Echidnas //, sometimes known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. The four extant species, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs. Although their diet consists largely of ants and termites, they are no more closely related to the true anteaters of the Americas than to any other placental mammal. They live in Australia and New Guinea. The echidnas are named after a monster in ancient Greek mythology.
Echidnas are small, 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 have elongated and slender snouts which 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, 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 short-beaked echidna's diet consists largely of ants and termites, while the Zaglossus species typically eats worms and insect larvae.
Echidnas and the platypus are the only egg-laying mammals, known as monotremes. The female lays a single soft-shelled, leathery egg 22 days after mating, and deposits it directly into her pouch. 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.
Neocortex makes up half of the echidna's brain, compared to one-third 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 REM sleep, 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.
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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. The heads used are swapped each time the mammal copulates. When not in use, the penis is retracted inside a preputial sac in the cloaca. The male echidna's penis is 7 centimeters long when erect, and its shaft is covered with penile spines.
Breeding season begins in late June and extends through September. Males will form lines up to ten individuals long that follow the female and attempt to mate. Two weeks after mating, a single fertilized egg 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.
Molecular clock data suggest echidnas split from platypuses between 19 and 48 million years ago, and that platypus-like fossils dating back to over 112.5 million years ago therefore represent basal forms, rather than close relatives of the modern platypus. This would imply that echidnas evolved from water-foraging ancestors that returned to living completely on the land, even though this put them in competition with marsupials. Consequently, oviparous reproduction in monotremes may have given them an advantage over marsupials, a view consistent with present ecological partitioning between the two groups.
Echidnas are classified into three genera. The genus Zaglossus includes three extant species and two species known only from fossils, while only one extant species from the genus Tachyglossus is known. The third genus, Megalibgwilia, is known only from fossils.
The three living Zaglossus species are endemic to New Guinea. They are rare and are hunted for food. They forage in leaf litter on the forest floor, eating earthworms and insects. The species are:
- Western long-beaked echidna (Z. bruijni), of the highland forests;
- Sir David's long-beaked echidna (Z. attenboroughi), described in 1961 and preferring a still higher habitat;
- Eastern long-beaked echidna (Z. bartoni), of which four distinct subspecies have been identified.
The two fossil species are:
The short-beaked echidna (Tachyglossus aculeatus) is found in southeast New Guinea, and also occurs in almost all Australian environments, from the snow-clad Australian Alps to the deep deserts of the Outback, essentially anywhere ants and termites are available. It is smaller than the Zaglossus species, and it has longer hair.
The genus Megalibgwilia is known only from fossils:
Echidna in popular culture
- The echidna appears on the reverse of the Australian 5-cent coin.
- Knuckles the Echidna is a red echidna featured in the video game series Sonic the Hedgehog. Knuckles made his debut in Sonic the Hedgehog 3 for the Sega Mega Drive and Sega Genesis.
- Millie, an echidna, was a mascot for the 2000 Summer Olympics.
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