The muscles of this echidna have a number of unusual features. There is an enormous muscle that is just beneath the skin and covers the entire body. By contraction of various parts of this muscle the Short-beaked Echidna can change shape. The most common shape change is achieved by rolling itself into a ball when threatened, protecting its belly and presenting a defensive array of sharp spines. It has one of the shortest spinal cords of any mammal, extending only as far as the thorax.
The tongue of the short-beaked echidna is the animal's only way of catching prey. It can stick out up to 180 mm outside the snout. The tongue is sticky because of the presence of glycoprotein-rich mucous. This mucus both lubricates movement in and out of the snout and helps to catch ants and termites, which stick to it. The protruded tongue is stiffened by the rapid flow of blood, allowing it to penetrate wood and soil. Retraction requires the contraction of two internal muscles. When the tongue is retracted, the prey is caught on backward-facing keratinous "teeth" along the roof of the buccal cavity. This allows the animal to both capture and grind food. The tongue moves with great speed, and has been measured to move in and out of the snout 100 times a minute.
Many physiological adaptations fit the animal to its lifestyle. It burrows, and can tolerate high levels of carbon dioxide. Its ear is sensitive to low-frequency sound, which may be ideal for detecting sounds emitted by termites and ants underground. The leathery snout is covered in mechano- and thermoreceptors. These receptors provide information about the surrounding environment. The echidna has a well-developed olfactory system, which may be used to detect mates and prey, and its other senses perform well. Its brain and central nervous system have been extensively studied for comparison with placental mammals.
The short-beaked echidna has the largest prefrontal cortex, relative to body size, of any mammal. It takes up 50% of the volume of the cortex, compared to 29% for humans. This suggests good decision-making in its constant search for insect nests, and for a mate when reproducing.
Like all monotremes, the echidna has only one orifice for the passage of faeces, urine and reproductive products, which is known as the cloaca. The male has internal testes, no external scrotum and a highly unusual penis with four knobs on the tip. The gestating female has a pouch on its underside, where it raises its young.
The laying of eggs and the cloaca are basal traits which are present in all early amniotes, including reptiles, birds and early mammals.
The solitary Short-beaked Echidna looks for a mate between May and September; the precise timing of the mating season varies with geographic location. Both males and females give off a strong odour during the mating season. During courtship—observed for the first time in 1989—males locate and pursue females. Trains of up to ten males may follow a single female in a courtship ritual that may last for up to four weeks; the duration of the courtship period varies with location. In cooler parts of their range, such as Tasmania, females may mate within a few hours of arousal from hibernation.
Before mating, the male smells the female, paying particular attention to the cloaca. The male is often observed to roll the female onto her side and then assumes a similar position so that the two animals are abdomen to abdomen. Each mating results in the production of a single egg, and females are known to mate only once during the breeding season; each mating is successful.
Fertilisation occurs in the oviduct. Gestation takes between 21 and 28 days, during which time the female constructs a nursery burrow. Following the gestation period, a single rubbery-skinned egg between 13 and 17 millimetres in diameter is laid directly into a small, backward-facing pouch that has developed on her abdomen. Ten days after it is laid, the egg hatches within the pouch. The embryo develops an "egg tooth" during incubation, which it uses to tear open the egg; the tooth disappears soon after hatching.
Hatchlings are about 1.5 cm long and weigh between 0.3 and 0.4 grams. After hatching, young Echidnas are known as "puggles". Hatchlings attach themselves to their mothers' milk areolae, a specialised patch on the skin that secretes milk (monotremes have no nipples). The way in which puggles drink the milk is not yet known, but they have been observed drinking large amounts during each feeding period, since mothers may leave them unattended in the burrow for between five and ten days. The principal components of the milk are types of lactose. The milk has a high iron content, which gives it a pink colour.
Juveniles are eventually ejected from the pouch at around two to three months of age, because of the continuing growth in the length of their spines. Suckling gradually decreases until juveniles are weaned at about six months of age. The duration of lactation is about 200 days, and the young leave the burrow between 180 and 240 days.
The age of sexual maturity is uncertain, but may be four to five years. A twelve-year field study, published in 2003, found that the Short-beaked Echidna reached sexual maturity between five and 12 years of age, and that the frequency of reproduction varies from once every two years to once every six years. The Short-beaked Echidna can live as long as 45 years in the wild.
No systematic study of the ecology of the Short-beak Echidna has been published. There have been studies of several aspects of their ecological behaviour. Short-beaked Echidnas live alone and apart from the burrow created for rearing young; they have no fixed shelter or nest site. They do not have a home territory, but range over a wide area. Short-beaked Echidnas are usually active in the daytime; however, they have problems in hot weather, because they have no sweat glands and do not pant. Therefore, in warm weather they change their pattern of activity, becoming crepuscular (active at dawn or dusk) or nocturnal (active in the night). They can tolerate cold temperatures, and hibernate during the winter in very cold regions.
Short-beaked Echidnas can live anywhere where there is a good supply of food. Short-beaked Echidnas find food by smell, using sensors in the tip of their beak, and regularly feast on ants and termites. They are powerful diggers, using their clawed front paws to dig out prey and dig burrows for shelter. They may rapidly dig themselves into the ground if they cannot find cover when in danger.
In Australia they are most common in forested areas where there are many termite-filled fallen logs. In agricultural areas, they are most likely to be in uncleared scrub; they may be in grassland, arid areas, and in the outer suburbs of the capital cities. Little is known about their distribution in New Guinea. They have been found in southern New Guinea between Merauke in the west, to the Kelp Welsh River, east of Port Moresby, where they may be in open woodland.
The Short-beaked Echidna is common in most of temperate Australia and lowland New Guinea, and is not listed as endangered. In Australia, the number of Short-beaked Echidnas has been less affected by land clearing than have some other species, since Short-beaked Echidnas do not require a specialised habitat beyond a good supply of ants and termites. Despite their spines, they are eaten by birds, the Tasmanian Devil, cats, foxes and dogs. They were also eaten by indigenous Australians and the early Europen settlers of Australia. The most common threats to the animal in Australia are cars and habitat destruction. These have led to localised extinction. Infection with the introduced parasite "Spirometra erinaceieuropaei" is fatal for the Echidna. The Wildlife Preservation Society of Queensland runs an Australia-wide survey called "Echidna Watch" to monitor the species in Australia.