Editing Opossum (section)

== Characteristics ==
[[File:Monodelphis domestica skeleton - ZooKeys 465-10.png|thumb|Skeleton of the [[gray short-tailed opossum]] (''Monodelphis domestica'')]]
Didelphimorphs are small to medium-sized marsupials that grow to the size of a house cat. They tend to be semi-[[arboreal]] [[omnivore]]s, although there are many exceptions. Most members of this order have long [[snout]]s, a narrow [[Cranium|braincase]], and a prominent [[sagittal crest]]. The [[dental formula]] is: {{DentalFormula|upper=5.1.3.4|lower=4.1.3.4|total=50}} teeth. By [[mammal]]ian standards, this is an unusually full jaw. The incisors are very small, the canines large, and the molars are [[Dental anatomy#Cusp|tricuspid]].

Didelphimorphs have a [[plantigrade]] stance (feet flat on the ground) and the hind [[Foot|feet]] have an [[Thumb|opposable digit]] with no [[claw]]. Like some [[New World monkey]]s, some opossums have [[Prehensility|prehensile tails]]. Like most marsupials, many females have a pouch. The tail and parts of the feet bear [[scute]]s. The [[stomach]] is simple, with a small [[cecum]].<ref name = "Krause"/> Like most marsupials, the male opossum has a forked [[penis]] bearing twin [[Glans penis|glande]]s.<ref>{{cite journal|doi=10.1515/mamm.1997.61.2.161|url=https://www.researchgate.net/publication/240754313 |title=Penis morphology as a distinctive character of the murine opossum group (Marsupialia Didelphidae): A preliminary report |journal=Mammalia |volume=61 |issue=2 |year=1997 |last1=Martinelli |first1=P.M. |last2=Nogueira |first2=J.C. |s2cid=84674547 }}</ref><ref>{{cite journal|url=http://ctcusp.org/pdf/references2013/2013_37.pdf |pmid=23362127|year=2013|last1=De Barros|first1=M. A.|title=Marsupial morphology of reproduction: South America opossum male model|journal=Microscopy Research and Technique|volume=76|issue=4|pages=388–97|last2=Panattoni Martins|first2=J. F.|last3=Samoto|first3=V. Y.|last4=Oliveira|first4=V. C.|last5=Gonçalves|first5=N.|last6=Mançanares|first6=C. A.|last7=Vidane|first7=A.|last8=Carvalho|first8=A. F.|last9=Ambrósio|first9=C. E.|last10=Miglino|first10=M. A.|doi=10.1002/jemt.22178|s2cid=27200317}}</ref><ref name="Krause">Krause, William J.; Krause, Winifred A. (2006).[https://web.missouri.edu/~krausew/Histology/Home_files/opossum.pdf  ''The Opossum: Its Amazing Story''] {{webarchive|url=https://web.archive.org/web/20121211214644/https://web.missouri.edu/~krausew/Histology/Home_files/opossum.pdf |date=2012-12-11 }}. Department of Pathology and Anatomical Sciences, School of Medicine, University of Missouri, Columbia, Missouri. p. 39</ref>

Although all living opossums are essentially opportunistic [[omnivores]], different species vary in the amount of meat and vegetation they include in their diet. Members of the [[Caluromyinae]] are essentially [[frugivorous]]; whereas the [[lutrine opossum]] and [[Patagonian opossum]] primarily feed on other animals.<ref>{{cite book|last1=Vieira|first1=Emerson R.|last2=De Moraes|first2=D. Astua|publisher=Csiro Publishing|year= 2003|isbn=978-0-643-06634-2|title=Predators with Pouches: the biology of carnivorous marsupials|pages=267–280 |chapter=Carnivory and insectivory in Neotropical marsupials}}</ref> The [[water opossum]] or yapok (''Chironectes minimus'') is particularly unusual, as it is the only living semi-aquatic marsupial, using its webbed hindlimbs to dive in search of freshwater mollusks and crayfish.<ref>{{cite journal|title=''Chironectes minimus''|first=Larry G.|last=Marshall|journal=[[Mammalian Species]]|year=1978|volume=109|pages=1–6|doi=10.2307/3504051|issue=99|jstor=3504051|doi-access=free}}</ref> The extinct ''[[Thylophorops]]'', the largest known opossum at {{Cvt|4-7|kg}}, was a macropredator.<ref name="Goin 2009 35–46">{{cite journal|title=A new large didelphid of the genus ''Thylophorops'' (Mammalia: Didelphimorphia: Didelphidae), from the late Tertiary of the Pampean Region (Argentina)|first=Francisco J.|last=Goin|author2=Natalia Zimicz|author3=Martin de los Reyes|author4=Leopoldo Soibelzon|url=http://biostor.org/reference/19000|journal=[[Zootaxa]]|year=2009|volume=2005|pages=35–46|doi=10.11646/zootaxa.2005.1.3|hdl=11336/67625|hdl-access=free|archive-date=2013-12-17|access-date=2013-12-17|archive-url=https://web.archive.org/web/20131217052732/http://biostor.org/reference/19000|url-status=dead}}</ref><ref>{{cite journal|doi=10.1007/s10914-011-9175-9|title=The Evolution of the Cenozoic Terrestrial Mammalian Predator Guild in South America: Competition or Replacement?|journal=Journal of Mammalian Evolution|volume=20|pages=3–21|year=2011|last1=Prevosti|first1=Francisco J.|last2=Forasiepi|first2=Analía|last3=Zimicz|first3=Natalia|s2cid=15751319|hdl=11336/2663|hdl-access=free}}</ref><ref>{{cite journal|doi=10.1080/08912963.2015.1089868|title=Mammalian predator–prey relationships and reoccupation of burrows in the Pliocene of the Pampean Region (Argentina): New ichnological and taphonomic evidence|journal=Historical Biology|volume=28|issue=8|pages=1026–1040|year=2015|last1=Cenizo|first1=Marcos|last2=Soibelzon|first2=Esteban|last3=Magnussen Saffer|first3=Mariano|s2cid=83862150}}</ref> Most opossums are [[scansorial]], well-adapted to life in the trees or on the ground, but members of the [[Caluromyinae]] and [[Glironia|Glironiinae]] are primarily arboreal, whereas species of ''[[Metachirus]]'', ''[[Monodelphis]]'', and to a lesser degree ''[[Didelphis]]'' show adaptations for life on the ground.<ref>{{cite journal|title=Phylogenetic analysis of postcranial skeletal morphology in didelphid marsupials|first=David A.|last=Flores|journal=[[Bulletin of the American Museum of Natural History]]|year=2009|volume=320|pages=1–81|doi=10.1206/320.1|hdl=2246/5953|s2cid=54177473|hdl-access=free}}</ref> ''[[Metachirus nudicaudatus]]'', found in the upper [[Amazon basin]], consumes fruit seeds, small [[vertebrate]] creatures like birds and reptiles and [[invertebrate]]s like [[crayfish]] and [[snail]]s, but seems to be mainly [[insectivorous]].<ref>{{cite book |last1=Gardner |first1=Alfred |title=Mammals of South America Volume 1 |publisher=University of Chicago Press |page=34}}</ref>

[[File:Opossum (Mar 2021).jpg|thumb|Virginia opossum on top of a fence]]

=== 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/ |url-status=dead |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|url-status=dead|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>

=== Behavior ===
[[File:Opossum2.jpg|left|thumb|Virginia opossum [[apparent death|feigning death]], or "playing possum"]]

Opossums are usually solitary and nomadic, staying in one area as long as food and water are easily available. Some families will group together in ready-made burrows or even under houses. Though they will temporarily occupy abandoned burrows, they do not dig or put much effort into building their own. As nocturnal animals, they favor dark, secure areas. These areas may be below ground or above.<ref>{{cite journal|author=Hamilton, W. J. Jr. |year=1958|title= Life history and economic relations of the opossum (''Didelphis marsupialis virginiana'') in New York State|journal= Cornell Univ. Agric. Exp. Sta. Memoirs |volume=354|pages=1–48}}</ref><ref>{{Cite web|url=http://wdfw.wa.gov/living/opossums.html|title=Opossums – Living with Wildlife {{!}} Washington Department of Fish & Wildlife|website=wdfw.wa.gov|access-date=2017-05-14}}</ref>

[[File:Baby opossum.jpg|right|thumb|Juvenile Virginia opossum hissing defensively]]
When threatened or harmed, they will "[[Apparent death|play possum]]", mimicking the appearance and smell of a sick or dead animal. This physiological response is involuntary (like fainting), rather than a conscious act.  In the case of baby opossums, however, the brain does not always react this way at the appropriate moment, and therefore they often fail to "play dead" when threatened. When an opossum is "playing possum", the animal's lips are drawn back, the teeth are bared, saliva foams around the mouth, the eyes close or half-close, and a foul-smelling fluid is secreted from the [[anal glands]]. The stiff, curled form can be prodded, turned over, and even carried away without reaction. The animal will typically regain consciousness after a period of a few minutes to four hours, a process that begins with a slight twitching of the ears.<ref>[http://www.opossumsocietyus.org/injured_opossum.html Found an Orphaned or injured Opossum?]. Opossumsocietyus.org. Retrieved on 2012-05-03.</ref>

Some species of opossums have [[prehensile tail]]s, although dangling by the tail is more common among juveniles. An opossum may also use its tail as a brace and a fifth limb when climbing. The tail is occasionally used as a grip to carry bunches of leaves or bedding materials to the nest.<ref name="GonzálezClaramunt2000">{{cite journal|last1=González|first1=E.M.|last2=Claramunt|first2=S.|title=Behaviors of captive short-tailed Opossums, ''Monodelphis dimidiata'' (Wagner, 1847) (Didelphimorphia, Didelphidae)|journal=Mammalia|volume=64|issue=3|year=2000|pages=271–286 |doi=10.1515/mamm.2000.64.3.271|s2cid=84782113}}</ref> A mother will sometimes carry her young upon her back, where they will cling tightly even when she is climbing or running.

Threatened opossums (especially males) will growl deeply, raising their pitch as the threat becomes more urgent. Males make a clicking "smack" noise out of the side of their mouths as they wander in search of a mate, and females will sometimes repeat the sound in return. When separated or distressed, baby opossums will make a sneezing noise to signal their mother.  The mother in return makes a clicking sound and waits for the baby to find her. If threatened, the baby will open its mouth and quietly hiss until the threat is gone.<ref>{{Cite web|title=Opossum Sounds & Noises: What Do Opossums Sound Like?|url=https://www.crittercontrol.com/wildlife/opossums/opossum-noises|access-date=2022-01-24|website=www.crittercontrol.com}}</ref>