Editing Marsupial (section)

==Anatomy==
[[File:Koala climbing tree.jpg|thumb|[[Koala]] <br />(''Phascolarctos cinereus'')]]

Marsupials have typical mammalian characteristics—e.g., mammary glands, three [[Middle ear|middle ear bones]], (and ears that usually have [[tragus (ear)|tragi]],<ref>{{cite journal | vauthors = Stannard HJ, Dennington K, Old JM  | title = The external ear morphology and presence of tragi in Australian marsupials | journal = Ecology and Evolution | volume = 10 | pages = 9853–9866 | year = 2020 | issue = 18 | doi = 10.1002/ece3.6634| pmid = 33005349 | pmc = 7520188 | bibcode = 2020EcoEv..10.9853S }}</ref> varying in hearing thresholds<ref>{{cite journal | vauthors = Old JM, Parsons CL, Tulk, ML | title = Hearing thresholds of small native Australian mammals - red-tailed phascogale (''Phascogale calura''), kultarr (''Antechinomys laniger'') and spinifex hopping-mice (''Notomys Alexis'') | journal = Zoological Journal of the Linnean Society | volume = 190 | pages = 342–351 | year = 2020 | doi = 10.1093/zoolinnean/zlaa003}}</ref>), true [[hair]] and bone structure.<ref name="hair">{{cite book|url={{Google books|plainurl=yes|id=udCnKce9hfoC|page=97}}| vauthors = Feldhamer GA, Drickamer LC, Vessey SH, Merritt JF, Krajewski C |year=2007|title=Mammalogy: Adaptation, Diversity, Ecology|edition=3rd|location=Baltimore|publisher=Johns Hopkins University Press|isbn=978-0-8018-8695-9|oclc=124031907}}</ref> However, striking differences including anatomical features separate them from [[eutheria]]ns.

Most female marsupials have a front [[Pouch (marsupial)|pouch]], which contains multiple nursing [[teat]]s. Marsupials have other common structural features. [[ossification|Ossified]] [[patella]]e are absent in most modern marsupials (with exceptions)<ref>{{cite journal | vauthors = Samuels ME, Regnault S, Hutchinson JR | title = Evolution of the patellar sesamoid bone in mammals | journal = PeerJ | volume = 5 | pages = e3103 | year = 2017 | pmid = 28344905 | pmc = 5363259 | doi = 10.7717/peerj.3103 | doi-access = free }}</ref> and [[epipubic bone]]s are present. Marsupials (and [[monotreme]]s) also lack a gross communication ([[corpus callosum]]) between the right and left brain hemispheres.{{sfn|Nowak|1999}}

=== Skull and teeth ===
Marsupials exhibit distinct cranial features compared to placentals. Generally, their skulls are relatively small and compact. Notably, they possess frontal holes known as [[foramen]] lacrimale situated at the front of the orbit. Marsupials have enlarged cheekbones that extend further to the rear, and their lower jaw's angular extension (''processus angularis'') is bent toward the center. The hard palate of marsupials contains more openings than that of placentals.

Teeth differ significantly. Most Australian marsupials outside the order Diprotodontia have a varying number of incisors between their upper and lower jaws. Early marsupials had a dental formula of 5.1.3.4/4.1.3.4 per quadrant, consisting of five (maxillary) or four (mandibular) incisors, one canine, three premolars, and four molars, totaling 50 teeth. While some taxa, like the opossum, retain this original tooth count, others have reduced numbers.

For instance, members of the Macropodidae family, including kangaroos and wallabies, have a dental formula of 3/1 – (0 or 1)/0 – 2/2 – 4/4. Many marsupials typically have between 40 and 50 teeth, more than most placentals. In marsupials, the second set of teeth only grows in at the site of the third premolar and posteriorly; all teeth anterior to this erupt initially as permanent teeth.

=== Torso ===
Few general characteristics describe their skeleton. In addition to unique details in the construction of the ankle, [[epipubic bone]]s (''ossa epubica'') are observed projecting forward from the pubic bone of the pelvis. Since these are present in males and pouchless species, it is believed that they originally had nothing to do with reproduction, but served in the muscular approach to the movement of the hind limbs. This could be explained by an original feature of mammals, as these epipubic bones are also found in [[monotreme]]s. Marsupial reproductive organs differ from placentals. For them, the reproductive tract is doubled. Females have two [[uteri]] and two [[vagina]]s, and before birth, a birth canal forms between them, the median vagina.{{sfn|Nowak|1999}} In most species, males have a split or double penis lying in front of the scrotum,<ref name="Renfree1987"/> which is not [[Homology (biology)|homologous]] to the placental scrota.<ref>{{Cite book |last1=Armati |first1=Patricia J. |url=https://books.google.com/books?id=x3S5v971Nk0C&dq=scrotum&pg=PA115 |title=Marsupials |last2=Dickman |first2=Chris R. |last3=Hume |first3=Ian D. |date=2006-08-17 |publisher=Cambridge University Press |isbn=978-1-139-45742-2 |language=en}}</ref>

A pouch is present in most species. Many marsupials have a permanent bag, while in others such as the [[shrew opossum]] the pouch develops during gestation, where the young are hidden only by skin folds or in the maternal fur. The arrangement of the pouch is variable to allow the offspring to receive maximum protection. Locomotive kangaroos have a pouch opening at the front, while many others that walk or climb on all fours open in the back. Usually, only females have a pouch, but the male [[water opossum]] has a pouch that protects his genitalia while swimming or running.

=== General and convergences ===
{{Multiple image
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| footer = The [[sugar glider]], a marsupial, (left) and [[flying squirrel]], a placental, (right) are examples of [[convergent evolution]].
}}
Marsupials have adapted to many habitats, reflected in the wide variety in their build. The largest living marsupial, the [[red kangaroo]], grows up to {{convert|1.8|m|ftin}} in height and {{convert|90|kg|lbs}} in weight. Extinct genera, such as ''[[Diprotodon]]'', were significantly larger and heavier. The smallest marsupials are the [[marsupial mice]], which reach only {{convert|5|cm|in}} in body length.

Some species resemble placentals and are examples of [[convergent evolution]]. This convergence is evident in both brain evolution<ref>{{cite journal | vauthors = Todorov OS, Blomberg SP, Goswami A, Sears K, Drhlík P, Peters J, Weisbecker V | title = Testing hypotheses of marsupial brain size variation using phylogenetic multiple imputations and a Bayesian comparative framework | journal = Proceedings. Biological Sciences | volume = 288 | issue = 1947 | pages = 20210394 | date = March 2021 | pmid = 33784860 | pmc = 8059968 | doi = 10.1098/rspb.2021.0394 }}</ref> and behaviour.<ref>{{cite book | vauthors = Todorov OS |contribution=Marsupial Cognition |date=2019 | title = Encyclopedia of Animal Cognition and Behavior |pages=1–8 | veditors = Vonk J, Shackelford T |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-47829-6_1167-1 |isbn=978-3-319-47829-6 |s2cid=242256517 }}</ref> The extinct [[thylacine]] strongly resembled the placental wolf, hence one of its nicknames "Tasmanian wolf". The ability to glide evolved in both marsupials (as with [[sugar glider]]s) and some placentals (as with [[flying squirrel]]s), which developed independently. Other groups such as the kangaroo, however, do not have clear placental counterparts, though they share similarities in lifestyle and ecological niches with [[ruminant]]s.

===Body temperature===
Marsupials, along with [[monotremes]] ([[platypuses]] and [[echidnas]]), typically have lower body temperatures than similarly sized [[placentals]] ([[eutherians]]),<ref>{{cite book| title=Abstract: Thermoregulation in marsupials and monotremes, chapter of Marsupials and monotremes: nature's enigmatic mammals| date=2015| author1=Gaughan, John B.|author2=Hogan, Lindsay A.|author3=Wallage, Andrea| publisher=Nova Science Publishers, Incorporated| url=https://espace.library.uq.edu.au/view/UQ:380292| isbn=9781634834872| accessdate=2022-04-20}}</ref> with the averages being {{convert|35|°C}} for marsupials and {{convert|37|°C}} for placentals.<ref name="DC">{{cite web |url=http://www.bio.davidson.edu/courses/anphys/1999/White/thermal.htm |author=White |title=Thermal Biology of the Platypus |publisher=Davidson College |year=1999 |access-date=2006-09-14 |df=dmy-all |url-status=dead |archive-url=https://web.archive.org/web/20120306024923/http://www.bio.davidson.edu/courses/anphys/1999/White/thermal.htm |archive-date=March 6, 2012}}</ref><ref name="Sherman">{{cite web |url=http://faculty.bennington.edu/~sherman/comp.%20anim.%20physiol./control%20systems%20part%202.pdf |title=Control Systems Part&nbsp;2 |access-date=6 July 2016 |archive-date=8 October 2016 |archive-url=https://web.archive.org/web/20161008154505/http://faculty.bennington.edu/~sherman/comp.%20anim.%20physiol./control%20systems%20part%202.pdf |url-status=dead }}</ref> Some species will bask to conserve energy <ref>{{cite journal|last1=Stannard|first1= Hayley J.|last2= Fabian|first2= Megan|last3= Old|first3= Julie M.|year=2015|title=To bask or not to bask: Behavioural thermoregulation in two species of dasyurid, ''Phascogale calura'' and ''Antechinomys laniger''|journal= Journal of Thermal Biology|volume= 53|pages= 66–71| doi= 10.1016/j.jtherbio.2015.08.012|pmid= 26590457|bibcode= 2015JTBio..53...66S}}</ref>

==={{anchor|Reproductive_System}}Reproductive system===
{{see also|Kangaroo#Reproduction and life cycle}}{{anchor|Reproduction}}[[File:Kangaroo and joey03.jpg|thumb|150px|Female [[eastern grey kangaroo]] with a [[joey (marsupial)|joey]] in her pouch]]
Marsupials' reproductive systems differ markedly from [[Mammalian reproductive system|those of placentals]].<ref name="ShortBalaban1994">{{cite book| vauthors = Short RV, Balaban E |title=The Differences Between the Sexes|url={{google books |plainurl=y |id=zunYrumtsR8C}}|date=1994|publisher=Cambridge University Press|isbn=978-0-521-44878-9}}</ref><ref name=DaMR/> During embryonic development, a [[choriovitelline placenta]] forms in all marsupials. In [[bandicoots]], an additional [[chorioallantoic placenta]] forms, although it lacks the [[chorionic villi]] found in [[eutherian]] placentas.

Both sexes possess a [[cloaca]],<ref name=DaMR/> although modified by connecting to a urogenital sac and having a separate anal region in most species.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 396–399|isbn= 978-0-03-910284-5}}</ref> The [[urinary bladder|bladder]] of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.<ref name=DaMR/>

===={{anchor|Male}}Males====
[[File:The_cyclopædia_of_anatomy_and_physiology_(1847)_(20821803985).jpg|thumb|left|100px|Reproductive tract of a male [[Macropodidae|macropod]]]]
Most male marsupials, except for [[macropodidae|macropod]]s<ref name="Staker2014">{{cite book| vauthors = Staker L |title=Macropod Husbandry, Healthcare and Medicinals—Volumes One and Two|url={{google books|plainurl=y |id=37e1AwAAQBAJ}}|date=30 June 2014|publisher=Lynda Staker|isbn=978-0-9775751-2-1}}</ref> and [[marsupial mole]]s,<ref>
[https://www.jstor.org/stable/pdf/2452234.pdf On the Habits and Affinities of the New Australian Mammal, Notoryctes typhlops]
E. D. Cope
''The American Naturalist''
Vol. 26, No. 302 (February 1892), pp. 121–128</ref> have a [[wikt:bifurcated|bifurcated]] penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas.{{sfn|Nowak|1999}}<ref name=DaMR>{{cite web|url=http://www.biology.iastate.edu/InternationalTrips/1Australia/Australia%20papers/Discoveries%20about%20Marsupial%20Rep |publisher=Iowa State University Biology Dept. |title=Discoveries about Marsupial Reproduction | vauthors = King A |year=2001 |access-date=22 November 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120905123347/http://www.biology.iastate.edu/InternationalTrips/1Australia/Australia%20papers/Discoveries%20about%20Marsupial%20Rep |archive-date=5 September 2012 }}</ref><ref>{{Cite book | doi=10.1007/978-1-349-02721-7_24|chapter = Reproduction in male marsupials|title = The Biology of Marsupials| pages=411–457|year = 1977| vauthors = Setchell BP | isbn=978-1-349-02723-1}}</ref><ref name="Renfree1987">{{cite book| vauthors = Renfree M, Tyndale-Biscoe H |title=Reproductive Physiology of Marsupials|url={{google books |plainurl=y |id=HpjovN0vXW4C}}|date=1987|publisher=Cambridge University Press|isbn=9780521337922}}</ref> The penis is used only during [[copulation (zoology)|copulation]], and is separate from the [[urinary tract]].<ref name="Renfree1987" /><ref name="DaMR" /> It curves forward when erect,<ref>{{cite book| vauthors = Sadleir R |title=The Reproduction of Vertebrates|url={{google books |plainurl=y |id=PVP-wS9UXwoC}}|date=1973|publisher=Elsevier Science|isbn=978-0-323-15935-7}}</ref> and when not erect, it is retracted into the body in an S-shaped curve.<ref name="Renfree1987"/> Neither marsupials nor monotremes possess a [[baculum]].{{sfn|Nowak|1999}} The shape of the [[glans penis]] varies among marsupial species.<ref name="Renfree1987"/><ref name="Society1978">{{cite book|author=Australian Mammal Society|title=Australian Mammal Society|url={{google books |plainurl=y |id=N_ifwszrgFsC|page=73}}|date=1978|publisher=Australian Mammal Society|pages=73–}}</ref><ref name="OsgoodHerrick1921">{{cite book| vauthors = Osgood WH, Herrick CJ |title=A monographic study of the American marsupial, Caēnolestes ...|url={{google books |plainurl=y |id=8Ag9AAAAYAAJ|page=64}}|year=1921|publisher=University of Chicago|pages=64–}}</ref><ref name="The Urologic and Cutaneous Review">{{cite book|title=The Urologic and Cutaneous Review|url={{google books |plainurl=y |id=nApEAAAAYAAJ|page=677}}|year=1920|publisher=Urologic & Cutaneous Press|pages=677–}}</ref>

The shape of the urethral grooves of the males' genitalia is used to distinguish between ''[[Monodelphis brevicaudata]]'', ''[[Monodelphis domestica|M. domestica]]'', and ''[[Monodelphis americana|M. americana]]''. The grooves form two channels that form the ventral and dorsal folds of the erectile tissue.<ref>{{cite journal|vauthors = Nogueira J, Castro AS, Câamara EC, Câmara BO |title= Morphology of the Male Genital system of ''Chironectes minimus'' and Comparison to other didelphid marsupials|journal= Journal of Mammalogy|volume= 85|issue= 5|pages= 834–841|doi=10.1644/207|year= 2004|s2cid= 85595933}}</ref> Several species of [[dasyurid]] marsupials can also be distinguished by their penis morphology.<ref>{{Cite journal| vauthors = Woolley PA, Westerman M, Krajewski C |date=December 2007|title=Interspecific Affinities within the Genus Sminthopsis (Dasyuromorphia: Dasyuridae) Based on Morphology of the Penis: Congruence with Other Anatomical and Molecular Data|journal=Journal of Mammalogy|volume=88|issue=6|pages=1381–1392|doi=10.1644/06-mamm-a-443r.1|issn=0022-2372|doi-access=free}}</ref> Marsupials' only accessory sex glands are the [[prostate]] and [[bulbourethral gland]]s.<ref name="Rodger&Hughes1973">{{cite journal| vauthors = Rodger JC, Hughes RL|title=Studies of the accessory glands of male marsupials|journal=Australian Journal of Zoology|year=1973|volume=21|issue=3|page=303|doi=10.1071/ZO9730303|hdl=1959.4/70011 |url=https://unsworks.unsw.edu.au/fapi/datastream/unsworks:71547/SOURCE01?view=true|hdl-access=free}}</ref> Male marsupials have one to three pairs of bulbourethral glands.<ref>{{Cite book| vauthors = Vogelnest L, Portas T |url=https://books.google.com/books?id=396VDwAAQBAJ&q=bulbourethral|title=Current Therapy in Medicine of Australian Mammals |date=2019-05-01|publisher=Csiro Publishing|isbn=978-1-4863-0753-1|language=en}}</ref> [[Ampullae of vas deferens]], [[seminal vesicle]]s or coagulating glands are not present.<ref>{{cite journal | vauthors = Rodger JC | title = Comparative aspects of the accessory sex glands and seminal biochemistry of mammals | journal = Comparative Biochemistry and Physiology. B, Comparative Biochemistry | volume = 55 | issue = 1 | pages = 1–8 | date = January 1976 | pmid = 780045 | doi = 10.1016/0305-0491(76)90164-4 }}</ref><ref name="Hunsaker1977">{{cite book |url={{google books |plainurl=y |id=cESCLrRJGm0C}} |title=The Biology of Marsupials |vauthors=Hunsaker II D |publisher=Elsevier Science |year=1977 |isbn=978-0-323-14620-3}}</ref> The prostate is proportionally larger in marsupials than in placentals.<ref name="Renfree1987" /> During the breeding season, the male [[tammar wallaby]]'s prostate and bulbourethral gland enlarge. However, the weight of the testes does not vary seasonally.<ref name="Inns1982">{{cite journal | vauthors = Inns RW | title = Seasonal changes in the accessory reproductive system and plasma testosterone levels of the male tammar wallaby, Macropus eugenii, in the wild | journal = Journal of Reproduction and Fertility | volume = 66 | issue = 2 | pages = 675–680 | date = November 1982 | pmid = 7175821 | doi = 10.1530/jrf.0.0660675 | doi-access = free }}</ref>

===={{anchor|Female}}Females====
{{see also|Birth#Marsupials}}
[[File:Cambridge Natural History Mammalia Fig 048.png|thumb|300px|Female reproductive anatomy of several marsupial species]]
Female marsupials have two lateral [[vagina]]s, which lead to separate [[uteri]], both accessed through the same orifice.<ref>{{Cite book |last=Nowak |first=Ronald M. |url=https://books.google.com/books?id=ldXtY8ppxSQC&pg=PA14 |title=Walker's Marsupials of the World |date=2005-09-12 |publisher=JHU Press |isbn=978-0-8018-8211-1 |language=en}}</ref> A third canal, the median vagina, is used for birth. This canal can be transitory or permanent.{{sfn|Nowak|1999}} Some marsupial species [[female sperm storage|store sperm]] in the [[oviduct]] after mating.<ref name="PlantZeleznik2014">{{cite book| vauthors = Plant TM, Zeleznik AJ |title=Knobil and Neill's Physiology of Reproduction|url=https://books.google.com/books?id=I1ACBAAAQBAJ|date=15 November 2014|publisher=Academic Press|isbn=978-0-12-397769-4}}</ref>

Marsupials give birth very early in gestation; after birth, newborns crawl up their mothers' bodies and attach themselves to a teat, which is located on the underside of the mother, either inside a pouch called the [[Pouch (marsupial)|marsupium]], or externally. Mothers often lick their fur to leave a trail of scent for the newborn to follow to increase their chances of reaching the marsupium. There they remain for several weeks. Offspring eventually leave the marsupium for short periods, returning to it for warmth, protection, and nourishment.<ref name="The Conversation">{{cite web|last1=Stannard|first1= Hayley J.|last2=Old |first2=Julie M. |title=Wallaby joeys and platypus puggles are tiny and undeveloped when born. But their mother's milk is near-magical|url=https://theconversation.com/wallaby-joeys-and-platypus-puggles-are-tiny-and-undeveloped-when-born-but-their-mothers-milk-is-near-magical-207726|publisher=The Conversation|year=2023}}</ref><ref name="Marsupial and monotreme milk – a re">{{cite journal|last1=Stannard|first1=Hayley J.|last2=Miller|first2=Robert D.|last3=Old|first3=Julie M.|title=Marsupial and monotreme milk – a review of its nutrients and immune properties|journal=PeerJ|year=2020|volume=8|page=e9335|doi=10.7717/peerj.9335 |pmid=32612884 |pmc=7319036 |doi-access=free }}</ref>

====Early development====

[[File:Jeune M. rufogriseus poche.jpg|thumb|A [[red-necked wallaby]] joey inside its mother's pouch]]
Gestation differs between marsupials and [[Placentalia|placentals]]. Key aspects of the first stages of placental embryo development, such as the [[inner cell mass]] and the process of compaction, are not found in marsupials.<ref>{{cite journal | vauthors = Frankenberg SR, de Barros FR, Rossant J, Renfree MB | title = The mammalian blastocyst | journal = Wiley Interdisciplinary Reviews. Developmental Biology | volume = 5 | issue = 2 | pages = 210–232 | year = 2016 | pmid = 26799266 | doi = 10.1002/wdev.220 | s2cid = 22001725 }}</ref> The [[cleavage (embryo)|cleavage]] stages of marsupial development are vary among groups and aspects of marsupial early development are not yet fully understood.

Marsupials have a short [[gestation]] period—typically between 12 and 33 days,<ref name =KathleenSmith>{{cite journal |last1= Smith|first1=Kathleen K. |last2=Keyte |first2=Anna L. |date=2020 |title=Adaptations of the Marsupial Newborn: Birth as an Extreme Environment |url= |journal=The Anatomical Record |volume= 303|issue=2 |pages=235–249 |doi=10.1002/ar.24049 |pmid=30548826 |s2cid=56484546 |access-date=|doi-access=free }}</ref> but as low as 10 days in the case of the [[stripe-faced dunnart]] and as long as 38 days for the [[long-nosed potoroo]].<ref>{{cite journal |last1=Drews |first1=Barbara  |last2=Roellig |first2=Kathleen  |last3= Menzies|first3=Brandon R.|last4=Shaw |first4=Geoff |last5=Buentjen |first5=Ina |last6=Herbert |first6=Catherine A. |last7=Hildebrandt |first7=Thomas B. |last8=Renfree |first8=Marilyn B.|date=15 March 2013 |title=Ultrasonography of wallaby prenatal development shows that the climb to the pouch begins in utero |url= |journal= Scientific Reports|volume=3 |issue= 1458|page=1458 |doi=10.1038/srep01458 |pmid=23492830 |pmc=3597997 |bibcode=2013NatSR...3.1458D |access-date=}}</ref> The baby (joey) is born in a [[Fetus|fetal]] state, equivalent to an 8–12 week human fetus, blind, furless, and small in comparison to placental newborns: sizes range from 4-800g+.<ref name =KathleenSmith/> A newborn can be categorized in one of three grades of development. The least developed are found in [[dasyurid]]s, intermediates are found in [[didelphid]]s and [[Peramelidae|peramelids]], and the most developed are [[Macropodidae|macropods]].<ref>{{Cite journal|title=Morphology and evolution of the oral shield in marsupial neonates including the newborn monito del monte (Dromiciops gliroides, Marsupialia Microbiotheria) pouch young - PMC|year=2017 |pmc=5472534 |last1=Schneider |first1=N. Y. |last2=Gurovich |first2=Y. |journal=Journal of Anatomy |volume=231 |issue=1 |pages=59–83 |doi=10.1111/joa.12621 |pmid=28620997 }}</ref> The newborn crawls across its mother's fur to reach the [[pouch (marsupial)|pouch]],<ref>{{Cite book|url=https://books.google.com/books?id=rR9XPnaqvCMC&dq=%22The+pouch+is+like+an+external+womb%22&pg=PA224|title=The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution|first1=Richard|last1=Dawkins|first2=Yan|last2=Wong|date=9 February 2005|publisher=Houghton Mifflin Harcourt|isbn=9780618619160 |via=Google Books}}</ref> where it latches onto a [[teat]]. It does not emerge for several months, during which time it relies on its mother's milk for essential nutrients, growth factors and immunological defence.<ref>{{cite journal | vauthors = Stannard HJ, Miller RD, Old JM | title = Marsupial and monotreme milk – a review of its nutrients and immune properties | journal = PeerJ | volume = 8 | pages = e9335 | date = 2020 | doi = 10.7717/peerj.9335 | pmid = 32612884 | pmc = 7319036 | doi-access = free }}</ref> Genes expressed in the [[eutherian]] placenta needed for the later stages of fetal development are expressed in females in their mammary glands during lactation.<ref>{{Cite web|url=http://med.stanford.edu/news/all-news/2017/09/baby-marsupials-drink-placenta-to-enhance-development.html|title=Baby marsupials 'drink' placenta to enhance development|first=Krista|last=Conger|website=Stanford Medicine News Center|date=12 September 2017}}</ref> After this period, the joey spends increasing periods out of the pouch, feeding and learning survival skills. However, it returns to the pouch to sleep, and if danger threatens, it seeks refuge in its mother's pouch.

An early birth removes a developing marsupial from its mother's body much sooner than in placentals; thus marsupials lack a complex [[placenta]] to protect the [[embryo]] from its mother's [[immune system]]. Though early birth puts the newborn at greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as the fetus cannot compromise the mother in bad seasons. Marsupials are [[altricial]] animals, needing intensive care following birth ([[cf.]] [[precocial]]). Newborns lack histologically mature immune tissues<ref name="Old-2003">{{cite journal | vauthors = Old JM, Deane EM | title = The lymphoid and immunohaematopoietic tissues of the embryonic brushtail possum (''Trichosurus vulpecula'') | journal = Anatomy and Embryology | volume = 206 | issue = 3 | pages = 193–197 | date = 2003 | doi = 10.1007/s00429-002-0285-2| pmid = 12592570 | s2cid = 546795 }}</ref><ref>{{cite journal | vauthors = Old JM, Selwood L, Deane EM | title = A histological investigation of the lymphoid and immunohaematopoietic tissues of the adult stripe-faced dunnart (''Sminthopsis macroura'') | journal = Cells Tissues Organs | volume = 173 | issue = 2 | pages = 115–121 | date = 2003 | doi = 10.1159/000068946| pmid = 12649589 | s2cid = 46354564 }}</ref><ref>{{cite journal | vauthors = Old JM, Selwood L, Deane EM | title = Development of the lymphoid tissues of the stripe-faced dunnart (''Sminthopsis macroura'') | journal = Cells Tissues Organs | volume = 175 | issue = 4 | pages = 192–201 | date = 2003 | doi = 10.1111/j.0021-8782.2004.00310.x| pmid = 15255959 | pmc = 1571326 }}</ref> and are highly reliant on their mother's immune system for immunological protection.<ref>{{cite journal | vauthors = Old JM, Deane EM | title = Development of the immune system and immunological protection in marsupial pouch young | journal = Developmental and Comparative Immunology | volume = 24 | issue = 5 | pages = 445–454 | date = 2000 | doi = 10.1016/S0145-305X(00)00008-2| pmid = 10785270 }}</ref>

Newborns front limbs and facial structures are much more developed than the rest of their bodies at birth.<ref>{{cite journal | vauthors = Sears KE | title = Differences in the timing of prechondrogenic limb development in mammals: the marsupial-placental dichotomy resolved | journal = Evolution; International Journal of Organic Evolution | volume = 63 | issue = 8 | pages = 2193–2200 | date = August 2009 | pmid = 19453378 | doi = 10.1111/j.1558-5646.2009.00690.x | s2cid = 42635687 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Smith KK | title = Early development of the neural plate, neural crest and facial region of marsupials | journal = Journal of Anatomy | volume = 199 | issue = Pt 1-2 | pages = 121–131 | year = 2001 | pmid = 11523813 | pmc = 1594995 | doi = 10.1046/j.1469-7580.2001.19910121.x }}</ref><ref name="Old-2003"/> This requirement has been argued to have limited the range of locomotor adaptations in marsupials compared to placentals. Marsupials must develop grasping forepaws early, complicating the evolutive transition from these limbs into [[hoof|hooves]], [[wing]]s, or [[flipper (anatomy)|flippers]]. However, several marsupials do possess atypical forelimb morphologies, such as the hooved forelimbs of the [[pig-footed bandicoot]], suggesting that the range of forelimb specialization is not as limited as assumed.<ref>Larry Vogelnest, Graeme Allan, Radiology of Australian Mammals</ref>

Joeys stay in the pouch for up to a year or until the next joey arrives. Joeys are unable to regulate their body temperature and rely upon an external heat source. Until the joey is well-furred and old enough to leave the pouch, a pouch temperature of {{convert|30|-|32|C|F}} must be constantly maintained.

Joeys are born with "oral shields", soft tissue that reduces the mouth opening to a round hole just large enough to accept the teat. Once inside the mouth, a bulbous swelling on the end of the teat attaches it to the offspring till it has grown large enough to let go. In species without pouches or with rudimentary pouches these are more developed than in forms with well-developed pouches, implying an increased role in ensuring that the young remain attached to the teat.<ref>{{cite journal | vauthors = Schneider NY | title = The development of the olfactory organs in newly hatched monotremes and neonate marsupials | journal = Journal of Anatomy | volume = 219 | issue = 2 | pages = 229–242 | date = August 2011 | pmid = 21592102 | pmc = 3162242 | doi = 10.1111/j.1469-7580.2011.01393.x }}</ref><ref>{{Cite journal|title=Comparative anatomy of neonates of the three major mammalian groups (monotremes, marsupials, placentals) and implications for the ancestral mammalian neonate morphotype - PMC|year=2017 |pmc=5696127 |last1=Ferner |first1=K. |last2=Schultz |first2=J. A. |last3=Zeller |first3=U. |journal=Journal of Anatomy |volume=231 |issue=6 |pages=798–822 |doi=10.1111/joa.12689 |pmid=28960296 }}</ref>