Echidnas (/ɪˈkɪdnəz/), sometimes known asspiny anteaters,[1] are quill-covered[2]monotremes (egg-layingmammals) belonging to thefamilyTachyglossidae/tækiˈɡlɒsɪdiː/, living inAustralia andNew Guinea. The fourextant species of echidnas and theplatypus are the only living mammals that lay eggs and the only surviving members of theorder Monotremata.[3] The diet of some species consists of ants and termites, but they are not closely related to the Americantrue anteaters or tohedgehogs. Their young are called puggles.
Echidnas evolved between 20 and 50 million years ago and descend from an amphibious,platypus-like monotreme.
Etymology
Echidnas are possibly named afterEchidna, a creature fromGreek 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'.[4]
Asynonym for the echidna, in a mid-19th century British Encyclopaedia, is, “hedgehog of the Colonists at Sidney [sic]”.[5]
Physical characteristics
Echidnas are medium-sized, solitary mammals covered with coarse hair andspines.[6] The spines are modified hairs and are made ofkeratin, 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 ashedgehogs andporcupines. They are usually black or brown in coloration. There have been several reports ofalbino echidnas with pink eyes and white spines.[6] They have elongated and slendersnouts, orproboscises, that function as both mouth and nose, and which haveelectrosensors to find earthworms, termites, ants, and other burrowing prey.[7] This is similar to theplatypus, 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. Theexternal ear is created by a largecartilaginous 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 fromplatypus-like ancestors. When swimming, they expose their snout and some of their spines, and are known to journey to water to bathe.[9]
Theshort-beaked echidna's diet consists mostly of ants and termites, while theZaglossus (long-beaked) species typically eat worms and insect larvae.[11] The tongues oflong-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]
Habitat
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 andwombats. Individual echidnas have large, mutually overlapping territories.[12]
Anatomy
Echidnas and platypuses are the only egg-laying mammals, themonotremes.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 singlecloaca opening for urination, defecation, and mating.[6]
Male echidnas have non-venomousspurs on the hind feet, similar to the venomous male platypus.[13]
Due to their lowmetabolism 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.[14]
The echidna's brain is halfneocortex,[15] compared to 80% of a human brain.[16][17] Contrary to previous research, the echidna does enterREM 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.[18]
Reproduction
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)[19] and is about 1.4 centimetres (0.55 in) long. Whilehatching, the baby echidna opens the leather shell with a reptile-likeegg tooth.[20] Hatching takes place after 10 days ofgestation; the young echidna, called a puggle,[21][22] born larval and fetus-like, then sucksmilk from thepores of the twomilk patches (monotremes have noteats) and remains in the pouch for 45 to 55 days,[23] 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.[24] 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 itcopulates, it uses the pair of heads opposite those it used the previous time.[25][26] When not in use, the penis is retracted inside apreputial sac in the cloaca. The male echidna's penis is 7 centimetres (2.8 in) long when erect, and its shaft is covered withpenile spines.[27] These may be used toinduce ovulation in the female.[28]
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 ofelectrically stimulated ejaculation in order toobtain semen samples but this has only resulted in the penis swelling.[26]
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 ten males, the youngest trailing last, and some males switching between lines.[12]
Threats
Echidnas are very timid. When frightened, they attempt to partially bury themselves and curl into a ball similar to ahedgehog. Strong front arms allow echidnas to dig in and hold fast against a predator pulling them from the hole.
Their many predators includeferal cats,foxes,domestic dogs, andgoannas. 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]
In June 2024, scientists reported a first-of-its-kind encounter when they witnessed a tiger shark regurgitating a whole echidna off the coast of an island near Australia. Sharks are known to eat a wide range of animals, and occasionally objects, according to a news release by the researchers from James Cook University in North Queensland who also noted that sharks had previously been observed eating rocks for no apparent reason.[29]
Evolution
Short-beaked echidna skeleton
The divergence between oviparous (egg-laying) and viviparous (offspring develop internally) mammals is believed to date to theTriassic period.[30] Most findings from genetics studies (especially of nuclear genes) are in agreement with the palaeontological dating, but some other evidence, like mitochondrial DNA, give slightly different dates.[31]
Molecular clock data suggest echidnas split from platypuses between 19 and 48 million years ago, so thatplatypus-like fossils dating back to over 112.5 million years ago representbasal forms, rather than close relatives of the modern platypus.[32] This would imply that echidnas evolved fromwater-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.[20]
Further evidence ofwater-foraging ancestors can be found in some of the echidna's anatomy, includinghydrodynamic streamlining, dorsally projecting hind limbs acting as rudders, and locomotion founded on hypertrophied humeral long-axis rotation, which provides an efficient swimming stroke.[32]
Oviparous reproduction in monotremes may give them an advantage overmarsupials in some environments.[32] Their observed adaptive radiation contradicts the assumption that monotremes are frozen in morphological andmolecular evolution.
It has been suggested that echidnas originally evolved inNew 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]
Taxonomy
Cladogram of Tachyglossidae by Upham et al. 2019[34][35]
Echidnas are a smallclade with two extantgenera and fourspecies.[36] The genusZaglossus includes three extant and two fossil species, with only one extant species from the genusTachyglossus.
Sir David's long-beaked echidna (Z. attenboroughi), discovered by Western science in 1961 (described in 1998) and preferring a still higher habitat;[37][38]
In Australia, the short-beaked echidna may be found in many environments, including urban parkland, such as the shores ofLake Burley Griffin inCanberra, as depicted here.
Theshort-beaked echidna (Tachyglossus aculeatus) is found in southern, southeast and northeastNew Guinea, and also occurs in almost all Australian environments, from the snow-cladAustralian Alps to the deep deserts of theOutback, essentially anywhere ants and termites are available. It is smaller than theZaglossus species, and it has longer hair.
Despite the similar dietary habits and methods of consumption to those of an anteater, there is no evidence supporting the idea that echidna-like monotremes have been myrmecophagous (ant or termite-eating) since theCretaceous. The fossil evidence of invertebrate-feedingbandicoots and rat-kangaroos, from around the time of the platypus–echidnadivergence and pre-datingTachyglossus, show evidence that echidnas expanded into new ecospace despite competition from marsupials.[39]
M. hacketti (previously classified in the genusZaglossus) from Pleistocene ofWestern Australia.
As food
TheKunwinjku people ofWestern Arnhem Land (Australia) call the echidnangarrbek,[41] and regard it as a prized food and "good medicine".[note 1][42] The echidna is hunted at night, gutted, and filled with hot stones and mandak (Persoonia falcata) leaves.[43] According toLarrakia elders Una Thompson and Stephanie Thompson Nganjmirra, once captured, an echidna is carried attached to the wrist like a thick bangle.[citation needed]
^Dunbar, R.I.M. (1993). "Coevolution of neocortical size, group size and language in humans".Behavioral and Brain Sciences.16 (4):681–735.doi:10.1017/S0140525X00032325.S2CID145583615.
^Dunbar, R.I.M."The Social Brain Hypothesis"(PDF).University of Colorado at Boulder, Department of Psychology and Neuroscience. Archived fromthe original(PDF) on 12 April 2016. Retrieved4 January 2014.
^Kuruppath, Sanjana; Bisana, Swathi; Sharp, Julie A; Lefevre, Christophe; Kumar, Satish; Nicholas, Kevin R (11 August 2012). "Monotremes and marsupials: Comparative models to better understand the function of milk".Journal of Biosciences.37 (4):581–588.doi:10.1007/s12038-012-9247-x.hdl:10536/DRO/DU:30047989.PMID22922184.S2CID15026875.Developmental stages of echidna: (A) Echidna eggs; (B) Echidna puggle hatching from egg...
^Musser AM (2003). "Review of the monotreme fossil record and comparison of palaeontological and molecular data".Comp. Biochem. Physiol. A.136 (4):927–42.doi:10.1016/s1095-6433(03)00275-7.PMID14667856.
