Editing Marsupial (section)

====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>