Echidna

Echidnas Temporal range: Miocene–Holocene PreꞒ Ꞓ O S D C P T J K Pg N
Short-beaked echidna
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Monotremata
Superfamily:	Ornithorhynchoidea
Family:	Tachyglossidae Gill, 1872
Type genus
Tachyglossus Illiger, 1811
Species
Genus Tachyglossus    T. aculeatus Genus Zaglossus    Z. attenboroughi    Z. bruijnii    Z. bartoni Genus †Megalibgwilia    †M. owenii    †M. robusta Genus †Murrayglossus    †M. hacketti
Echidna range   Short-beaked echidna   Western long-beaked echidna   Eastern long-beaked echidna   Sir David's long-beaked echidna

Echidnas (/ɪˈkɪdnəz/), sometimes known as spiny anteaters,^[1] are quill-covered^[2] monotremes (egg-laying mammals) belonging to the family Tachyglossidae /tækiˈɡlɒsɪdiː/, living in Australia and New Guinea. The four extant species of echidnas and the platypus are the only living mammals that lay eggs and the only surviving members of the order Monotremata.^[3] The diet of some species consists of ants and termites, but they are not closely related to the American true anteaters or to hedgehogs. Their young are called puggles.

Echidnas evolved between 20 and 50 million years ago, descending from a platypus-like monotreme.^[4] This ancestor was aquatic, but echidnas adapted to life on land.^[4]

Echidnas are possibly named after Echidna, a creature from Greek mythology who was half-woman, half-snake, as the animal was perceived to have qualities of both mammals and reptiles.^{[citation needed]} An alternative explanation is a confusion with Ancient Greek: ἐχῖνος, romanized: ekhînos, lit. 'hedgehog, sea urchin'.^[5]

Echidnas are medium-sized, solitary mammals covered with coarse hair and spines.^[6] The spines are modified hairs and are made of keratin, the same fibrous protein that makes up fur, claws, nails, and horn sheaths in animals.^[2]

Superficially, they resemble the anteaters of South America and other spiny mammals such as hedgehogs and porcupines. They are usually black or brown in coloration. There have been several reports of albino echidnas with pink eyes and white spines.^[6] They have elongated and slender snouts that function as both mouth and nose, and which have electrosensors to find earthworms, termites, ants, and other burrowing prey.^[7] This is similar to the platypus, which has 40,000 electroreceptors on its bill, but the long-beaked echidna has only 2,000, while the short-beaked echidna, which lives in a drier environment, has no more than 400 at the tip of its snout.^[8]

Echidnas have short, strong limbs with large claws, and are powerful diggers. Their hind claws are elongated and curved backwards to aid in digging. Echidnas have tiny mouths and toothless jaws, and feed by tearing open soft logs, anthills and the like, and licking off prey with their long, sticky tongues. The ears are slits on the sides of their heads under the spines. The external ear is created by a large cartilaginous funnel, deep in the muscle.^[6] At 33 °C (91.4 °F), echidnas also possess the second-lowest active body temperature of all mammals, behind the platypus.

Despite their appearance, echidnas are capable swimmers, as they evolved from platypus-like ancestors. When swimming, they expose their snout and some of their spines, and are known to journey to water to bathe.^[9]

The first European drawing of an echidna was made in Adventure Bay, Tasmania by HMS Providence's third lieutenant George Tobin during William Bligh's second breadfruit voyage.^[10]

The short-beaked echidna's diet consists mostly of ants and termites, while the Zaglossus (long-beaked) species typically eat worms and insect larvae.^[11] The tongues of long-beaked echidnas have sharp, tiny spines that help them capture their prey.^[11] They have no teeth, so they break down their food by grinding it between the bottoms of their mouths and their tongues.^[12] Echidnas' faeces are 7 cm (3 in) long and are cylindrical in shape; they are usually broken and unrounded, and composed largely of dirt and ant-hill material.^[12]

Like all mammals, echidnas feed their young on milk, which contains various factors to sustain their growth and development.^[13][14]

Echidnas do not tolerate extreme temperatures; they shelter from harsh weather in caves and rock crevices. Echidnas are found in forests and woodlands, hiding under vegetation, roots or piles of debris. They sometimes use the burrows (both abandoned and in use) of animals such as rabbits and wombats. Individual echidnas have large, mutually overlapping territories.^[12]

Echidnas and platypuses are the only egg-laying mammals, the monotremes. The average lifespan of an echidna in the wild is estimated at 14–16 years. Fully grown females can weigh about 4.5 kilograms (9.9 lb), the males 33% larger, at about 6 kilograms (13 lb).^[12] Though the internal reproductive organs differ, both sexes possess an identical single cloaca opening for urination, defecation, and mating.^[6]

Male echidnas have non-venomous spurs on the hind feet, similar to the venomous male platypus.^[15]

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.^[16]

The echidna's brain is half neocortex,^[17] compared to 80% of a human brain.^[18][19] Contrary to previous research, the echidna does enter REM sleep, but only in a comfortable temperature around 25 °C (77 °F). At lower or higher temperatures of 15 °C (59 °F) and 28 °C (82 °F), REM sleep is suppressed.^[20]

The female lays a single soft-shelled, leathery egg 22 days after mating, and deposits it directly into her pouch. An egg weighs 1.5 to 2 grams (0.05 to 0.07 oz)^[21] and is about 1.4 centimetres (0.55 in) long. While hatching, the baby echidna opens the leather shell with a reptile-like egg tooth.^[22] Hatching takes place after 10 days of gestation; the young echidna, called a puggle,^[23][24] born larval and fetus-like, 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,^[25] 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. Puggles will stay within their mother's den for up to a year before leaving.^[12]

Male echidnas have a four-headed penis.^[26] 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.^[27][28] 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.^[29] These may be used to induce ovulation in the female.^[30]

It is a challenge to study the echidna in its natural habitat, and they show no interest in mating while in captivity. Prior to 2007, no one had 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 this has only resulted in the penis swelling.^[28]

Breeding season begins in late June and extends through September. During mating season, a female may be followed by a line or "train" of up to 10 males, the youngest trailing last, and some males switching between lines.^[12]

Echidnas are very timid. When frightened, they attempt to partially bury themselves and curl into a ball similar to a hedgehog. Strong front arms allow echidnas to dig in and hold fast against a predator pulling them from the hole.

Their many predators include feral cats, foxes, domestic dogs, and goannas. Snakes pose a large threat when they slither into echidna burrows and prey on the spineless young puggles.

They are easily stressed and injured by handling. Some ways to help echidnas include picking up litter, causing less pollution, planting vegetation for shelter, supervising pets, reporting hurt echidnas, and leaving them undisturbed.^[12]

The divergence between oviparous (egg-laying) and viviparous (offspring develop internally) mammals is believed to date to the Triassic period.^[31] Most findings from genetics studies (especially of nuclear genes) are in agreement with the paleontological dating, but some other evidence, like mitochondrial DNA, give slightly different dates.^[32]

Molecular clock data suggest echidnas split from platypuses between 19 and 48 million years ago, so that platypus-like fossils dating back to over 112.5 million years ago represent basal forms, rather than close relatives of the modern platypus.^[4] This would imply that echidnas evolved from water-foraging ancestors that returned to land living, which put them in competition with marsupials.^{[further explanation needed]} Although extant monotremes lack adult teeth (platypuses have teeth only as juveniles), many extinct monotreme species have been identified based on the morphology of their teeth.^[7] Of the eight genes involved in tooth development, four have been lost in both platypus and echidna, indicating that the loss of teeth occurred before the echidna-platypus split.^[22]

Further evidence of water-foraging ancestors can be found in some of the echidna's anatomy, including hydrodynamic streamlining, dorsally projecting hind limbs acting as rudders, and locomotion founded on hypertrophied humeral long-axis rotation, which provides an efficient swimming stroke.^[4]

Oviparous reproduction in monotremes may give them an advantage over marsupials in some environments.^[4] Their observed adaptive radiation contradicts the assumption that monotremes are frozen in morphological and molecular evolution.

It has been suggested that echidnas originally evolved in New Guinea when it was isolated from Australia and from marsupials. This would explain their rarity in the fossil record, their abundance in present times in New Guinea, and their original adaptation to terrestrial niches, presumably without competition from marsupials.^[33]