Echidnas are medium-sized, solitary mammals covered with coarse hair and spines. Superficially, they resemble the anteaters of South America and other spiny mammals such as hedgehogs and porcupines. They are usually black or brown in colour. There have been several reports of albino echidnas, their eyes pink and their spines white. They have elongated and slender snouts that function as both mouth and nose. Like the platypus, they are equipped with electrosensors, but while the platypus has 40,000 electroreceptors on its bill, the long-billed echidna has only 2,000 electroreceptors, and the short-billed echidna, which lives in a drier environment, has no more than 400 located at the tip of its snout. They have very short, strong limbs with large claws, and are powerful diggers. Echidnas have tiny mouths and toothless jaws. The echidna feeds by tearing open soft logs, anthills and the like, and using its long, sticky tongue, which protrudes from its snout, to collect prey. The echidnas' ears are slits on the sides of their heads that are usually unseen due to the fact that they are blanketed by their spines. The external ear is created by a large cartilaginous funnel, deep in the muscle.
The short-beaked echidna's diet consists largely of ants and termites, while the Zaglossuss species typically eats worms and insect larvae. They have no teeth, and break down their food by grinding it between the bottoms of their mouths and their tongues.
Echidnas do not tolerate extreme temperatures; they use caves and rock crevasses to shelter from harsh weather conditions. Echidnas are found in forests and woodlands, hiding under vegetation, roots or piles of debris. They sometimes use the burrows of animals such as rabbits and wombats. Individual echidnas have large, mutually overlapping territories. Echidnas are capable swimmers. When swimming, they expose their snout and some of their spines. They are known to journey to water in order to groom and bathe themselves. The tongues of long-beaked echidnas have sharp, tiny spines that help them capture their prey. Echidnas faeces are 7 centimetres long and are cylindrical in shape; they are usually broken and unrounded, and composed largely of dirt and ant-hill material.
Echidnas and the platypus are the only egg-laying mammals, known as monotremes. The average lifespan of an echidna in the wild is estimated around 16 years. When fully grown a female can weigh up to 4.5 kilograms (9.9 lbs) and a male can weigh up to 6 kilograms (13.2 lbs). The echidnas' sex can be inferred from their size, as males are 25% larger than females on average. The reproductive organs also differ, but both sexes have a single opening called a cloaca, which they use to urinate, release their faeces and to mate. Male echidnas have non-venomous spurs on the hind feet.
The neocortex makes up half of the echidna's brain, compared to 80% of a human brain. Due to their low metabolism and accompanying stress resistance, echidnas are long-lived for their size; the longest recorded lifespan for a captive echidna is 50 years, with anecdotal accounts of wild individuals reaching 45 years. Contrary to previous research, the echidna does enter REMsleep, but only when the ambient temperature is around 25 °C (77 °F). At temperatures of 15 °C (59 °F) and 28 °C (82 °F), REM sleep is suppressed.
The female lays a single soft-shelled, leathery egg 22 days after mating, and deposits it directly into her pouch. While hatching, the baby echidna opens the leather shell with a reptile-like egg tooth. Hatching takes place after 10 days; the young echidna then sucks milk from the pores of the two milk patches (monotremes have no nipples) and remains in the pouch for 45 to 55 days, at which time it starts to develop spines. The mother digs a nursery burrow and deposits the young, returning every five days to suckle it until it is weaned at seven months.
Male echidnas have a four-headed penis. During mating, the heads on one side "shut down" and do not grow in size; the other two are used to release semen into the female's two-branched reproductive tract. Each time it copulates, it alternates heads in sets of two. When not in use, the penis is retracted inside a preputial sac in the cloaca. The male echidna's penis is 7 centimetres (2.8 in) long when erect, and its shaft is covered with penile spines. These may be used to induce ovulation in the female.
It is a challenge to study the echidna in their natural habitat and they show no interest in mating while in captivity. Therefore, no one has ever seen an echidna ejaculate. There have been previous attempts, trying to force the echidna to ejaculate through the use of electrically stimulated ejaculation in order to obtain semen samples but has only resulted in the penis swelling.
Breeding season begins in late June and extends through September. Males will form lines up to ten individuals long, the youngest echidna trailing last, that follow the female and attempt to mate. During a mating season an echidna may switch between lines. This is known as the "train" system. Two weeks after mating, a single fertilized egg, weighing 380 milligrams and being about 1.4 centimetres long, is implanted in a rear-facing pouch that has developed on the female, where it is held for ten days before hatching. The young echidna, called a puggle, is then held in the pouch for two to three months before being expelled. Puggles will stay within their mother's den for up to a year before leaving.
Echidnas are very timid animals. When they feel endangered they attempt to bury themselves or if exposed they will curl into a ball, both methods using their spines to shield them. Strong front arms allow echidnas to continue to dig themselves in whilst holding fast against a predator attempting to remove them from the hole. Although they have a way to protect themselves, the echidnas still face many dangers. Some predators include wild cats, foxes, domestic dogs and goannas. Snakes pose as a large threat to the echidna species because they slither into their burrows and prey on the young spineless puggles. Some precautions that can be taken include keeping the environment clean by picking up litter and causing less pollution, planting vegetation for echidnas to use as shelter, supervising pets, reporting hurt echidnas or just leaving them undisturbed. Merely grabbing them may cause stress, and not picking them up correctly may even result in injury.