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Monotremes[1]
Temporal range: Early Cretaceous (Barremian) – Present
PlatypusWestern long-beaked echidnaShort-beaked echidnaEastern long-beaked echidna
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Mammaliaformes
Class: Mammalia
Order: Monotremata
C.L. Bonaparte, 1837[2]
Subgroups

Monotremes (/ˈmɒnətriːmz/) are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The five extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified by structural differences in their brains, jaws, digestive tracts, reproductive tracts, and other body parts, compared to the more common mammalian types. Although they are different from other living mammals in that they lay eggs, female monotremes are like other mammals in that they nurse their young with milk.

Monotremes have been considered by some authors to be members of Australosphenida, a clade that contains extinct mammals from the Jurassic and Cretaceous of Madagascar, South America, and Australia, but this categorization is disputed and their taxonomy is under debate.

All extant species of monotremes are indigenous to Australia and New Guinea, although they were also present during the Late Cretaceous and Paleocene epochs in southern South America, implying that they were also present in Antarctica, though remains have not yet been found there.

The name monotreme derives from the Greek words μονός (monós 'single') and τρῆμα (trêma 'hole'), referring to the cloaca.

General characteristics

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Like other mammals, monotremes are endothermic with a high metabolic rate, though not as high as other mammals; have hair on their bodies; produce milk through mammary glands to feed their young; have a single bone in their lower jaw; and have three middle ear bones.

In common with marsupials, monotremes lack the connective structure (corpus callosum) which in placentals is the primary communication route between the right and left brain hemispheres.[4] The anterior commissure does provide an alternate communication route between the two hemispheres, though, and in monotremes and marsupials it carries all the commissural fibers arising from the neocortex, whereas in placental mammals the anterior commissure carries only some of these fibers.[5]

Platypus
Short-beaked echidna
Diagram of a monotreme egg. (1) shell; (2) yolk; (3) yolk sac; (4) allantois; (5) embryo; (6) amniotic fluid; (7) amniotic membrane; and (8) membrane

Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have a "tribosphenic" form of molars (with the occlusal surface formed by three cusps arranged in a triangle), which is one of the hallmarks of extant mammals. Some recent work suggests that monotremes acquired this form of molar independently of placentals and marsupials,[6] although this hypothesis remains disputed.[7] Tooth loss in modern monotremes might be related to their development of electrolocation.[8]

Monotreme jaws are constructed somewhat differently from those of other mammals, and the jaw opening muscle is different. As in all true mammals, the tiny bones that conduct sound to the inner ear are fully incorporated into the skull, rather than lying in the jaw as in non-mammalian cynodonts and other pre-mammalian synapsids; this feature, too, is now claimed to have evolved independently in monotremes and therians,[9] although, as with the analogous evolution of the tribosphenic molar, this hypothesis is disputed.[10][11] Nonetheless, findings on the extinct species Teinolophos confirm that suspended ear bones evolved independently among monotremes and therians.[12] The external opening of the ear still lies at the base of the jaw.

The sequencing of the platypus genome has also provided insight into the evolution of a number of monotreme traits, such as venom and electroreception, as well as showing some new unique features, such as monotremes possessing five pairs of sex chromosomes which collectively behave as a single XY sex-determination system — during spermatogenesis, the ten sex chromosomes of the male form an alternating chain of X and Y chromosomes that recombine at the ends of consecutive chromosomes, and all the X or all the Y chromosomes are inherited together. One of the X chromosomes resembles the Z chromosome of birds,[13] suggesting that the two sex chromosomes of marsupial and placentals evolved after the split from the monotreme lineage.[14] Additional reconstruction through shared genes in sex chromosomes supports this hypothesis of independent evolution.[15] This feature, along with some other genetic similarities with birds, such as shared genes related to egg-laying, is thought to provide some insight into the most recent common ancestor of the synapsid lineage leading to mammals and the sauropsid lineage leading to birds and modern reptiles, which are believed to have split about 315 million years ago during the Carboniferous.[16][17] The presence of vitellogenin genes (a protein necessary for egg yolk formation) is shared with birds; the presence of this symplesiomorphy suggests that the common ancestor of monotremes, marsupials, and placentals was oviparous, and that this trait was retained in monotremes but lost in all other extant mammal groups. DNA analyses suggest that although this trait is shared and is synapomorphic with birds, platypuses are still mammals and that the common ancestor of extant mammals lactated.[18]

The monotremes also have extra bones in the shoulder girdle, including an interclavicle and coracoid, which are not found in other mammals. Monotremes retain a reptile-like gait, with legs on the sides of, rather than underneath, their bodies. The monotreme leg bears a spur in the ankle region; the spur is not functional in echidnas, but contains a powerful venom in the male platypus. This venom is derived from β-defensins, proteins that are present in mammals that create holes in viral and bacterial pathogens. Some reptile venom is also composed of different types of β-defensins, another trait shared with reptiles.[16] It is thought to be an ancient mammalian characteristic, as many non-monotreme archaic mammal groups also possess venomous spurs.[19]

Reproductive system

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Further information: Echidna § Reproduction, and Platypus § Reproduction

The key anatomical difference between monotremes and other mammals gives them their name; monotreme means "single opening" in Greek, referring to the single duct (the cloaca) for their urinary, defecatory, and reproductive systems. Like birds and reptiles, monotremes have a single cloaca.[20] Marsupials have a separate genital tract, whereas most placental females have separate openings for reproduction (the vagina), urination (the urethra), and defecation (the anus). In monotremes, only semen passes through the penis while urine is excreted through the male's cloaca.[21] The monotreme penis is similar to that of turtles and is covered by a preputial sac.[22][23] Male monotremes do not have a prostate or seminal vesicles.[24]

Monotreme eggs are retained for some time within the mother and receive nutrients directly from her, generally hatching within ten days after being laid – much shorter than the incubation period of sauropsid eggs.[25][26] Much like newborn marsupials (and perhaps all non-placentals[27]), newborn monotremes, called "puggles",[28] are larval- and fetus-like and have relatively well-developed forelimbs that enable them to crawl around. Monotremes lack teats, so puggles crawl about more frequently than marsupial joeys in search of milk. This difference raises questions about the supposed developmental restrictions on marsupial forelimbs.[clarification needed][29]

Rather than through teats, monotremes lactate from their mammary glands via openings in their skin. All five extant species show prolonged parental care of their young, with low rates of reproduction and relatively long life-spans.

Monotremes are also noteworthy in their zygotic development: most mammalian zygotes go through holoblastic cleavage, where the ovum splits into multiple, divisible daughter cells. In contrast, monotreme zygotes, like those of birds and reptiles, undergo meroblastic (partial) division. This means that the cells at the yolk's edge have cytoplasm continuous with that of the egg, allowing the yolk and embryo to exchange waste and nutrients with the surrounding cytoplasm.[16]

Physiology

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Monotreme female reproductive organs
Male platypus reproductive system. 1. Testes, 2. Epididymis, 3. Bladder, 4. Rectum, 5. Ureter, 6. Vas Deferens, 7. Genito-urinary sinus, 8. Penis enclosed in a fibrous sheath, 9. Cloaca, 10. Opening in the ventral wall of the cloaca for the penis.

Monotremes' metabolic rate is remarkably low by mammalian standards. The platypus has an average body temperature of about 31 °C (88 °F) rather than the averages of 35 °C (95 °F) for marsupials and 37 °C (99 °F) for placentals.[30][31] Research suggests this has been a gradual adaptation to the harsh, marginal environmental niches in which the few extant monotreme species have managed to survive, rather than a general characteristic of extinct monotremes.[32][33]

Monotremes may have less developed thermoregulation than other mammals, but recent research shows that they easily maintain a constant body temperature in a variety of circumstances, such as the platypus in icy mountain streams. Early researchers were misled by two factors: firstly, monotremes maintain a lower average temperature than most mammals; secondly, the short-beaked echidna, much easier to study than the reclusive platypus, maintains normal temperature only when active; during cold weather, it conserves energy by "switching off" its temperature regulation. Understanding of this mechanism came when reduced thermal regulation was observed in the hyraxes, which are placentals.

The echidna was originally thought to experience no rapid eye movement sleep (REM).[34] However, a more recent study showed that REM sleep accounted for about 15% of sleep time observed on subjects at an environmental temperature of 25 °C (77 °F). Surveying a range of environmental temperatures, the study observed very little REM at reduced temperatures of 15 °C (59 °F) and 20 °C (68 °F), and also a substantial reduction at the elevated temperature of 28 °C (82 °F).[35]

Monotreme milk contains a highly expressed antibacterial protein not found in other mammals, perhaps to compensate for the more septic manner of milk intake associated with the absence of teats.[36]

During the course of evolution, the monotremes have lost the gastric glands normally found in mammalian stomachs as an adaptation to their diet.[37] As such, by some definitions, they do not have stomachs as an organ,[38] although the term is widely used in studies of monotreme anatomy.[39][40] Monotremes synthesize L-ascorbic acid only in the kidneys.[41]

Both the platypus and echidna species have spurs on their hind limbs. The echidna spurs are vestigial and have no known function, while the platypus spurs contain venom.[42] Molecular data show that the main component of platypus venom emerged before the divergence of platypus and echidnas, suggesting that the most recent common ancestor of these taxa was also possibly a venomous monotreme.[43]

Taxonomy

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The traditional "Theria hypothesis" states that the divergence of the monotreme lineage from the Metatheria (marsupial) and Eutheria (placental) lineages happened prior to the divergence between marsupials and placentals, and this explains why monotremes retain a number of primitive traits presumed to have been present in the synapsid ancestors of later mammals, such as egg-laying.[44][45][46] Most morphological evidence supports the Theria hypothesis, but one possible exception is a similar pattern of tooth replacement seen in monotremes and marsupials, which originally provided the basis for the competing "Marsupionta" hypothesis in which the divergence between monotremes and marsupials happened later than the divergence between these lineages and the placentals. Van Rheede (2005) concluded that the genetic evidence favors the Theria hypothesis,[47] and this hypothesis continues to be the more widely accepted one.[48]

Monotremes are conventionally treated as comprising a single order Monotremata. The entire grouping is also traditionally placed into a subclass Prototheria, which was extended to include several fossil orders, but these are no longer seen as constituting a group allied to monotreme ancestry. A controversial hypothesis now relates the monotremes to a different assemblage of fossil mammals in a clade termed Australosphenida, a group of mammals from the Jurassic and Cretaceous of Madagascar, South America and Australia, that share tribosphenic molars.[6][49] However, in a 2022 review of monotreme evolution, it was noted that Teinolophos, the oldest (Barremian ~ 125 million years ago) and the most primitive monotreme differed substantially from non-monotreme australosphenidans in having five molars as opposed to the three present in non-monotreme australosphenidians. Aptian and Cenomanian monotremes of the family Kollikodontidae (113–96.6 ma) have four molars. This suggests that the monotremes are likely to be unrelated to the australosphenidan tribosphenids.[50]

The time when the monotreme line diverged from other mammalian lines is uncertain, but one survey of genetic studies gives an estimate of about 220 million years ago,[51] while others have posited younger estimates of 163 to 186 million years ago (though the already eutherian Juramaia is dated to 161–160 million years ago). Teinolophos like modern monotremes displays adaptations to elongation and increased sensory perception in the jaws, related to mechanoreception or electroreception.[50]

An echidna excavating a defensive burrow on French Island

Molecular clock and fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19–48 million years ago,[52] but another putting it at 17–89 million years ago.[53] It has been suggested that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.[50]

The precise relationships among extinct groups of mammals and modern groups such as monotremes are uncertain, but cladistic analyses usually put the last common ancestor (LCA) of placentals and monotremes close to the LCA of placentals and multituberculates, whereas some suggest that the LCA of placentals and multituberculates was more recent than the LCA of placentals and monotremes.[54][55]

Phylogeny of a 5,911 species 31 gene supertree[56][57]
Monotremata

Fossil monotremes

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See also: Evolution of mammals
A model of the extinct monotreme Steropodon at the Australian Museum

The first Mesozoic monotreme to be discovered was the Cenomanian (100–96.6 Ma) Steropodon galmani from Lightning Ridge, New South Wales.[58] Biochemical and anatomical evidence suggests that the monotremes diverged from the mammalian lineage before the marsupials and placentals arose. The only Mesozoic monotremes are Teinolophos (Barremian, 126 Ma), Sundrius and Kryoryctes (Albian, 113–108 Ma), and Dharragarra, Kollikodon, Opalios, Parvopalus, Steropodon, and Stirtodon (all Cenomanian, 100.2–96.6 Ma) from Australian deposits, and Patagorhynchus (Maastrichtian) from Patagonian deposits in the Cretaceous, indicating that monotremes were diversifiying by the early Late Cretaceous.[59] Monotremes have been found in the latest Cretaceous and Paleocene of southern South America, so one hypothesis is that monotremes arose in Australia in the Late Jurassic or Early Cretaceous, and that some migrated across the Antarctic land bridge to South America, both of which were still united with Australia at that time.[60][61] This direction of migration is the opposite of that hypothesized for Australia's other dominant mammal group, the marsupials, which likely migrated across Antarctica to Australia from South America.[62]

In 2024, a prominent assemblage of early monotremes was described from the Cenomanian deposits (100–96.6 Ma) of the Griman Creek Formation in Lightning Ridge, New South Wales. One of these, the fossil jaw fragment of Dharragarra, is the oldest known platypus-like fossil.[50][3][63] The durophagous Kollikodon, the pseudotribosphenic Steropodon, and Stirtodon, Dharragarra, Opalios, and Parvopalus occur in the same Cenomanian deposits. Oligo-Miocene fossils of the toothed platypus Obdurodon have also been recovered from Australia, and fossils of a 63 million-year old platypus relative occur in southern Argentina (Monotrematum), see fossil monotremes below. The extant platypus genus Ornithorhynchus in also known from Pliocene deposits, and the oldest fossil tachyglossids are Pleistocene (1.7 Ma) in age.[50]

Fossil species

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A 100 million-year-old Steropodon jaw on display at the American Museum of Natural History, New York City, USA
Platypuses swimming at Sydney Aquarium

Excepting Ornithorhynchus anatinus, all the animals listed in this section are known only from fossils. Some family designations are hesitant, given the fragmentary nature of the specimens.[3]

References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Monotreme

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Monotremes are the sole extant order of egg-laying mammals, comprising a primitive subclass of mammals known as , which diverged from other mammalian lineages during the era. They are distinguished by their via leathery-shelled eggs, a for and , and the absence of nipples, with secreted through openings onto the fur or skin to nourish hatchlings. Despite these reptilian-like traits, monotremes exhibit key mammalian characteristics, including fur (though lacking whiskers), a four-chambered heart, three bones, a single lower , relatively low metabolic rates, and endothermy, albeit with lower body temperatures than other mammals. The order Monotremata includes only five living species: the duck-billed (Ornithorhynchus anatinus) in the family , which is semiaquatic and inhabits eastern and , and four species of echidnas (spiny anteaters) in the family Tachyglossidae, which are terrestrial and distributed across and . These animals display specialized adaptations, such as the platypus's webbed feet, electroreceptive bill for detecting prey, and venomous spurs in males, while echidnas feature elongated snouts, strong claws for digging, and spines integrated with fur for defense. Reproduction involves laying one to three small eggs (about 13–15 mm in diameter) after a period, with incubation lasting around 10–12 days in a or pouch-like fold; the young, called puggles, hatch underdeveloped and nurse for several months until weaning at 16–20 weeks. Fossils indicate monotremes originated in the (~126 million years ago) in what is now , with ancient relatives once widespread across , including a recently discovered species from , though modern species are now endemic to following a diversification gap of about 35 million years. Their unique blend of ancestral and derived traits provides critical insights into mammalian , particularly in , where their multiple , for example the platypus has a diploid number of 52, and repertoire show a mosaic of reptilian and mammalian features.

Taxonomy and Evolution

Living Species and Classification

Monotremata is the sole order of extant egg-laying mammals, known as monotremes, and comprises only five living divided into two families. These species represent the most basal lineage among modern mammals, characterized by their unique reproductive strategy of laying leathery eggs rather than giving birth to live young. The order Monotremata is classified under the subclass , though this grouping is sometimes debated in modern , and includes the family with a single species, the (Ornithorhynchus anatinus), and the family Tachyglossidae with four species of echidnas. The Tachyglossidae encompasses the (Tachyglossus aculeatus) and three species of long-beaked echidnas in the genus Zaglossus: Z. bruijnii, Z. attenboroughi, and Z. bartoni. The name Monotremata, meaning "single opening," refers to the shared by monotremes for and , a trait distinguishing them from other mammals. Historically, the of monotremes began with the description of the by in 1800, who named it Ornithorhynchus paradoxus (later synonymized with O. anatinus), highlighting its anomalous features among mammals. The order Monotremata was formally established by in 1837, grouping the and echidnas based on shared primitive traits. In contemporary , molecular and genomic has solidified Monotremata as a monophyletic clade distinct from and basal to (marsupials and placentals), diverging around 187 million years ago during the . The living monotreme species exhibit limited geographic ranges confined to . The (Ornithorhynchus anatinus) is endemic to the eastern seaboard of , including , inhabiting freshwater rivers, streams, and lakes in forested and agricultural areas. The (Tachyglossus aculeatus) has the broadest distribution among monotremes, occurring across , , , and southeastern , adapting to diverse habitats from deserts to rainforests. The three long-beaked echidna species (Zaglossus spp.) are restricted to and adjacent islands, primarily in highland forests and mountainous regions of Papua New Guinea and , with each species occupying distinct elevational zones in the island's central cordillera and peninsulas.

Phylogenetic Position

Monotremes occupy a basal position within the mammalian class Mammalia, forming the sister group to , which encompasses both marsupials () and placentals (). This relationship is robustly supported by genomic analyses, including nuclear gene sequences and insertions/deletions, which place monotremes as the outgroup to therians. Molecular dating estimates the divergence of monotremes from therians at approximately 166 million years ago during the . Several morphological features underpin the basal phylogenetic placement of monotremes, reflecting their retention of ancestral traits relative to therians. These include , characterized by egg-laying reproduction, a single serving urinary, digestive, and reproductive functions, and a distinctive with additional reptilian-like elements such as the interclavicle and bones, which provide enhanced rigidity. The hypothesis proposes that monotremes are more closely related to a of ancient mammals, including and forms like Ausktribosphenos, than to northern therians, based on shared tribosphenic molar structures and morphology. This view has sparked debate, with some analyses suggesting Australosphenida may be polyphyletic and that monotremes derive from earlier pretribosphenic stock, while others uphold the southern affinity but affirm the overall monotreme-therian split. Recent genomic studies from the 2020s have illuminated monotreme-specific adaptations through expanded gene families. For instance, the genome exhibits proliferation of defensin-like peptide genes (OavDLP) associated with production in males, a trait lost in echidnas. Similarly, expansions in vomeronasal type-1 receptor (V1R) gene families support the evolution of electroreception in the bill, contrasting with reduced genes and highlighting sensory divergence within monotremes.

Fossil Record and Origins

The fossil record of monotremes provides compelling evidence of their ancient origins within the Mesozoic era, with the earliest known remains dating to the Early Cretaceous period in Australia. Teinolophos trusleri, described from a single molar tooth discovered in the Flat Rocks locality of New South Wales, represents the oldest undisputed monotreme, with an age estimated at 121–112.5 million years ago (Ma). This diminutive species, likely rat-sized, confirms that monotremes had already diverged from other mammal lineages by the Early Cretaceous and possessed specialized dental features indicative of an insectivorous diet. Subsequent discoveries from the same Lightning Ridge opal fields in have expanded our understanding of early monotreme diversity. Steropodon galmani, known from opalized jaw fragments dated to approximately 100 Ma, is another () form that exhibits tribosphenic-like molars adapted for crushing prey, suggesting a semi-aquatic similar to modern . More recent excavations in 2024 at Lightning Ridge have yielded three additional monotreme species—Opalios splendens, Dharragarra aurora, and Parvopalus clytiei—from the same deposits, indicating a surprisingly high diversity of egg-laying mammals in around 100 Ma, including forms with mixed platypus- and echidna-like traits. In contrast, later fossils such as Obdurodon dicksoni from the Miocene Riversleigh World Heritage Area (approximately 23–5 Ma) reveal larger, toothed platypus relatives with robust skulls and spoon-shaped bills, highlighting the persistence of dental structures that were eventually lost in extant species. The evolutionary timeline of monotremes underscores their deep divergence from therian mammals (marsupials and placentals) during the period, with molecular estimates placing the split around 187 Ma, predating the oldest fossils by tens of millions of years. Following this divergence, monotremes underwent significant radiation in , particularly in what is now , after the supercontinent's fragmentation began around 180 Ma; by the , they had adapted to diverse niches in isolation on the Australian continent. The loss of functional teeth in modern monotremes, such as the and echidnas, likely occurred during the Oligocene-Miocene transition, as evidenced by the toothed persisting into the before the emergence of edentulous forms. Fossils from outside Australia further illuminate monotreme migration patterns across . In 2023, pascuali was described from an isolated humerus in the (, ~70 Ma) of , , representing the earliest known monotreme from and suggesting a broader Gondwanan distribution during the . sudamericanum, identified from isolated teeth in early (Tiupampan, ~61–63 Ma) sediments of , , is another South American monotreme and suggests that these mammals inhabited southern continents when land connections between and still existed, which persisted until approximately 34 Ma in the Eocene. This distribution implies a Gondwanan origin and dispersal before fully severed connections, with no subsequent evidence of monotremes surviving in after the .

Morphology and Anatomy

External Features

Monotremes exhibit distinctive external features that set them apart from other mammals, including a combination of coverage and specialized appendages adapted to their respective aquatic or terrestrial lifestyles. All species possess dense for insulation, but echidnas additionally feature spines integrated into their pelage for defense against predators. The platypus (Ornithorhynchus anatinus) displays a streamlined body covered in waterproof fur ranging from dark brown to reddish brown, with lighter underfur, aiding its semiaquatic existence. Its most notable external trait is the soft, leathery, duck-like bill, which facilitates electroreceptive foraging in murky waters. The limbs feature five digits: the front feet are fully webbed for swimming propulsion, while the hind feet are partially webbed and serve as rudders, complemented by strong claws for burrowing on land. Adults measure 37–63 cm in length and weigh 0.6–3 kg, with southern populations tending to be larger. In contrast, echidnas (family Tachyglossidae) have a more robust, cylindrical body covered in coarse fur interspersed with hollow spines—yellowish in short-beaked (Tachyglossus aculeatus) and white, gray, or black in long-beaked (genus Zaglossus)—providing a defensive mechanism when the animal curls into a ball. Their limbs are short and stout, ending in five clawed digits optimized for digging into soil for shelter and prey extraction; hind claws curve backward to enhance this burrowing efficiency. Size varies by : short-beaked echidnas reach 30–45 cm in length and 2.5–7 kg, while long-beaked echidnas extend to 60–100 cm and 5–10 kg. Sexual dimorphism is pronounced in the platypus, where males are larger and possess venomous spurs on their hind ankles connected to crural glands, used during breeding season, whereas females lack functional spurs. Echidnas show minimal external differences between sexes. Unlike therian mammals, all monotremes lack true teats; instead, females eject milk through pores in the skin of the abdominal mammary glands, which the young lap up from fur patches.

Internal Anatomy and Adaptations

Monotremes possess a and structure that retains several primitive mammalian traits akin to those in reptilian ancestors. The lower , or , is primarily composed of the , which is slender and bears only a vestige of the coronoid process, differing from the more robust form in therian mammals. During , monotremes hatch lacking a fully functional (TMJ); the developing provide transient support to the lower jaw until the mammalian dentary-squamosal TMJ fully develops. The in monotremes is distinctive for retaining elements lost in other mammals, including a fused to the and a separate epicoracoid bone. These structures, homologous to reptilian components, serve as origins for key muscles such as the supracoracoideus and subcoracoideus, facilitating humeral flexion and adduction essential for locomotion and . Adult monotremes lack , a condition unique among mammals, and instead utilize paired horny pads in the upper and lower jaws to grind and crush prey such as . This edentulous state is a derived feature, as evidenced by fossil relatives like Obdurodon dicksoni that possessed functional molars; the loss in modern forms is linked to adaptations for electroreception, which enlarged neural canals and reduced space for roots. Juveniles, however, develop temporary milk teeth during early postnatal stages, which are resorbed and replaced by the horny pads as the animals mature. Monotremes exhibit a highly derived digestive system with a simple, non-acidic lacking typical glandular structures and, in the , a pyloric . This adaptation is linked to the ancient inactivation of the Nkx3.2 gene, which regulates development, resulting in reduced acid secretion and protein digestion suited to their invertebrate-based diets. The of monotremes displays reptilian-like organization, most notably in the absence of a ; interhemispheric communication occurs via an enlarged , which is topographically organized to connect homologous cortical regions. Olfactory structures are prominently expanded, featuring a large and pyriform cortex that occupy a substantial portion of the telencephalon, underscoring the reliance on chemosensory cues for and prey detection. Monotremes have a four-chambered heart, aligning with other mammals in providing separated pulmonary and systemic circulations, yet it incorporates primitive reptilian elements such as a well-developed sinuatrial that regulates blood flow into the right atrium. While ventricular septation is complete, certain atrial features echo reptilian conditions, including myocardial extensions from the that partially integrate with the atrial walls, potentially allowing minor hemodynamic adjustments during diving or activity.

Physiology

Metabolic and Thermoregulatory Processes

Monotremes exhibit basal metabolic rates (BMR) that are notably lower than those of eutherian mammals, typically ranging from 60% to 70% of equivalent-sized placentals, which enables energy conservation in fluctuating environmental conditions. For instance, the displays a BMR approximately 70-80% of eutherian values, while echidnas show even lower rates at 25-40%, reflecting adaptations to their respective and terrestrial lifestyles. This reduced metabolic demand contributes to their overall energy efficiency, particularly in habitats with variable food availability and temperatures. Torpor and hibernation patterns further underscore these metabolic adaptations in monotremes. The platypus does not enter hibernation or daily , maintaining stable activity without significant drops in metabolic rate during winter. In contrast, echidnas, particularly the short-beaked species, undergo seasonal during winter, where body temperature can plummet to 4-9°C—close to ambient levels—accompanied by profound metabolic suppression. These arousals occur periodically every 2-3 weeks, allowing brief periods of normothermia before returning to , a strategy that minimizes energy expenditure in colder months. Thermoregulation in monotremes involves specialized mechanisms to manage their relatively low core body temperatures. The sustains a core temperature of around 32°C in both air and water, achieved through behavioral adjustments like burrowing and reliance on dense insulation rather than high metabolic production. Echidnas employ a nasal counter-current heat exchange system, where blood vessels in the nasal passages conserve during exhalation in cold conditions, supplemented by evaporative cooling via mucus bubbles at the tip during heat stress to prevent overheating without panting or sweating. Monotreme milk composition supports rapid neonatal growth through elevated levels, differing markedly from therian mammals. Unlike the lactose-dominant of eutherians and marsupials, monotreme features high concentrations of complex carbohydrates such as fucose-rich oligosaccharides, including sialyl-lactose in echidna and difucosyl-lactose in , which provide an efficient energy source for egg-hatching young. This emphasis, comprising a significant portion of solids, facilitates quick development in the absence of a placental connection.

Sensory Systems

Monotremes exhibit specialized sensory adaptations that reflect their semi-aquatic or terrestrial lifestyles, with the platypus (Ornithorhynchus anatinus) demonstrating particularly advanced electrosensory and mechanosensory capabilities in its bill for detecting prey in murky waters. The platypus bill contains approximately 40,000 mucous gland electroreceptors, which are distributed across its skin and function to detect weak electric fields generated by the muscular contractions of hidden prey, such as crustaceans and small fish. These electroreceptors are present in far smaller numbers in echidnas (family Tachyglossidae), with estimates ranging from about 400 in short-beaked echidnas to around 2,000 in long-beaked echidnas, limiting their electrosensory role compared to the platypus and reflecting differences in terrestrial versus semi-aquatic foraging. This sensory modality is unique among mammals and supports the platypus's foraging efficiency in low-visibility environments. Complementing electroreception, the platypus bill features push-rod mechanoreceptors that enable tactile detection of prey movements and vibrations underwater. These specialized structures, consisting of elongated sensory cells connected to endings, form push-rod circuits that transmit mechanical stimuli from the bill's surface, allowing the animal to localize objects through subtle changes even when its eyes, ears, and nostrils are sealed during dives. Echidnas possess similar push-rod mechanoreceptors in their snouts, adapted for probing and detecting or via and texture, though these are less specialized for aquatic use. Vision in monotremes is generally poor, particularly in the platypus, where small eyes and a structure suited to low-light conditions provide limited acuity, especially underwater where nictitating membranes obscure sight. This visual limitation is compensated by a robust ; the platypus possesses an expanded set of genes, enabling acute smell for navigating and locating food on land, while echidnas rely heavily on olfaction for foraging, with an enlarged processing scents to detect buried from afar. Hearing in monotremes is facilitated by a structure with three that evolved independently from those in therian mammals (marsupials and placentals), featuring a distinct configuration that supports sensitivity to low-frequency sounds but lacks the full range of ultrasonic detection found in many therians. Although not a sensory organ per se, the venom delivery system in male platypuses integrates with defensive behaviors, where hind-leg spurs connected to crural glands inject a complex venom containing defensin-like peptides (DLPs) that induce intense, prolonged without causing lethality in humans or typical predators. This toxin, produced seasonally during breeding, disrupts ion channels and causes , serving primarily as a competitive tool among males rather than a predatory .

Reproduction and Development

Reproductive Anatomy

Monotremes possess a , a single multifunctional opening that serves as the common exit for the digestive, urinary, and reproductive systems, distinguishing them from therian mammals which have separate orifices for these functions. This structure facilitates egg-laying, urination, and defecation through a in females and a similar arrangement in males. In males, the testes are internal and undescended, a primitive trait retained from reptilian ancestors. The (Ornithorhynchus anatinus) features a single that everts during for delivery, while is expelled separately via the . In contrast, the (Tachyglossus aculeatus) has a unique with four rosette-like at the tip, formed by a quadrifurcate ; only two become erect and functional per event, and the organ is used exclusively for , not urination. Female monotremes have paired ovaries and two separate oviducts that converge into a single opening into the , with no true or placental development, reflecting their oviparous mode. The mammary glands lack nipples or teats; instead, milk is secreted through specialized pores in hairless areolar patches on the or chest, where it accumulates for young to lap up. These glands enlarge during to support the immunologically naive hatchlings. Breeding in monotremes is generally seasonal, with the exhibiting induced triggered by in spring to summer (August–October in ). Echidnas also show induced , primarily during winter to spring (June–September), though some populations in warmer regions display more variable or extended activity. Egg development occurs within the oviducts, where albumen and shell layers form prior to laying.

Egg-Laying, Incubation, and Parental Care

Monotremes are unique among mammals in their oviparous , laying leathery-shelled eggs rather than giving birth to live young. The eggs are small, typically measuring 11 to 16 mm in diameter, with a soft, flexible shell composed primarily of keratin-like proteins that lacks the seen in avian eggs. This leathery texture protects the while allowing through its porous structure. Females usually lay one to three eggs per , depending on the , after a period of about 21 days in the and 21 to 23 days in echidnas. Incubation methods vary between the and echidnas, reflecting adaptations to their respective habitats. In the (Ornithorhynchus anatinus), the female lays the s in a concealed and curls her body around them to incubate them for approximately 10 days, using her to cover and warm the clutch while periodically leaving to . This external incubation maintains a stable temperature through direct contact with the mother's body heat. In contrast, echidna (Tachyglossus and Zaglossus) lay a single directly into a temporary abdominal pouch formed by muscular contraction; the is incubated within this pouch for about 10 to 11 days until hatching, after which the pouch serves as a nursery for the young for an additional 45 to 50 days in some populations, allowing extended development in a protected environment. Upon hatching, the premature young—known as puggles—are tiny, blind, and hairless, weighing less than 2 grams and measuring about 1.5 to 2 cm in length. Lacking nipples, monotreme mothers secrete milk from specialized mammary glands through pores in the skin, forming patches that the puggles actively lick or lap up for nourishment; this process relies on the young's instinctive behavior to stimulate milk ejection without suckling. In platypuses, the female remains vigilant at the burrow entrance, guarding and nursing the puggles for up to four months until they are independent, making frequent foraging trips but returning to nurse multiple times daily. Echidna mothers carry the puggle in the pouch initially, then deposit it in a secure nursery burrow around 50 days post-hatching, visiting periodically to nurse until weaning at 150 to 200 days, ensuring the young's survival through targeted parental investment.

Ecology and Behavior

Habitat, Distribution, and Conservation

Monotremes are endemic to Australia and New Guinea, with no native populations occurring elsewhere in the world. The platypus (Ornithorhynchus anatinus) is distributed across eastern Australia, from Cooktown in Queensland southward through New South Wales and Victoria to Tasmania, primarily east of the Great Dividing Range. The short-beaked echidna (Tachyglossus aculeatus) has the broadest range among monotremes, occurring across all of mainland Australia, Tasmania, and much of New Guinea. The three species of long-beaked echidnas (Zaglossus spp.) are confined to New Guinea. Monotremes occupy diverse habitats suited to their ecological niches. The platypus inhabits freshwater systems, including rivers, creeks, streams, and lakes with suitable bank vegetation for burrowing, often in areas with stable water flow and abundant macroinvertebrates. The short-beaked echidna thrives in a wide array of environments, from arid deserts and grasslands to temperate forests and woodlands, where it shelters in hollow logs, rock crevices, or self-dug burrows. Long-beaked echidnas prefer montane and highland forests in , typically at elevations between 1,500 and 4,000 meters, including mossy tropical forests and alpine grasslands with dense understory cover. Conservation statuses vary among monotreme species, reflecting differences in threats and population resilience. The is classified as Near Threatened on the , with populations declining due to ongoing habitat degradation. The is listed as Least Concern, owing to its wide distribution and adaptability, though some subspecies face localized pressures. Among the long-beaked echidnas, Zaglossus bartoni is Vulnerable, while Z. attenboroughi and Z. bruijnii are Critically Endangered, driven by severe population reductions. Attenborough's long-beaked echidna (Z. attenboroughi) was rediscovered in 2023 after more than 60 years without confirmed sightings, with photographic evidence and genetic confirmation published in 2025, underscoring the urgency of intensified conservation measures. Major threats to monotremes include habitat loss from land clearing, , and , which fragment ecosystems and reduce available resources. For the , climate change exacerbates risks by altering water flows through droughts and floods, potentially leading to 47–66% declines in abundance over 50 years under current conditions. Long-beaked echidnas face intense hunting pressure for and , compounded by habitat destruction from logging and in New Guinea's highlands. Conservation efforts focus on habitat protection, threat mitigation, and population recovery. In , reintroduction programs for the have been implemented in areas like to restore local populations, supported by river rehabilitation projects to improve and flow. Recent 2020s research on , including whole-genome sequencing of monotremes, informs breeding and translocation strategies to enhance and resilience against . For long-beaked echidnas, initiatives emphasize anti-poaching patrols and expansion in montane forests.

Diet and Foraging

Monotremes exhibit specialized diets that reflect their evolutionary adaptations to distinct ecological niches, with the platypus favoring aquatic prey and echidnas targeting terrestrial . These mammals lack teeth in adulthood, relying instead on structural modifications for prey capture and ingestion. Foraging behaviors are predominantly nocturnal, minimizing competition and predation risks while aligning with prey availability. The platypus (Ornithorhynchus anatinus) is a semi-aquatic monotreme whose diet consists primarily of aquatic , such as yabbies (freshwater like Cherax species), larvae, and worms. It employs electroreception through its bill to detect the electrical signals from prey muscle contractions, allowing it to effectively in murky waters. Daily foraging involves multiple dives, reaching depths of up to 3 meters and lasting 30-60 seconds each, with individuals consuming 20-30% of their body weight in prey per day during active periods. In contrast, echidnas, which are terrestrial monotremes, primarily consume and , accounting for approximately 90% of their dietary intake in short-beaked species (Tachyglossus aculeatus). They use a long, sticky to lap up from soil crevices, with the tongue extending up to 18 cm and retracting rapidly to capture prey. Long-beaked echidnas (Zaglossus ) supplement this with earthworms and other soft-bodied , adapting to forested habitats where such prey is abundant. Foraging occurs nocturnally on the ground, with echidnas using their strong foreclaws to dig into ant and termite mounds or overturn logs. Both and echidna foraging techniques emphasize efficiency in low-light conditions, with the conducting nocturnal swims in rivers and streams, and echidnas performing digs that can excavate up to 3 kg of per session. Key adaptations include horny, padded bills or snouts for crushing prey against the , followed by direct swallowing without mastication, which suits their soft-bodied diets and reduces energy expenditure on dental maintenance.

and

Monotremes exhibit a predominantly solitary , with individuals maintaining distinct ranges and minimal interactions outside of the breeding season. Females typically possess well-established, smaller ranges centered around or areas, while males roam over larger territories and engage in for access to receptive females. This asocial lifestyle is evident in both the (Ornithorhynchus anatinus) and echidnas (family Tachyglossidae), where adults rarely form lasting pairs or groups, though multiple individuals may occasionally share the same waterway or without overt aggression. In platypuses, territorial defense is facilitated by located on the shoulders and neck, which produce a musky that intensifies during the breeding period; individuals rub these secretions onto logs, rocks, or entrances to mark boundaries and signal presence to conspecifics. Communication among monotremes relies heavily on chemical cues rather than vocal or visual signals, reflecting their cryptic and solitary habits. Both platypuses and echidnas utilize glandular secretions for social and reproductive signaling; for instance, male echidnas possess crural and femoral glands that enlarge seasonally, producing scents potentially used in mate attraction or territorial marking via cloacal dragging. Recent observations have revealed that short-beaked echidnas (Tachyglossus aculeatus) produce subtle vocalizations, including cooing, grunting, and wheezing sounds, primarily during encounters, suggesting a limited acoustic component to their communication repertoire. Platypuses emit low-frequency growls, squeaks, or snuffles when disturbed, particularly underwater, though the functional role of these sounds in social interactions remains unclear and may serve more for individual distress signaling than inter-individual exchange. Olfactory cues thus appear paramount, with scent marking enabling indirect communication in their low-density populations. Daily activity patterns in monotremes are adapted to avoid predation and optimize use, with most displaying nocturnal or crepuscular rhythms. Platypuses are primarily active at night, foraging in streams and returning to burrows during daylight, while long-beaked echidnas (Zaglossus spp.) follow a similar nocturnal schedule in their forested habitats. Short-beaked echidnas, in contrast, exhibit more flexible patterns, being mostly nocturnal but occasionally active during the day in warmer regions. To cope with seasonal resource scarcity and cold s, echidnas frequently enter —a state of metabolic suppression—or prolonged lasting weeks to months, particularly in temperate or arid zones; body can drop to near ambient levels (around 5–10°C), with bouts triggered by environmental cues. Platypuses do not hibernate but may reduce activity in winter, relying on their semi-aquatic lifestyle for . These rhythms underscore the monotremes' low metabolic demands and opportunistic tied to environmental conditions. Reproductive behaviors in monotremes are brief and focused on aggregations, with social interactions peaking seasonally. In platypuses, breeding occurs from late winter to spring, during which males compete aggressively using spurs on their hind legs to deliver painful stings to rivals, establishing dominance and access to females; this , produced by crural glands, causes intense swelling and incapacitation but is not lethal to humans. Echidnas also breed seasonally, with males forming temporary "trains" of up to 11 individuals pursuing a receptive female in a circular , involving nose-to-tail following and occasional vocalizations; while not strictly monogamous, some pairs may remain associated briefly post- before resuming solitary lives. These encounters highlight male intrasexual competition driven by large testes relative to body size, indicative of in promiscuous systems across the group.

References

  1. https://animaldiversity.org/accounts/Monotremata/
  2. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Biology_%28Kimball%29/19%253A_The_Diversity_of_Life/19.01%253A_Eukaryotic_Life/19.1.15%253A_Monotremes
  3. https://ucmp.berkeley.edu/mammal/monotreme.html
  4. https://www.tandfonline.com/doi/pdf/10.1080/03115518.2022.2025900
  5. https://www.nature.com/articles/s42003-023-04498-7
  6. https://www.nature.com/articles/s41586-020-03039-0
  7. https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=180092
  8. https://www.science.smith.edu/departments/biology/VHAYSSEN/msi/pdf/i0076-3519-585-01-0001.pdf
  9. https://animaldiversity.org/accounts/Ornithorhynchus_anatinus/
  10. https://animaldiversity.org/accounts/Tachyglossus_aculeatus/
  11. https://animaldiversity.org/accounts/Zaglossus_bartoni/
  12. https://www.nature.com/articles/nature06936
  13. https://academic.oup.com/mbe/article/23/3/587/1110306
  14. https://www.sciencedirect.com/science/article/abs/pii/S1095643303002757
  15. https://www.pnas.org/doi/10.1073/pnas.0706385105
  16. https://www.tandfonline.com/doi/full/10.1080/03115518.2022.2025900
  17. https://digimorph.org/specimens/Teinolophos_trusleri/216750/
  18. https://www.researchgate.net/publication/258998114_Monotreme_nature_of_the_Australian_Early_Cretaceous_mammal_Teinolophos
  19. https://australian.museum/learn/australia-over-time/extinct-animals/steropodon-galmani/
  20. https://www.tandfonline.com/doi/full/10.1080/03115518.2024.2348753
  21. https://australian.museum/learn/australia-over-time/extinct-animals/obdurodon-dicksoni/
  22. https://www.researchgate.net/publication/225075243_First_discovery_of_monotremes_in_South_America
  23. https://platypus.asn.au/the-platypuss-fossil-history/
  24. https://animaldiversity.org/accounts/Tachyglossidae/
  25. https://australian.museum/learn/animals/mammals/platypus/
  26. https://pmc.ncbi.nlm.nih.gov/articles/PMC2267822/
  27. https://pmc.ncbi.nlm.nih.gov/articles/PMC10078425/
  28. https://elifesciences.org/articles/57860
  29. https://par.nsf.gov/servlets/purl/10144102
  30. https://pmc.ncbi.nlm.nih.gov/articles/PMC5061491/
  31. https://onlinelibrary.wiley.com/doi/10.1111/j.1469-7580.2009.01060.x
  32. https://doi.org/10.1098/rsob.240071
  33. https://pmc.ncbi.nlm.nih.gov/articles/PMC1978492/
  34. https://enpubs.faculty.ucdavis.edu/wp-content/uploads/sites/209/2015/03/Krubitzer_1999-What-can-monotremes-tell-us-about-brain-evolution.pdf
  35. https://pmc.ncbi.nlm.nih.gov/articles/PMC6590421/
  36. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0063651
  37. https://doi.org/10.1071/ZO9790511
  38. https://pmc.ncbi.nlm.nih.gov/articles/PMC5399094/
  39. https://pubmed.ncbi.nlm.nih.gov/2566425/
  40. https://pmc.ncbi.nlm.nih.gov/articles/PMC7319036/
  41. https://pubmed.ncbi.nlm.nih.gov/8828862/
  42. https://espace.library.uq.edu.au/view/UQ:143598/UQ143598_OA.pdf
  43. https://ielc.libguides.com/sdzg/factsheets/short-beaked-echidna/characteristics
  44. https://pmc.ncbi.nlm.nih.gov/articles/PMC1191811/
  45. https://pubmed.ncbi.nlm.nih.gov/9720114/
  46. https://pmc.ncbi.nlm.nih.gov/articles/PMC1692312/
  47. https://pubmed.ncbi.nlm.nih.gov/15705848/
  48. https://pubmed.ncbi.nlm.nih.gov/10417345/
  49. https://pmc.ncbi.nlm.nih.gov/articles/PMC11684381/
  50. https://karger.com/sxd/article/2/3/115/296216/Reproductive-Biology-in-Egg-Laying-Mammals
  51. https://pubmed.ncbi.nlm.nih.gov/33915542/
  52. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0053686
  53. https://www.researchgate.net/publication/323192886_Female_control_of_reproductive_behaviour_in_the_platypus_Ornithorhynchus_anatinus_with_notes_on_female_competition_for_mating
  54. https://www.wildlife.vic.gov.au/__data/assets/pdf_file/0020/92441/Short-Beaked-Echidna.pdf
  55. https://ielc.libguides.com/sdzg/factsheets/short-beaked-echidna/reproduction
  56. https://platypus.asn.au/platypus-reproduction/
  57. https://animals.sandiegozoo.org/animals/echidna
  58. https://www.researchgate.net/publication/345383754_Maternal_care_of_platypus_nestlings_Ornithorhynchus_anatinus
  59. https://www.austmus.gov.au/platypus
  60. https://www.environment.nsw.gov.au/topics/animals-and-plants/native-animals/native-animal-facts/land-mammals/echidnas
  61. https://nre.tas.gov.au/wildlife-management/fauna-of-tasmania/mammals/echidnas-and-platypus/platypus/platypus-in-tasmania
  62. https://www.qld.gov.au/environment/plants-animals/animals/discovering-wildlife/platypus
  63. https://figshare.utas.edu.au/ndownloader/files/40899773
  64. https://www.ipswich.qld.gov.au/About-Council/Initiatives/Environment/Wildlife/Platypus-Conservation
  65. https://library.dbca.wa.gov.au/FullTextFiles/925289.pdf
  66. https://www.nespthreatenedspecies.edu.au/media/atdhz0lv/azm_species-profile_short-beaked-echidna_v1.pdf
  67. https://www.edgeofexistence.org/species/attenboroughs-long-beaked-echidna/
  68. https://palmoildetectives.com/2021/02/03/eastern-long-beaked-echidna-zaglossus-bartoni/
  69. https://wwf.org.au/what-we-do/species/platypus/
  70. https://wildlife.org.au/platypuswatch-news-the-perilous-future-of-queenslands-platypus/
  71. https://www.mammaldiversity.org/taxon/1000002/
  72. https://www.mammaldiversity.org/taxon/1000004/
  73. https://animaldiversity.org/accounts/Zaglossus_bruijni/
  74. https://www.nature.com/articles/s44185-025-00086-6
  75. https://d3n8a8pro7vhmx.cloudfront.net/auscon/pages/18230/attachments/original/1624235948/Platypus_Assessment_report_v8.pdf?1624235948
  76. https://www.unsw.edu.au/newsroom/news/2020/01/platypus-on-brink-of-extinction
  77. https://news.mongabay.com/2020/01/severe-drought-and-other-climate-impacts-are-driving-the-platypus-towards-extinction/
  78. https://animaldiversity.org/accounts/Zaglossus_attenboroughi/
  79. https://www.environment.vic.gov.au/__data/assets/pdf_file/0030/484086/01-Platypus-PRR-FinalSign-1.pdf
  80. https://www.adelaide.edu.au/newsroom/news/list/2021/01/12/genomes-reveal-new-insights-into-iconic-aussie-animals
  81. https://www.unimelb.edu.au/newsroom/news/2021/january/genomes-reveal-insights-into-much-loved-aussie-animals
  82. https://openknowledge.fao.org/bitstreams/733f91eb-8dc0-4f01-8d2a-db62c41e8410/download
  83. https://www.researchgate.net/publication/259345527_Behaviour_and_ecology_of_monotremes
  84. https://ielc.libguides.com/sdzg/factsheets/platypus/behavior
  85. https://animals.sandiegozoo.org/animals/platypus
  86. https://ielc.libguides.com/sdzg/factsheets/short-beaked-echidna/behavior
  87. https://www.abc.net.au/news/2023-10-02/echidna-sounds-recorded-for-first-time-by-curtin-university/102917730
  88. https://pubmed.ncbi.nlm.nih.gov/11923076/
  89. https://journals.biologists.com/jeb/article/210/4/586/17232/Cooling-rates-and-body-temperature-regulation-of
  90. https://pmc.ncbi.nlm.nih.gov/articles/PMC4014732/
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