmoturbinals (Augee et al. 2006)
    • Increased surface area enables heightened smelling ability
  • Large, complex olfactory nerves and bulb (Nicol 2015a)
    • Bulb is folded for increased surface area
  • Also see “Scent and scent marking” in Communication
• Mechanoreception (Augee et al. 2006)
  • Echidnas possess mechanoreceptors in high densities
  • Mechanoreceptors respond to mechanical stimuli, such as touch, pressure, sound waves, and motion
  • Push rods
    • Unique structures found in echidnas and the platypus
    • Possibly allow for the passage of mechanical stimuli to sensory receptors under the echidna’s tough, keratinized skin
• Electroreception
  • Echidnas can detect voltage 1,000 times smaller than humans can detect with the tip of their tongue (Augee et al. 2006)
  • Hundreds of glands around the snout tip secrete mucous, improving the echidna’s sensitivity to electrical signals conducted through the soil (Augee et al. 2006)
  • It is unknown how significant electroreception is in locating prey (Nicol 2015a, and as noted)
    • Smell and mechanoreceptors likely more important senses for echidnas
    • Echidna has only 400 electroreceptors whereas the platypus has 40,000 (Pettigrew 1999)

Metabolism

• Lowest metabolism of any mammal (Nicol 2017)
  • Basal metabolic rates are about one third that of a placental mammal (Augee et al. 2006)
  • Lowest annual energy requirements of any mammal (Augee et al. 2006)
• Low oxygen needs (consumption) at rest (Clemente et al. 2016)
• Tolerate asphyxia (breathing restrictions that result in low oxygen, high carbon dioxide concentrations in the blood) (Augee et al. 2006)
  • May encounter these conditions when in a burrow, feeding inside a termite mound, or during a natural disaster
  • Heart rate and blood circulation slow to conserve oxygen for vital organs (heart, brain)

Brain

• Large and structurally complex (Nicol and Andersen 2007a; Nicol 2015b, except as noted)
  • Many folds in the cerebral cortex
    • Many structures said to be comparable to placental mammals (Hassiotis et al. 2003)
    • Area and volume similar to a domestic cat (Augee et al. 2006)
  • Very large prefrontal cortex (Augee et al. 2006)
    • Takes up 50% of the cerebral cortex—a larger proportion than any other animal, including humans (Russell and Burke 2016)
    • May be involved in foraging and capitalizing on cyclical/seasonal food resources (‘forward planning’) (Augee et al. 2006)
• Large, densely packed brain cells, compared to the platypus (Augee et al. 2006)
• “Positions of the sensory, visual and auditory areas…are unlike those described in any other mammal” (Augee et al. 2006)
  • Much sensory area devoted to the snout and tongue
• Can perform high level cognitive tasks, including conceptual learning (e.g., Russell and Burke 2016)

Body temperature and thermal stress

• Low body temperature compared to other mammals: approximately 29.5-32.5°C (85.1-90.5°F) (Barker et al. 2016)
  • Falls to 10-29°C (50-84°F) during torpor (Morrow and Nicol 2009)
• Body temperature fluctuates throughout the day (Grigg et al. 2004)
• Echidnas can lower and control body temperature to cope with extreme cold or lack of food (Augee et al. 2006)
• Can direct blood towards or away from the skin to increase or decrease heat loss (Augee et al. 2006)
• Use microclimates (e.g., rock caves, burrows, hollow logs) and behaviors (e.g., sun-basking) to regulate their body temperature (Augee et al. 2006)
  • Sun-basking appears to be more common in managed care (Grigg et al. 2004)
  • Cool down by swimming and seeking cool shelters (e.g., abandoned burrows of other species, rock caves, and hollow logs) (Nicol 2015a)
• Tolerate low temperatures well, but are susceptible to heat stress; no sweat glands (Augee et al. 2006; Nicol 2015a)
  • Can survive temperatures of 35-40 ºC (95-104 ºF) in shelters for up to 10 hours (Barker et al. 2016)
• Also see Other Behaviors

Water retention

(Augee et al. 2006)

• Structure of an echidna’s snout helps to prevent water loss
• Some water loss through the skin
• Urine
  • Not super-concentrated, as in desert rodents
  • Have dwarf nephrons similar to those found in lizards and snakes; however, excretions mainly comprised of urea (typical of mammals)
• Feces are usually dry

Stomach unusual for a mammal

(Augee et al. 2006, except as noted)

• No secretory glands
  • Like placental anteaters (SC Nicol, personal communication, 2017)
• pH of gastric liquids is close to neutrality (6.2-7.4), rather than acidic, as in most mammals
  • Like placental anteaters (SC Nicol, personal communication, 2017)
• See “Digestion and scat” in Feeding

The spines of the short-beaked echidna are modified hairs that can be moved individually for defense. The echidna's tapered nose is used for feeding on termites and ants underground.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.

Image note: This is a cropped image.

Echidnas are superb diggers.

Their snout and claws are adapted for penetrating and moving soil. Even their corneas have special layers of kertain to protect their eyes from ant bites and debris, while digging.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.