Echidna

Echidnas, also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, 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 New Guinea and Australia. The echidnas are named after a monster in ancient Greek mythology.

Description
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 snouts which have the functions of both mouth and nose. Their snouts are elongated and slender. 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 a tiny mouth and a toothless jaw. 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.

The long-beaked echidnas have sharp, tiny spines on their tongues that help capture their prey.

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 ten days; the young echidna, called a puggle, 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 puggle, returning every five days to suckle it until it is weaned at seven months. The average wild echidna can grow as old as 16 years.

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.

Contrary to previous research, the echidna does enter REM sleep, albeit 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.



Evolution
Molecular clock and fossil dating suggest echidnas split from platypuses 19–48 million years ago. Echidnas evolved from water-foraging ancestors which returned to living completely on the land, even though this put them in competition with marsupials. Because of this, it has been suggested that "oviparous reproduction in monotremes confers advantages over marsupials, a view consistent with present ecological partitioning between monotremes and marsupials."

Taxonomy
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.

Zaglossus
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 (Zaglossus bruijni), of the highland forests
 * Sir David's long-beaked echidna (Zaglossus attenboroughi), described in 1961 and preferring a still higher habitat
 * Eastern long-beaked echidna (Zaglossus bartoni), of which four distinct subspecies have been identified

The two fossil species are:


 * †Zaglossus robustus
 * †Zaglossus hacketti

Tachyglossus
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.

Megalibgwilia
The genus Megalibgwilia is known only from fossils:


 * Megalibgwilia ramsayi from Late Pleistocene sites in Australia
 * Megalibgwilia robusta from Miocene sites in Australia

Echidna in pop culture
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.