Physical characteristics

Together with the platypus, echidnas are the world's only monotremes, or egg-laying mammals. They are sometimes referred to as "spiny anteaters" because they look like the hedgehog and the porcupine in that they are covered by sharp spines.

Echidnas have a domed shaped back with short stubby tail, no obvious neck, and a flat belly. Back and sides are covered with spines (modified hairs) of varying sizes and lengths. Fine to course hair covers the legs and belly and surrounds the spines. The panniculus carnosus, a muscle located under the skin and around the body allows echidnas to assume contortionist shapes from very round to nearly flat. This muscle also permits movement of individual spines and helps form the pouch in reproductively active females.

Short and long-beaked echidnas are easily distinguished by differences in size, body mass, and length of the beak. Appearance of ears and eyes are also different. Adult Za-glossus range 24-40 in (60-100 cm) in length, weigh

A short-beaked echidna (Tachyglossus aculeatus) forages. (Photo by Animals Animals ©Roger W. Archibald. Reproduced by permission.)

13.2-35.2 lb (6-16 kg), and have a 4.2 in (10.5 cm) beak, often displaying a downward curve. Adult Tachyglossus are 12-20 in (30-50 cm) long, weigh 5.5-15.4 lb (2.5-7 kg), and have a 2.1 in (5.5 cm) straight beak. Whereas Zaglossus often have small distinct pinna, Tachyglossus generally have no external ear. Eyes of Tachyglossus are nearly obscured by hairs, but surrounded by bare wrinkled skin in Zaglossus. Contrary to lore, echidnas see well and can learn using visual cues.

The thick, woolly hair and short spines of Zaglossus are most like the Tasmanian subspecies of Tachyglossus, T. a. se-tosus. Pelage density, color, and spine length differ between subspecies. The arid dwelling T. a. acanthion tend to have longer, thinner spines and less hair than the other mainland subspecies T. a. aculeatus. Pelage of Kangaroo Island, T. a. multiaculeatus, and Tasmania, T. a. setosus, subspecies varies from light straw-colored to very dark whereas mainland subspecies are uniformly dark. Albinism has been reported in most subspecies.

Front and back limbs have five toes, with one to three grooming claws on each hind foot. Articulation of the rotated hind limbs in the pelvic girdle gives echidnas extreme dexterity to scratch between spines on any part of the body. Arrangement of the muscles in relation to the short, stout limbs gives echidnas enormous strength for digging and climbing. The limbs of Zaglossus are twice as long as those of Tachyglossus.

Echidna body temperature is low compared to other mammals, 87.8-91.4°F (31-33°C) and individuals use torpor (lowering of body temperature and metabolic rate) at any time of the year. There is only one opening, the cloaca, for excretion of urinary, fecal, and reproductive products out of the body. It is not possible to tell the gender of an echidna by external features. All genitalia are located internally. Both males and females may retain a spur on the inside of the hind foot and both can contract abdominal muscles to form a pseudo-pouch. Echidnas can live in excess of 50 years.

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