View source for Opossum

=== Reproduction and life cycle ===
{{further|Marsupial#Reproductive system}}

As marsupials, female opossums have a reproductive system that includes a bifurcated [[vagina]] and a divided [[uterus]]; many have a [[Pouch (marsupial)|pouch]].<ref>Campbell, N. & Reece, J. (2005) ''Biology''. Pearson Education Inc.</ref> The average estrous cycle of the [[Virginia opossum]] is about 28 days.<ref name="Opossum Society of the United States">[http://www.opossumsocietyus.org/opossum_reproduction_and_life_cycle.htm Reproduction – Life Cycle]. opossumsocietyus.org.</ref>  Opossums do possess a [[placenta]],<ref name=Enders2005>{{cite journal |author1=Enders, A.C.  |author2=Enders, R.K.  |name-list-style=amp | year = 2005 | title = The placenta of the four-eyed opossum (''Philander opossum'') | journal = The Anatomical Record | volume = 165 | issue = 3 | pages = 431–439 | doi = 10.1002/ar.1091650311|pmid=5346723  |s2cid=85011250 }}</ref> but it is short-lived, simple in structure, and, unlike that of placental mammals, not fully functional.<ref name=Krause1985>{{cite journal |author1=Krause, W.J.  |author2=Cutts, H.  |name-list-style=amp | year = 1985 | title = Placentation in the Opossum, ''Didelphis virginiana'' | journal = Acta Anatomica | volume = 123 | issue = 3 | pages = 156–171 | doi = 10.1159/000146058 | pmid = 4061035}}</ref> The young are therefore born at a very early stage, although the [[gestation]] period is similar to that of many other small marsupials, at only 12 to 14 days.<ref name=EoM>{{cite book |editor= Macdonald, D. |author= O'Connell, Margaret A. |year= 1984 |title= The Encyclopedia of Mammals |publisher= Facts on File |location= New York |pages= [https://archive.org/details/encyclopediaofma00mals_0/page/830 830–837] |isbn= 978-0-87196-871-5 |url-access= registration |url= https://archive.org/details/encyclopediaofma00mals_0/page/830 }}</ref> They give birth to litters of up to 20 young.<ref>{{cite web |title=Opossums {{!}} National Geographic |url=https://www.nationalgeographic.com/animals/mammals/group/opossums/ |archive-url=https://web.archive.org/web/20170221153623/http://www.nationalgeographic.com/animals/mammals/group/opossums/ |archive-date=February 21, 2017 |website=Animals |language=en |date=11 November 2010}}</ref> Once born, the offspring must find their way into the marsupium, if present, to hold on to and nurse from a teat. Baby opossums, like their Australian cousins, are called joeys.<ref>{{cite journal|pmid=25266360|year=2015|last1=Mellor|first1=D. J.|title=Survival implications of the development of behavioral responsiveness and awareness in different groups of mammalian young|journal=New Zealand Veterinary Journal|volume=63|issue=3|pages=131–40|last2=Lentle|first2=R. G.|doi=10.1080/00480169.2014.969349|s2cid=24045688}}</ref> Female opossums often give birth to very large numbers of young, most of which fail to attach to a [[teat]], although as many as 13 young can attach,<ref name=smithsonian /> and therefore survive, depending on species. The young are [[Weaning|weaned]] between 70 and 125 days, when they detach from the teat and leave the pouch. The opossum lifespan is unusually short for a mammal of its size, usually only one to two years in the wild and as long as four or more years in captivity. [[Senescence]] is rapid.<ref>[http://www.opossum.org/facts.htm Opossum Facts]. opossum.org.</ref>
Opossums are moderately [[Sexual dimorphism|sexually dimorphic]] with males usually being larger, heavier, and having larger [[Canine tooth|canines]] than females.<ref name=smithsonian>[http://www.mnh.si.edu/mna/image_info.cfm?species_id=65 Virginia Opossum. ''Didelphis virginiana'']. Smithsonian National Museum of Natural History.</ref> The largest difference between the opossum and non-marsupial mammals is the bifurcated penis of the male and bifurcated vagina of the female (the source of the term ''didelphimorph'', from the Greek ''didelphys'', meaning "double-wombed").<ref>{{cite web|title=Possum Hunt|url=http://projects.scsc.k12.ar.us/index.php?page=possum-hunt|archive-url=https://web.archive.org/web/20131110190430/http://projects.scsc.k12.ar.us/index.php?page=possum-hunt|archive-date=2013-11-10}}</ref> Opossum spermatozoa exhibit sperm-pairing, forming conjugate pairs in the [[epididymis]]. This may ensure that [[flagella]] movement can be accurately coordinated for maximal [[motility]]. Conjugate pairs dissociate into separate spermatozoa before fertilization.<ref>{{cite journal
 |author= Moore, H.D.
 |title= Gamete biology of the new world marsupial, the grey short-tailed opossum, monodelphis domestica
 |journal=Reproduction, Fertility, and Development
 |volume= 8
 |issue=4
 |pages=605–15
 |year=1996
 |doi= 10.1071/RD9960605
|pmid= 8870084
 }}</ref>